Subsections

Phytopathology, diseases of plants

Diseases of plants can cause great crop damages which may lead to hunger disasters of great part of populations. Basic knowledge of the diseases of plants and their cause are therefore important for everyone handling foods.
Despite all efforts to reduce crop damage due to plant diseases using agrochemicals in the past 30 years the loss of approximately 10% remained unchanged. As the crops have increased the total loss is increasing rapidly. This makes a better understanding of phytopathology necessary[856].
Phytopathology wants to bring a bectter understanding on the cause, the circumstances and the progress of diseases of plants as well the interaction of host-pathogenic agent.
Phytopathogenic agents are not pathogenic to mankind as moulds and bacteria known as phytopathogenic cannot grow at 37$^{o}$C or are eliminated by immunology of mankind. Phytopathogenic virus need plant louse or gnome zicada as vectors as they cannot penetrate by itself. Human skin is not attractive for these insects. The infection of these virus is therefore not possible.
However allergies to the spores of moulds such as Aspergillus and Penicillium sp. and mycotoxins are of great importance.

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The evolution of diseases in plants:

Diseases in plants develop in different phases:
Phases of a disease caused by mould:
Infection period: The infection period is the time between the first attack of the plant by the pathogen agent and the moment in which he has established itself in the tissue of the host.

Incubation period:The incubation period includes the infection period and ends with first macroscopic symptoms.

Fructification period: Is the time between Infection and the first reproduction of the agent in the host.

Some moulds have short periods and the whole life cycle takes only days such as:
Polymyxa
Phytium
Olpidium

Some moulds need over half a year to complete their life cycle such as:
Phases of a disease caused by other agents: viral agents,bacteria, mycoplasma-like organism (MLO) and Rickettsia-like Organism (RLO) present a great reproduction with high number of organism before the fist symptoms appear.
Vectors of phytopathogenic agents
Bacteria such as Xylella fastidiosa, spiroplasma, mycoplasma-like Organism (MLO) and Ricketsia-like organism (RLO) need the help of other living cells or small animals like gnome zicade as vectors to invade another host


The cause of plant damage

Condition for a disease of plants

Susceptible to the disease: The host must be susceptible to the disease. Some plants are resistant to different agents.

Organ specific activity: There must be an organ specific activity of the agent. Some agents can infest only specific organs like flowers or roots to which they are specialized. they cannot invade other parts of the plants.
Examples of organ specific activity are: Polymyxa betae for instance can only atta
Phase specific activity
Some agents have a specific activity to flowers. They have wait till the blossoms open to attack the host, other have a phase specific activity to leaves. they have to wait for the phase of leaves of a plant which is growing from a seed.
Survival of the agent
Some agents have developed survival strategies to overcome the time between one host to another. Virus cannot survive for long time outside a host cell. They need therefore a vector cell to get through the time between where there are no host cells available.The spores of the mould Polymxa can serve as vector for the beet necrotic yellow vein furovirus for a period of many years. Other agents have a saprophytic phase surviving in dead tissue of the host of the culture period to another season to start a new outbreak of disease. Saprophytic decomposition
Some saprophytes may decompose cell walls of dead material this activity also can include living phytopatological agents. Mucor and Rhizopus moulds can decompose cellulose of the cell of dead materials disclosing pathogen virus which had invaded these cells. The virus is now unprotected and will soon be damaged.
Agrochemicals can interfere in the cycle of nature destroying normal saprophytes which are enemies of harmful pathogenic agents.
Competition of nutrients
Saprophytic bacteria can multiply within an half hour. Pathogen mould need days to multiply being so in disadvantage on regard of nutritional resources.
Production of antibiotics Some moulds like Penicillium sp and Streptomyces sp =produce antibiotics which act as natural enemies of some phytopathogen agents. Interfering in the natural environment may destroy useful saprophytes boosting pathogens.
Virus with mycopathogenic activity. Some virus act toxic on pathogenic moulds. The protection of the natural environment is therefore important. Modern agricultural techniques should protect the complex interaction of th different biological systems.
Hyperparasites
A parasite with phytopathologic activities may be invaded on its turn, by other parasites ,like nematodes, bacteria and bacteriophages.

Parasitic plants can be half parasites having well developed leaves. They depend only from water and minerals from the xylem of the host (Loranthaceae, Viscaceae, Scrophulariaceae.
Full parasites have underdeveloped or no leaves at all. They invade the xilem and the phloem of their host and get there nourishing substances
Mistletoe    
Loranthus europaeus South Europe Oak tree
Arceuthobium sp North America, Himalaya conifer
Viscum album ssp album Europe Apple, poplar
Viscum album ssp.abietes Europe Fir tree
Convolvulaceae    
Cuscuta europea Europe Hop, shrubs, sugar beets
Cuscuta campestris worldwide Legume, sugar beet
Cuscuta reflexa Southeast Asia Citrus, Coffee, Litchi
Scrophulariaceae    
Striga asiatica worldwide,except Europe Maize,sorghum, millet, sugar cane
Orobanchaceae    
Orobanche cernua Mediterranean region Sunflower
  East Europe,Africa  
Orobanche crenata Mediterranean region Beans, lentil, chick pea
Orobanche ramosa Mediterranean region Tobaco, tomato, aubergine
  Africa, Zentral and North America  

Microbes as agents of plant diseases

Virus: have only one type of nucleic acid, most of the time RNA, seldom DNA. Energy system ATP does not exist. Virus depend on ribosomes of host cells.
Mycoplasma-Like Organism (MLO)
Rickettsia-Like Organism (RLO)
Bacteria
Moulds
MLO, RLO, bacteria and moulds have usually both types of nucleic acids (RNA and DNA). Energy system ATP exists and they can multiply without aid of a host cell.


