Pests and diseases
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Part 4: Crop protection
- Causers of diseases and pests
Deviations in the crop can be caused by:
- A. Non-animal parasitic organisms
If an infection is caused by non-animal parasitic organisms, they are called diseases.
- B. Animal parasitic organisms
If an infection is caused by animal parasitic organisms, they are called pests.
- C. Non-parasitic causes
- Soil influences
- Weather influences
- Air pollution
These disrupt the physiology of the crop. We talk of physiogenic diseases.
- D. Weeds
These compete with the crop for water, nutrients, light and space.
Note that bacteria are usually a secundairy corruption (eg after a fungi attack) or with wounds (weakened plants)
virusses work through direct contact, they can not survive long without the host, several species, eg persistent species remain in the host, also if transferred
 A. Non-animal parasitic organisms
Fungi are categorised to the lower plants. They contain no chlorophyll and are dependent for their nutrition on dead and living organic matter. They consist of microscopic filaments (hyphae) which are sometimes packed together as a visible tissue (mycelium). Molds propogate mainly through spores, which are very fine spheres that are dispersed by wind, water and also by animals. With sufficient moisture and warmth, the fungi develop rapidly. The damage that fungi cause is of major economic importance. We currently know some 3000 harmful fungi.
They usually have a full lifecycle on the course of a few days Conditions: hot and moist weather Weeds can be a host for fungi.
Commonly appearing on plants:
 Germing fungi
These can infect the seedlings in the nursery. The disease is also called smoldering. The young seedlings start to melt or fall over. Sometimes the plants don't rise above the soilsurface because the fungus has already struck them at the germination. It is best to hold off with sowing until the soil has warmed and dried enough. Also, do not sow too deep in the soil, nor too close together.
This fungus manifests itself from the soil at humid weather and first targets the dying plant parts of a crop. The parasite also targets wounds, it is mostly saprofytic, note that wounds can also be cause by a heavy impact of rain (cellular damage). Then it goes on to healthy plant parts. These will discolorate, and after this, they will exhibit a gray mold fluff, full of fresh spores for a new spreading.
With "Mildew", 2 diseases are meant: powdery mildew and downy mildew. Powdery mildew usually appears around the middle of July as woolly spots on top of the leaves. Moments later they are covered with a flour-like layer. The fungus draws moisture from the leaf cells, which makes the leaf discolor brownish and makes it dry-out. With the downy mildew, the symptoms are pale-yellow discolored spots on top of the leaf and white, powdery fungal fluff at the bottom. The leaves discolorate brown afterwards. Powdery_mildew occurs on dry, hot weather, the wooly fluff is everywhere, including the bottom of the leaf. Downy_mildew occurs with wet, hot weather
Prevention All measures aimed to let the crop dry-up faster, have a preventive effect on fungi. We can provide a wide spacing, a windy and weed-free place and adequate ventilation in the greenhouse. Both stressed plants (by supplying too much N) and crops with a growth arrest fall prey to fungi. Besides the essantial crop rotation, sprayings with rock meal are also possible. Sprayings with Equisetum arvense extract or pure sulfur are considered effective. At a beginning infestation, we remove the infected plants and burn them. Once the mold has well infected our crops, there really isn't much to do about it anymore without the use of chemical agents.
These microscopicly small organisms can also cause damage to plants (Erwinia and Pseudomona). It are these that cause root rot with tuber and bulb crops and various spot diseases or wilting in other plants. Against bacterial diseases, there are no effective pesticides. Modern varieties are often resistant and a crop rotation prevents damage.
Even under a regular microscope, these minuscule organisms are unobservable. In addition, viruses are so primitive that they live can not live outside of their hosts. Yet they are able to cause persistent diseases with symptoms such as growth inhibition and malformations. Viruses can be transferred at dozens of ways such as through aphids, fungi, touching, contaminated irrigation water, ... Viruses are almost impossible to combat. Preventative, one might think of few precautions to help prevent an infection such as:
- Burning diseased plants
- Buying healthy seeds/plants
- Growing varieties that are not susceptible to viruses
- Carefully combat weeds
- Effectively combat aphids
Also, we never put young plants or seed beds next to older plants or plants that already bear seeds. Modern varieties are usually resistant to viruses or at least tolerant.
 B. Animal parasitic organisms
Nematodes are always present in the soil. The colorless worms are less than 1 mm wide, and are not visible with the naked eye. Most species are decomposers of organic matter, only a few parasite on living plants and sucking their cells empty. The cyst-forming nematodes put their eggs in the soil in cysts (hard spheres filled with eggs, are observable with the naked eye). These cysts can survive years in the soil and awaken when a same vegetable species or a related one is grown. Such a nematode population can cause that a lot or a place fails, seemingly without any reason. We speak of soil fatigue. Infestation by nematodes is promoted by growing a same crop frequently in the same place, this is why we always apply a wide crop rotation. As a defense we can grow Tagetes (marigold). These plants secrete substances through their roots with a nematode-killing property and they are easy to use as a border planting or pre-cultivation.
Eals are a type of nematode. Eals exist in 2 varieties: parisitic and saprofytic (good-type) eals Several eals: some live in the roots, near the roots, cyste-eals (latter can survive for 5-10 years in the soil)
Eals are the greatest cause of "soil fatigue". If crop rotation is used, this pest can be avoided.
Insects belong to the arthropods. Insects have 3 pairs of legs and are usually winged. They usually also have antennas. During their lifetimes, most insects undergo a transformation or a metamorphosis. This can happen through the phase of nymph, where the young larvae already looks similar to the adult insect, but does not have wings yet (aphids, locusts, thrips). With a full metamorphosis the larvae does not yet look similar to the mature insect and a transition first occurs through the pupal stage, often at a hidden place in a self-spun cocoon (moth, beetle). 70% of all known species, are insects, they have a hard skeleton, reproduce