Phytopathogenic virus

Cauliflower mosaic (Caulimo): It is build of DNA.
Bacilus-shaped DNA (Badna)
Gemini virus
Phytoreo virus
Rhabdo virus
Tomato spotted wilt (Tospo)
Tenui virus
Cowpea mosaic virus (Como)
Faba virus
Nematode-transmitted polyeder (Nepo)
Potato virus Y (Poty)
Rye grass mosaic (Rymo)
Barley yellow mosaic (Bymo)
Southern bean mosaic (Sobemo)
Luteo virus
Tomato bushy stunt (Tombus)
Carnation mottle (Carmo)
Tobacco necrosis (Necro)
Diantho virus
Tobacco mosaic (Tobamo)
Tobaco rattle (Tobra)
Hordei virus
Fungus-transmitted, rod shaped virus (Furo)
Clostero virus
Turnip yellow mosaic (Tymo)
Alfafa mosaic (Alfamo)
Isometric labile ringspot (Ilar)
Brome mosaic (Bromo)
Cucumber mosaic (Cucumo)
Carnation latent (Carla)
Potato virus X (Potex)

Virus of economic importance

Cacao swollen shoot badnavirus: It is a DNA- virus.
Tomato leaf roll geminivirus: DNA- virus.
Cassava mosaic geminivirus: DNA- virus
Rice dwarf phytoreovirus: RNA- virus
Tomato spotted wilt tospovirus
Rice grassy stunt tenuivirus
Barley yellow dwarf luteovirus
Potato leaf roll luteovirus
Beet yellows closterovirus
Citrus tristeza closterovirus
Potato virus Y potyvirus
Barley yellow mosaic bymovirus
Tobacco mosaic tobamovirus
Grapevine fanleaf nepovirus
Beet necrotic yellow vein furovirus
Cucumber mosaic cucumovirus
Prunus necrotic ringspot ilarvirus
Alfafa mosaic alfamovirus
Tobacco rattle tobravirus

Pathogen germ Host Disease
     
Gram-negative    
Agrobacterium tumefaciens   tumor
Agrobacterium vitis vine  
Burkholderia solanacearum    
Burkholderia caryophylli clove  
Erwinia amylovora pear and apple  
Erwinia carotovora potatos  
Pseudomonas savastanoi bush bean  
Pseudomonas syringae    
Xanthomonas axonopodis    
Xanthomonas campestris cabbage  
Xantomonas hortorumpelargonium    
Xanthomonas oryzae rice  
Xanthomonas transluceus wheat  
     
Gram-positive    
Clavibacter michiganensis tomato  
Rhodococcus fascians pea  
Streptomyces scabies potato  

Phytopathogenic moulds

Aspiognomonia veneta: It causes antracnosis on plane-tree.
Cryphonectria parasitica: causes disease of chestnut-tree.
Diaporthe perniciosa: Causes necrosis of the bark from apple-tree
Leucostoma cincta: Causes a disease of the bark of peach-trees.
Cochliobolus sativus: Disease of cereals.
Cochliobolus victoriae: Victoria disease of oats.
Didymella aplanata: Disease of raspberry.
Leptosphaeria maculans: Disease of the roots of rape.
Mycosphaerella arachidicola: Spots on leaves of the peanut-plant.
Mycosphaerella graminicola: Spots on leaves of wheat.
Mycosphaerella musicola: Disease of banana.
Phaesphaeria nodorum: Disease of wheat.
Pyrenophora chaetomioides: Disease of oats.
Pyrenophora graminea: Disease of barley.
Pyrenophora teres:Disease of barley.
Venturia inequalis: Scab of apple.
Blumeria graminis Disease of cereals: It cannot synthesize purines which are part of nucleic acid . Purines must therefor be taken from the host. It is therefore an obligatory parasite
Leveillula taurica: Disease of tomatos.
Podosphaera leucotricha: Disease of appel.
Sphaerotheca fuliginia: Disease of cucumber.
Uncinula necator: Disease of grape plant.
Aspergillus flavus:Disease of peanuts.
Penicillium expansum Rot of apple.
Claviceps purpurea: Ergotism of wheat. Claviceps purpurea forms ascospores can infect only blossoms. The best conditions of weather for the production of blossoms of the wheat most of the time occurs at a different time of the releae
Builts black sclerots which are formed from mycelium Cibberella fujicuroi: Disease of rice.
Cibberella zeae:Disease of maize.
Nectria galligena: canker of apple-tree.
Diplocarpon rosae: Disease of roses.
Drepanopeziza ribis: Disease of redcurrant.
Mollisia acuformis: Disease of cereals.
Pezicula malicortis: disease of apple.
Pseudopezicola tracheiphila Disease of wine.
Pseudopeziza medicaginis: Disease of Spanish trefoil.
Magnaporthe grisea Rice blast.
Taphrina deformans: Disease of peach.
Taphrina pruni: Disease of prun.
Alternaria solani: Stains of the leaves of tomato-plant.
Septoria apiicola: Stains of the leaves of celeriae.
Alternaria sp.: Black rott of citrus fruit.
Aspergillus sp.: Black rott of peach and nectarine.
Aspergillus flavus produce aflatoxin B$_{1}$. 100mcg in the feed of mice over a long period o time induces liver cancer in all rats. In Africa Aspergillus flavua can be very high. The disease of liver cancer is in Africa the highest of the world because of contaminated vegetable foods. Botryodiplodia sp.: Rottenness of the banana-plant.
Botrytis sp.: Rottenness of fruits.
Cercospora musaeIs the cause of the Sigatoka disease of banana plantations.To fight the Sigatoka fungus a watery solution of chalk and copper sulphate was sprayed on the plants, later on mineral oil was sprayed by helicopters and crop airplanes.
Environment protection organizations have started a campaign to reduce toxic chemicals in banana plantations. The careless use of chemicals used against moulds and worms as well as weed killers being sprayed by crop planes over banana plantations produce sterility, skin diseases, asthma, damage kidneys and liver of the population living in this area.[873]
Cladosporium sp.: Rottenness of fruits.
Colletotrichum sp.: Rottenness of citrus fruits, avocados and papayas.
Diaporthe sp.: rottenness of citrus fruits.
Diplodia sp.: Rottenness of the stem of citrus fruits, avocados, and papayas. Wet rottenness of peach.
Fusarium sp.: Brown rottenness of citrus fruits, pineapple and wet rottenness of figs.
Fusarium oxysporum var.cubense This fungus is known as agent of the Panama disease of banana plants. It invades the roots of the plant and destroys a banana farm. In 1925 and 1935 the Fusarium fungus was avoided by getting the plantations one to one and a half meter under water. After six month the water was drained and the plantation could be used again for five to six years. This method could be used only in valleys or plains.
Very sensible to Fusarium are the banana sorts Gros Michel. In Brazil, in some parts of Africa and Australia the Cavendish sort is being cultivated, also Lactan and Robusta are sorts which are resistant to the fungus Fusarium.
Geotrichum sp.: Acid rottenness of citrus fruits and peaches.
Gibberella fujikuroi is the agent of the Bakanae disease of rice. The fungus Gibberella produces gibberilin acid which is a growth promoter. The growth which is induced by gibbeilin thin and the plant can scarcely carry its own weight. Gibberella also produces a substance which is toxic to the roots of rice. The rice plants die on account of these substances. Gloeosporium sp. Rottenness of pome fruits.
Leptosphaeria maculansIs worldwide the most important pathogen fungus which attacks rape. There are pathogen and not pathogen types.
Monilia sp.: Brown rottenness of fruits.
Monilia attacks cherry-trees, killing leaves and branches.
Nigrospora sp.: Rottenness of banana pulp.
Penicillium sp.: Blue and green rottenness of fruits.
Phomopsis sp.: Rottenness of the stem of citrus fruits and avocados.
Phytophthora sp.: Rottenness of citrus fruits, apple and strawberry.
Piricularia oryzae causes the Brusone disease blue dots which later change to brown color are the first symptoms. When the invasion takes place at an early stage of the growth of the plant no rice kernels are built. If it takes place later the stalk is bend under the weight of the grain in such a manner that the grain comes together with water or mud and gets lost.
To help against the Brusone disease solutions of mercury compounds and copper compounds were used despite their toxic effects. Rhizopus sp.: Wet rottenness of fruits.
Sclerotinia.: White rottenness of strawberry.
Sclerotinia sclerotiorum Is a fungus which causes great damage on rape. This fungus is in vitro highly sensible to alkenile-glucosinolates. Sclerotinia sclerotiorum produces during the invasion of a plant high amount of oxalic acid getting the pH down to 2,0 - 4,0. At the place of the invasion nitriles instead of alkenyles are formed. Trichoderma sp.: Brown-green rottenness of citrus fruits.
Venturia sp.: rottenness of fruits.

Garlic white rot on organic muck fields

[874]
Hovius and McDonald determine the efficacy of synthetic diallyl disulfide (DADS) to fight white rot of onions grown on organic muck soil. The authors concluded that DADS and to a lesser extent di-N-propyl disulfide (DPDS) effectively reduced the incidence of Allium white rot on onions grown on muck soils. Soil treatment with DADS.

According to the authors Sclerotium cepivorum Berk causes the disease white rot on several Allium family, such as Allium cepa L., Onions, leeks, garlic, chives, shallots and salad onions The disease is significant on organic soils.

Sclerotia germination and host infection are stimulated by alkenyl L-cysteine sulfoxides released by garlic and onions. Garlic root exudates are precursors of the volatile allyl and propyl sulfides that stimulate eruptive germination of the sclerotia.

Germination can be stimulated by products such as natural onion and garlic oil, or synthetic germination stimulants derived from petroleum, such as diallyl disulfide (DADS), a primary decomposition product of allicin.

When the sclerotia germinates, it develops a mycelium which becomes susceptible to hyperparasitism and lysis. In the absence of a host, the fungus dies without reproducing. Two applications of the synthetic germination stimulant DADS, but not DPDS or garlic oil in consecutive years resulted i9n significant reduction of onion infection on organic.

Soils should be treated when the soil temperatures are between 10 and 20$^{0}$C and will remain remain in this range for approximately 3 months with good uniform spread of the stimulant.

Mediterranean fruit fly

The Mediterranean fruit fly Ceratitis capitata attacks fruits, vegetables and nuts. The female Mediterranean fly mates once in her life span depositing the sperm in the female sperm-storing organ, the spermatheca. The female produces 250 - 1,200 eggs which can be fertilized with the stored sperms from the spermatheca.
The fertilized eggs are deposited beneath the outer surface of fruit and vegetables hatching into maggots which then feed on the pulp of the fruit or vegetable spoiling them. Matured maggots drop to the ground, form pupae and eventually emerge as new flies, completing the life cycle.
The fly originated in West Africa from where it spread out to North and South Africa. As the eggs are hidden under the skin of fruits the fly can easily be exported to all parts of the world. The fly is now being found in Europe, Middle East, Australia and South America.


Key program of USA against the Mediterranean fruit fly

In California the Mediterranean fruit fly was seen for the first time in 1975. USA has a key program against the fruit fly distributing traps with trimedlure as bait for the fly.

External abiotic sources of damage of plants

Plant diseases which spread by traffic

Hop mildew was introduced in North America in 1890 coming from Europe and Asia. Hop mildew destroyed the plantations of hop of east and middle west of USA. The mildew mould was found all over the country.
Asparagus rust reached America 1912. Quarantine of plants and seeds being imported are an important measure against uncontrolled spread of plant diseases.


Silage

[880] To avoid moulds and resulting aflatoxins in hay silage and forage as well as silage grains and feeds liquid concentrates of fermentation extracts of Lactobacillus acidophilus, lactis in the pre-bud stage of growth may be added. Alfalfa hay can thus be baled with 18 to 23% moisture.
1 ton square bales hay had the best results on proteins when obtained at 20% moisture.


Most frequent hazards to trees

[1201]
Beetles:
Beetles attack deciduous trees and coniferous forests.If they fed on leaves only the damage is not great, but some beetles such as bark beetles can destroy large parts of the plant beneath the bark. Other beetles transmit virus diseases.
Rhychaenus fragilis: Their larva undermine the leaves of the red beech and copper beech. the leaves turn out brown.
Elm sapwood beetle (Scolytus scolytus): is known to attack European elms. In America Elms are threatened by Hylurgopinus rufipes.
Big brown weevil (Hylobius abietis): This beetle gnaws round holes in the bark of young conifers causing their death.
Butterflies
If numerous butterflies are present complete defoliation of the growing ends may happen. Of importance are
Sackbeetle (Thyridopteryx ephemeraeformis): Male species with wings are harmless.Female species are wingless and live in a sack spun with leaves and leavestalks of USA shrubs and trees. They can cause defoliation of young trees.
Moonbird (Phalera bucephala): Larvae can cause complete defoliation at various deciduous trees.
Tamarack moth (Coleophora laricella): The larvae hollow out the needles of tamarack.
Western tent caterpillar (Malacosoma pluvialis): Their larvae live on hardwood trees of the Pacific coast of USA, inside of cocoons and nourish themselves with the young shoots of the plants.
Sawflies
Adult species are not harmful, but their larvae destroy leaves and needles heading to complete defoliation.
Larch sawfly (Pristiphora erichsonii): The larvae nourish from larch needles.causing destructive defoliation.
Willow sawfly (Pontania vesicator): It forms blister-like galls between the midrib and leave edge of willow trees. In these galls larvae develop.
Plant louse
They are small insects which may have wings or are wingless. attacking deciduous trees and coniferous plants.
Schizoneura lanuginosa: It sucks on elm leaves . not regular galls are formed.
Byrsocrypta ulmi (Byrsocrypta ulmi:) produces similar lesions as the galls of Schizoneura, but they are club formed.
Pemphigus bursarius: It attacks poplars deforming the petioles with galls.
Sitka spruce Picea sichensis: It feeds on the vascular system of spruce. Scale insect: Scale insects are generally covered by a waxlike protective cover. Only female species produce damage. They attack deciduous trees and coniferous plants.and fruits. Leaves and needles may die. Common scale insects are :
San-José-scale Insect, Pernicious scale Diaspidiotus (Quadraspidiotus) perniciosus sucks on leaves and fruits of apple-, plums- and peach trees.
The insect is small. it introduces its mouthpieces in the plant and feeds from the liquids. It secrets a white waxi material.
The University of California maintains the Statewide Integrated Pest Management Program, stressing that San Jose Scale has many natural enemies which, however, may be disrupted by broad-spectrum insecticides. Not using broad-spectrum sprays for 2 or 3 years may thus solve San Jose scale problems. [1202]
Beech scale (Cryptococcus fagi): It is a sap feeding insect which together with at least two Nectria fungi causes beech bark disease.
The American Beech (Fagus grandifolia) may be infested with the beech scale (Cryptococcus fagisuga Lind= fagi Baer).The tiny insects attack trunk and branches of the trees suckling sap of the inner bark.White wax covers the bodies of the scale so trees seem to be covered by white wool. Very small wounds caused by the scale enable the Nectria fungus to penetrate the tree killing areas of woody tissue. The tree will be girdled and die. Some infected trees will break off in heavy winds which is called `` beech snap ``.
The scale insect is tiny. It feeds on sap of the inner bark of the tree trunk and branches of American and European beech trees (Fagus sylvatica). The body of the scale is covered by a white wax which gives a look of white wool covering heavy infected parts of the tree. The scale insect causes small wounds and injuries which enables the Nectria fungus(Nectria galligen, Nectria coccinea var.faginata, Nectria ochroleuca) to penetrate the deeper tissues of the tree. leading to widespread damages.
The wind may transport scale insects and fungal spores, spreading the disease. The scale insect and the Nectria fungi can be transported from tree to tree in wind. The Nectria fungi species which are associated to beech bark disease are: Nectria galligena , Nectria coccinea var.faginataand Nectria ochroleuca.Beech scale alone does not kill the trees but after 3 to 6 years an invasion of Nectria fungi takes place which leads to the destruction of the plant.
Oistershell scale, Mussel scale (Lepidosaphes ulmi:) It feeds from the liquid of plants such as olive and apple trees causing incrustations on branches, twigs and fruits.It also leads to the splitting of the bark.
Anthracnose fungus (Colletotrichum sp.): is a disease of plantations of coffee, cashew, melons,citric fruits (Colletotrichum gloeosporioides) but also trees are affected, such as willow trees and poplar trees.Humidity accelerates the growth of the fungus.
Also economical important is Colletitrichum lindemuthianum (Sace and Magn.)which attacks black beans important food in Brazil. Armillaria mellea: Is a root disease which is a frequent cause of decay of trees in garden and in free nature. It survives as mycelium in the roots and wood of dead trees. The mushrooms of Armillaria grow in clusters at the base of dead or dying tree-trunks or near infected roots.
Plantations of coffee can be infested by Armillaria and Rosselinea spp. which has remained in roots and wood of trees of preceding forest.
Fruit tree red spider mite: Mites are spider like organisms which nourish from plant sap.
Economical important mite is the Fruit tree red mite Metatetranychus ulmi. it feeds by sucking the underside of leaves of vine and fruit trees like apple, pear, cherry-trees. The leaves turn brown and dry.
Bursaphelenchus xylophilus It is a nemathode which attacks pines. Its habitat is the southwest Asia and is present in USA, Japan China and Scandinavia. The nemathode is vectored from one pine to another by cerambycid loghorn beetles, also known as sawyers of the genus Monochamus. It can destroy 50 to 70 years old trees within few years.
In Germany,Saxony living nematodes Bursaphelenchus xylophilus were found for the first time among pallet wood of Chinese origin. As wood for pallets are of poor quality of dead trees are used pallets turn out to become a way of spread pests all over the world.


Diseases of potatoes

Tuber diseases:
Pink rot: is caused by the fungus Phytophthora erythroseptica which is soil-borne. Pink rot can be detected by a smell of ammonia prior of visual symptoms.
Phytium leak is caused by the fungi Phytium debaryanum and Phytium ultimum.Also known as water rot is caused by the fungus Phytium spp. living in the soil. The fungus invades the tubers through wounds made during harvest. It causes internal wet spongy rot with hollow cavities leaving behind only the tuber shells as thin paper skins.
Avoid overwatering near harvest. Avoid unnecessary damage to the tubers during harvest.
Fusariun dry rot:It is a postharvest disease of potatoes. It is caused by several soilborne fungus Fusarium. Infected tubers have wrinkled, sunken, brown to black lesions. The Fusarium fungi are common in soil and decaying plants as resistant spores. After low-temperature storage, internal tissues become firm and dry or even powdery.
Potato bacterial ring rot: is caused by Clavibacter michiganensis subsp. sepedonicus producing yellow areas which start on leave margins later turning brown. The leaves look like being burned.
Brown necrosis of the tubers are formed from the middle of the tuber, progressing to surface, leaving sometimes only hollow shells.
Rhizoctonia Canker: It is a disease of potatoes caused by the fungus Rhizoctonia solani. It is known as black scurf Hard black bodies called sclerotia are formed on the surface of the tubers. Delayed budding is caused by an attack of the buds by the fungus. Misshappen stems and weak plants with brown cankers at the base and belowground portions of the stem.
Silver scurf: Is a potato disease caused by the fungus Helmithosporium solani. It causes a metallic discoloration of the epiderma and causes weight loss during storage due to increased water loss of the tubers. Shrinking and flabbiness affects peeling of the tubers reducing consumer acceptance and rejection. It is a seed-borne disease.
Spongospora, Powdery Scale Is a disease of potatoes caused by Spongospora subterranea which can be the vector of the mop-top furovirus.
Net necrosis:
Foliar diseases
Early blight: Potato early blight is caused by the fungus Alternaria solani. It is a disease of stressed and senescing plants. Lesions of the leaves are circular with a target look. They turn out yellow and drop. Tubers develop dry rot lesions which appear sunken.
Infection is possible during wet and warm weather conditions (dew, rain or sprinkler irrigation). Tubers can be contaminated by lifting them through the surface soil.
Early blight can be reduced with with optimum growing conditions like fertilization,irrigation and other pest controls in order to strengthen the plants. Fungicide application is recommended only when the plants become diseased in a very early stage so the damage will be considerable.
Potato late blight[895] Potato blight is caused by Phytophthora infestans a fungus-like organism whose sporangia are dispersed by wind. To avoid damage caused by blight the potato farmers watch weather forecasts for the climatic conditions which favor the spread of the disease. Spraying fungicides may be unnecessary when blight spores are not present in the air current. More efforts are being done to control spore flight with air sampler in connection with flow cytometer. Particles collected by the sampler are stained and analyzed in the cytometer using laser light. Airborne potato blight spores are identified and counted against a background of other fungal spores, pollen and inert particles. This system is being developed by Dr. Gareth Griffith of the Institute of Biological Sciences at the University of Wales, Aberystwyth. The forecast of the potato blight disease which has caused the Irish Potato Famine, 1845-1847 leading to the death of 1 million people could be improved using data of the climatic forecast ( suitable conditions for pathogen growth) and detection of the sporangia in air (inoculum). With the help of these data the decision to spray the crops could could reduce excessive fungicides.

Potato late blight is caused by the fungus Phytophthora infestans and is the most important potato disease. Specially the US-8 genotype is strongly resistant to the fungicide mefenoxam and is together with the genotype US-11 very aggressive.
The spores of the fungi are carried by wind or other infection ways from one field to another. Once the fungi is established in the plant no chemical fungicide can kill it. Prevention is therefore the best way to prevent great damage. Fungicides must always be applied before the crop shows any signs of infection. The fungus hibernates in infected potato tubers as mycelium.New sprouts of the mycelium invades the cortical tissue of the tubers.Reaching the aerial part of the plant sporangiosphores will be created and which emerge through the stomata of leaves or stems.The sporangia which are then produced can infect other wet plants by means of wind and rain. Infection of tubers may not be seen during harvest, but it will go on during storage.
The sporangia can also spread on soil and tubers near the surface.
Prevention of potato late blight depends on the forecast of temperature and humidity of the specific region
When the relative humidity is below 80% the sporangia will lose its ability to germinate in 3 to 6 hours. Free moisture or dew makes germination possible.Best conditions for growth of the fungus is 100% of relative humidity.
Preventive application of fungicides are necessary if the environmental conditions are favorable for the disease. Mefenoxam is not recommended because Phytophthora infestans is resistant to it. If there is an infection of late blight the vines should be dead 2 to 3 weeks before harvest to avoid contamination of the tubers, as Phytophthora infestans does not survive for long in dead foliage.

Copper free compounds had either no or limited effects on blight compared with copper fungicides [896]
The EU project leaded by Professor Carlo Leifert aims the development of management of late blight in EU organic potato production. In this project it is being notet that there is a widespread view that a copper fungicide ban will have serious consequences for organic potato production unless effective alternatives are available.

Some conclusions of this project were:
Extracts of manure-based composts gave control of blight in potato leaf assays but not in the field. However, use of an adjuvant in combination with an autoclaved compost extract gave improved control and slightly higher yields in an experiment in 2003. Some micro-organisms, plant extracts and existing products showed promising effects on blight control. Efficacy of a range of commercial and novel anti-fungal compounds was unaffected by dose rate or formulation. Copper free compounds had either no or limited effects on blight compared with standard copper fungicides at normal rates but low doses of copper products were almost as effective.

Net necrosis and Potato leaf roll virus: Net necrosis is caused by the potato leaf roll virus (PLRV). It is a damage of the cells of the vascular tissue.of the tuber causing specific symptoms.
The virus can be spread by means of various aphid species such as the green peach aphid which can be killed with carbamate insecticides. Mechanical contact does not cause a transmission of the virus from plant to plant. it occurs only with a vector such as the aphids.

Phenology

Phenology is the study of the annual cycles of plant and animals and how they respond to seasonal changes in their environment. The uses of phrenology can be used in IPM: Desertification[1455] On the occasion of the World Day to Combat Desertification to be celebrated on June 17, 21006, worldwide farmers of the International Federation of Agricultural Producers presented a message, highlighting the essential role of farmers to combat desertification and their commitment to join efforts along with the international community and other relevant stakeholders to be more effective in this endeavour:
In a new policy statement, IFAP calls for increased investment in agriculture to turn dryland areas into economic ones through: investments in family farming and local food security which are the engines of economic growth; looking at all the assets of rural development; as well as development of people centered and rights based approaches to rural development.

Natural resources such as fertile topsoil, organic matter, plant cover and healthy crops are the most severely affected by desertification.

The United Nations Convention to Combat Desertification (UNCCD) presents a sustainable development and poverty reduction instrument. Unfortunately lack of investment for implementing the Convention due to a lack of political will and commitment hinders its activities. [1476]

Almost one-quarter of the earths land surface are defined as 'desert' , being home for over 500 million people. Degradation of fragile drylands are progressing due to overgrazing, overuse of land climate variations deforestation and poor irrigation methods.

Most of the 12 desert regions have a rainfall forecast of 10 to 20 per cent lower by the end of the century. Very threatened by drought are Chad, Iraq, Niger and Syria.

Soybean diseases

The Brazilian Census Bureau (IBGE) estimates an harvest of 59.2 million tonnes of soybeans in 2005-06, up from 51.1 million tonnes in 2004-05. Some crop losses due to dry weather in the previous season were reported.

Asian rust is another factor that could endanger some part of the Brazilian crop. Small and medium-sized farmers cannot pay the spray if rust attacks. Asian soybean rust was first found in Brazil in 2002 and has since spread to Brazil's major soy regions.

Extreme heat and drought in much of the Midwest in 2005 limited the spread of soybean rust to the southern United States (central Florida, reaching Kentucky and Texas ).X. B. Yang of the Department of Plant Pathology of the Iowa State University the disease developed slower in kudzu plants in southern states compared with those in South America. This might have been due to the biology of the U.S. Kudzu soy plant. However, the disease is still a serious threat to all U.S. soybean growing areas. [1478]

Asian soybean rust: It is a fungal disease caused by Phakopsora pachyrhizi. It can defoliate plants and reduce pod set, pod fill, seed quality and yield.

Similar looking diseases Diseases which are similar looking to asian soybean rust are: [1477]

Extreme heat and drought in some regions of Europe

The European Commission in a statement comments heat and drought in summer 2006 as follows:
"Frequent and persistent heat waves associated with dry conditions characterised the whole month of July. At the same time, the drought and the heat stress phenomena moved northward through the continent affecting particularly those areas where the winter crops were still at their sensitive stage (ripening/maturity),

Both in southern and in northern Europe, the spring-summer crops, in full vegetative to flowering phase, were suffering from the above-mentioned conditions. All this also had an impact on water reservoirs, reducing the irrigation resources mainly for grain maize, sugar beet and potatoes.

\includegraphics[width=260.56bp,height=250.67bp]{library/Corn.eps} Severe drought in German corn

Summer 2006 a severe drought makes corn leaves acquire a curly formate.

Genetic diversity within a species improves the ecosystem

[1479] [1480] Gregory Crutsinger and colleagues from the University of Tennessee in Knoxville found that increasing population genotypic diversity in a dominant old-field plant species, goldenrod (Solidago altissima), determined arthropod diversity and community structure and increased aboveground net primary productivity (ANPP), such as the insect diversity.
\includegraphics[width=251.56bp,height=238.67bp]{library/flowers.ps} Goldenrod (Solidago altissima)

The scientists counted and identified every insect on every plant. Crutsinger found that plants from plots that had the most genetic diversity ,12 genotypes, were bigger and contained not only the most insects but also the most species of insects. than plots with a single plant genotype.

The results of the study suggests that the goldenrods in the genetically diverse plots had provided better quality resources to the insects, with a resulting positive impact on the plants too.

Crutzinger thinks that tiny differences in plant characteristics, from leaf shape to stem thickness, might have promoted a wider diversity of pests and pollinators and spread out their effects over the plot, so that some plants are really nailed by insects and some are barely touched at all, The plants might also have picked nutrients out of the soil differently from each other, helping them make best use of the resources available.

Crutsinger concludes that the findings are significant for conservation biology and stresses the importance of genetic diversity within the crops.

Similar results had been reported by Youyong Zhu [1481]. He mixed a disease-susceptible with a disease-resistant variety of rice. Growing the two together boosted yields. Youyong Zhu concluded that crop heterogeneity is a possible solution to the vulnerability of monocultured crops to disease. Both theory and observation indicate that genetic heterogeneity provides greater disease suppression when used over large areas.

Insect Resistance Management

[1482] Insect resistance management (IRM) is the term used to describe practices aimed at reducing the potential for insect pests to become resistant to a pesticide. Bt IRM is of great importance because of the threat insect resistance poses to the future use of Bt plant-pesticides and Bt technology as a whole. Specific IRM strategies, such as the high dose/structured refuge strategy, will mitigate insect resistance to specific Bt proteins produced in corn, cotton, and potatoes.

Academic scientists, public interest groups, organic and other farmers have expressed concern that the widespread planting of these genetically transformed plants will hasten the development of resistance to pesticidal Bt endotoxins. Effective insect resistance management can reduce the risk of resistance development.

The insect resistance management section provides EPA's scientific assessment of various Bt plant-pesticide IRM strategies by reviewing the data and information available to the Agency. The Agency will use this assessment, the report of the FIFRA SAP meeting on October 18, 2000, and all public comments in its development of its risk management decisions for Bt plant- pesticides. The whole document can be downloaded at http://www.epa.gov/oscpmont/sap/meetings/2000/october/brad4_irm.pdf

Insect pest discussed in the EPA insect resistance management documet are:

Common Name Scientific Name Crop
Black Cutworm Agrotis ipsilon (Hufnagel) corn
Cotton Bollworm Helicoverpa zea (Boddie) cotton
Corn Ear Worm Helicoverpa zea (Boddie) corn
Colorado Potato Beetle Leptinotarsa decemlineata (Say) potato
Common Stalk Borer Papaipema nebris (Guen.) corn
European Corn Borer Ostrinia nubilalis (Huebner) corn
Fall Armyworm Spodoptera frugiperda (J. E. Smith) corn
Pink Bollworm Pectinophora gossypiella (Saunders) cotton
Southern Corn Stalk Borer Diatraea crambidoides (Grote) corn
Southwestern Corn Borer Diatraea grandiosella (Dyar) corn
Tobacco Budworm Heliothis virescens (Fabricius) cotton


Resistance management strategy [1482]
The 1998 Science Advisory Panel Subpanel agreed with EPA that an appropriate resistance management strategy is necessary to mitigate the development of insect resistance to Bt proteins expressed in transgenic crop plants.

The Subpanel recognized that resistance management programs should be based on the use of both a high dose and structured refuges designed to provide sufficient numbers of susceptible adult insects. The 1998 SAP also noted that insect resistance management strategies should be sustainable and to the extent possible, strongly consider grower acceptance and logistical feasibility.

High dose [1482]
The Subpanel defined a high dose as 25 times the amount of Bt delta-endotoxin necessary to kill susceptible individuals. The Agency has adopted this definition of high dose. A Bt plant-pesticide could be considered to provide a high dose if verified by at least two of the following five approaches:

1) Serial dilution bioassay with artificial diet containing lyophilized tissues of Bt plants using tissues from non-Bt plants as controls;

2) Bioassays using plant lines with expression levels approximately 25-fold lower than the commercial cultivar determined by quantitative ELISA or some more reliable technique;

3) Survey large numbers of commercial plants in the field to make sure that the cultivar is at the LD99.9 or higher to assure that 95% of heterozygotes would be killed (see Andow and Hutchison, 1998);

4) Similar to #3 above, but would use controlled infestation with a laboratory strain of the pest that had an LD50 value similar to field strains; and 5) Determine if a later larval instar of the targeted pest could be found with an LD50 that was about 25-fold higher than that of the neonate larvae. If so, the stage could be tested on the Bt crop plants to determine if 95% or more of the later stage larvae were killed.

Effective IRM is still possible even if the transformed plant does not express the Bt protein at a high dose. If the Bt plant is non-high dose, the IRM plan could include increased refuge size, increased scouting and monitoring, and/or prohibition of sales of non-high dose products in certain areas.

Structured refuge [1482]
A structured refuge is a non-Bt portion of a grower's field or set of fields that provides for the production of susceptible insects that may randomly mate with resistant insects that may emerge from Bt fields and dilute resistance. The size, placement, and management of the refuge is critical to the success of the high dose/structured refuge strategy to mitigate insect resistance to the Bt proteins produced in corn, cotton, and potatoes. The 1998 Subpanel defined structured refuges to "include all suitable non-Bt host plants for a targeted pest that are planted and managed by people.

These refuges could be planted to offer refuges at the same time when the Bt crops are available to the pests or at times when the Bt crops are not available." The Subpanel suggested that a production of 500 susceptible adults in the refuge that move into the transgenic fields for every adult in the transgenic crop area (assuming a resistance allele frequency of 5 x 10-2) would be a suitable goal. The placement and size of the structured refuge employed should be based on the current understanding of the pest biology data and the technology. The SAP also recognized that refuges should be based on regional pest control issues.

Insect resistant management imposed on registered Bt plant-pesticides [1482]
To address the very real concern of insect resistance to Bt proteins, EPA has imposed IRM requirements on registered Bt plant-pesticides. Sound IRM will prolong the life of Bt pesticides and universal adherence to the plans is to the advantage of growers, producers, researchers, and the American public. EPA's strategy to address insect resistance is two-fold:

1) mitigate anysignificant potential for pest resistance development in the field by instituting IRM plans, and

2) better understand the mechanisms behind pest resistance. Beginning with the first Bt plant-pesticide registration, the Agency has taken steps to manage insect resistance to Bt with IRM plans being an important part of the regulatory decision. The Agency identified (later confirmed by the 1995 SAP) seven elements that should be addressed in a Bt plant-pesticide resistance management plan:

1) knowledge of pest biology and ecology,
2) appropriate dose expression strategy,
3) appropriate refuge,
4) resistance monitoring and aremedial action plan should resistance occur,
5) employment of integrated pest management (IPM),
6) communication and education strategies on use of the product, and
7) development of alternative modes of action.

Developing an IRM plan [1482]
Key to developing an effective IRM plan is an understanding of the pest(s) biology, the dose ofthe protein expressed in the various plant tissues, and the size and placement of the refuge (a portion of the total acreage using non-Bt seed). It is believed that planting a refuge will delay the development of insect resistance by maintaining insect susceptibility. In addition to a structured refuge, IRM plans include additional field research, resistance monitoring for the development of resistance (and increased insect tolerance of the protein), grower education, a remedial action plan in case resistance is identified, annual reporting and communication. IRM plans will change as more scientific data become available. EPA, has in fact, changed IRM plans as new data has become available.

A summary of the Agency's risk assessment of insect resistance development and insect resistance management plans to mitigate resistance is provided below for Bt corn, Bt cotton, and Bt potato products. The detailed Agency risk assessments of insect resistance management are found in the following memoranda: A. Reynolds and R. Rose (OPP/BPPD) to M. Mendelsohn (OPP/BPPD), dated September 11, 2000; S. Matten (OPP/BPPD) to W. Nelson (OPP/BPPD), dated July 10, 2000; S. Matten (OPP/BPPD) to W. Nelson (OPP/BPPD), dated September 11, 2000; and S. Matten (OPP/BPPD) to W. Nelson and L. Hollis (OPP/BPPD), dated July 5, 2000.

Citrus canker:USDA APHIS study not supported by scientifically sound evidence.
The USDA APHIS evaluation of asymptomatic citrus fruit, (Citrus spp.) as a pathway for the introduction of citrus canker disease (Xanthomonas axonopodis pv. Citri) concludes that it is highly unlikely that citrus canker could be introduced on asymptomatic, commercially produced citrus fruit that has been treated with disinfectant dips and subject to other mitigations. Even if infected fruit were to enter a canker-free area with susceptible hosts, the establishment of citrus canker via this pathway appears to be unlikely. [1483]

The new Plant Health (PLH) Panel of the European Food Safety Authority (EFSA) evaluated a recent study on citrus canker disease published by the US Agriculture Department's Animal and Plant Health Inspection Services (APHIS) with special attention to its conclusion that citrus canker is not likely to spread by means of citrus fruit that show no signs of the disease. The PLH Panel concluded that key arguments in the study, and its conclusions, were not supported by scientifically sound evidence. [1484]

Citrus canker is an economically significant plant disease caused by the bacterium Xanthomonas axonopodis pv. citri (Xac) affecting most of the citrus growing areas in the world, including Florida. Once established, various control methods including spraying of copper compounds must be combined to reduce the damage by Xac. [1485]

Rigorous phytosanitary measures have insured that some areas, including Europe, are still free of the disease. These include the use of systems approaches. The current systems approach for the trade of citrus fruit requires that fruit are coming from disease-free area surrounded by a non-export buffer zone. The APHIS document proposes to modify the systems approach so that asymptomatic fruit coming from infected/contaminated areas are eligible for trade. [1485]

The APHIS proposal to allow fruit from contaminated groves to be traded is a major deviation from existing phytosanitary measures. The APHIS document analyses the five events that must occur for successful introduction and proposes a new systems approach to prevent entry and establishment. However, no new or additional studies are presented and the analysis of the evidence provided in the document does not justify a change in the regulations. None of the preventative measures in the systems approach proposed by APHIS are shown to give effective control of Xac. Therefore, it can be concluded that, where an initial inoculum load exists, the transmission of Xac in the scheme proposed by APHIS is more likely than with the current systems approach. [1485]

Cassava

As tapioca starch finds growing applications in global food industry the producing countries are highlighted: Nigeria is the largest producer of cassava, followed by Brazil, Thailand Indonesia and Congo. Floods, droughts and cassava mosaic virus threaten African tapioca yields, and low prices for cassava made Thailand farmers change from cassava to maize and sugar cane because of low prices of cassava. To counter this,Thai government plans to launch a futures market to stabilize the price. Bacteria harm cassava such as Phytomonas manihotis in Brazil, Bacterium cassava in Africa and Bacterium solanacearum in Indonesia. Insects like locusts, beetles and ants, rats, goats and wild pigs may devastate plantations.

Iceberg lettuce breeding for high resistance to fungus and microbes [1486] [1487]

Ryan J. Hayes, Krishna Subbarao and colleagues studied the disease verticillium wilt of iceberg lettuce. The lettuce collapses before firm nicely rounded heads are formed. The agent of the disease is the root-rotting fungus Verticillium dahliae.

The researchers made seeds of three new parent lettuces available to researchers and plant breeders the parent lines are meant for crossing with consumer-ready lettuces to boost the commercially grown lettuces' resistance to verticillium wilt.

Verticillium wilt

The Verticillium dahliae fungus infects roots of vulnerable plants, moving into leaves and causing them to discolour, then to eventually wilt and die. The fungus can also infect and kill hundreds of other kinds of plants, including strawberries and tomatoes.

Corky root

Corky root is a disease caused by the bacterium Sphingomonas suberifaciens which causes lettuce roots to develop ugly, yellow-to-brown lesions that harden to a corklike texture. Corky-root-infected plants may produce stunted heads 30 to 70 percent smaller than normal.

Lettuce mosaic

The lettuce mosaic, is caused by a virus of the same name. It results in stunted growth as well as unattractive mottling of leaves. Green peach aphids can spread the virus from plant to plant.
Mou and Carolee T.Bull working at ARS are searching for corky-root-resistance genes that are different from the one already working inside the new parent lettuces to give lettuce additional corky-root-resistance genes.

Resistant varieties

Breeding lettuces with natural resistance remains the most environmentally friendly, economical and sustainable option for combating the fungus and microbes.

The authors conclude that because of the existence of resistance-breaking race 2 isolates, this resistance may not be durable. Alternatively, targeted releases of race-1-resistant cultivars to fields with only race 1 pathogen genotypes may extend the life of these cultivars.

Verticillium dahliae count in soil

Verticillium dahliae can attack tomato, eggplant, pepper, potato, chrysanthemum, asters, fruit trees, strawberries, raspberries, roses, alfalfa, maple, elm and others. Damage to most fruit and vegetable crops if the Verticillium dahliae count in soil is high. This count is made when solanaceous crops such as potatoes, tomatoes, eggplant or peppers are to be grown. [1488]

Lupinus polyphyllus Lindl. is a perennial ornamental belonging to the Leguminosae family. It is grown in gardens for flower beds and borders. Garibaldi and colleagues found the fungus Verticillium dahliae to infect up to 30% of this legumonose in Northern Italy. [1489]

Olive plants

Mercado-Blanco and colleagues developed a nested-polymerase chain reaction (PCR) procedure based on the simultaneous amplification of both an ND- and a new D-specific marker by means of duplex, nested PCR. This procedure helps to certify pathogen-free planting material and accurate detection of defoliating (D) and nondefoliating (ND) Verticillium dahliae pathotypes infecting olive plant. [1490]

Jacobs and colleagues (1994) reported interspecific variation in susceptibility to Verticillium wilt caused by Verticillium albo-atrum Reinke and Berthier and Verticillium dahliae in redbud, and that Cercis yunnanensis is susceptible to the disease. [1491]

Sanogo and Clary (2003) report that Verticillium dahliae recovered from the weeds Physalis wrightii (Wright groundcherry), Anoda cristata (spurred anoda), and Proboscidea louisianica (devil's-claw),is pathogenic to Chile pepper.
Control of Wright groundcherry, spurred anoda, and devil's-claw may be important in the management of Verticillium dahliae in chile pepper. [1492]

Al-Rawahi and Hancock (1998) found the fungus Pythium oligandrum to be a parasite of Verticillium dahliae in dual culture, impeding the latter's ability to grow and form microsclerotia. Pepper grown in soil infested with V. dahliae, shoot and fruit weights were significantly higher in the presence of P. oligandrum than in its absence. When soil was infested only with P. oligandrum, fresh weights of shoots and fruits were 40 to 50% higher than when plants were grown in its absence. The authors find that improved plant health associated with soil treatment with P. oligandrum could be the result of complex interactions between pathogen, host, and mycoparasite. [1493]

Spink and Rowe (1989) evaluated Talaromyces flavus as a potential biological control agent of Verticillium dahliae on potato, but found no significant effect on disease development, tuber yield, or percent recovery of V. dahliae from rhizosphere or nonrhizosphere soil at harvest. [1494]

Page and colleagues (1991) studied the loss in irrigated alfalfa (Medicago sativa) attributable to Verticillium wilt (caused by Verticillium albo-atrum) was determined by comparing yields of resistant and susceptible cultivars grown in the presence of Verticillium wilt and developed equations for making economically sound decisions for managing alfalfa under the consederation of losses due to Verticillium albo-atrium [1495]


Red palm weevil may pose a risk to date production

[1496]
The red palm weevil (Rhynchophorus ferrugineus) became a risk to date palms pest lays its eggs on the bark of the date, coconut, oil, sago and other ornamental palms . Invasion spreaded from the Near East, the north of Africa and the European Meditarranean region. The weevil causes a total loss of the palms and rotting of the trunk which lead to the death of the tree. Adult females lay eggs in the crown of palm trees, larvae then penetrate the crown and later to most parts of the upper trunk, making tunnels of up to 1 m long. Pupation takes place in a cocoon under the bark.

Date palm is an important crop in north African countries and ornamental palms are widely planted as amenity trees in the whole Mediterranean area.

Phytosanitary measures taken in Israel according to Hamburger, Bitton and Nakache 2003: [1497]
  1. R. ferrugineus was declared a quarantine pest, and pheromone traps were installed.
  2. Mass trapping of 10-12 traps/ha was organized and to 1 trap/3 ha in non-infested areas).
  3. Chemical treatments of infested trees.
  4. Destruction of heavily infested trees.
  5. Preventive measures to avoid infestations (drip irrigation to maintain trunk dry, removal of offshoots, preventive sprays on trunks).
The authors concluded that mass pheromone trapping might have played a significant role in the suppression of red palm weevil populations in date plantations.

Dogs to find the weevil

[1498]
The tree being damaged by the weevil exudes a foul odour. Humans can eventually detect this smell but not soon enough to save the tree, which can be done, if caught in time, with Golden Retriever dogs are used in a joint Israeli-Arab project by the Shimon Peres Center for Peace, researchers. These dogs detect the presence of the grubs in the early stages of infestation when an injection of insecticide still may save the palm.

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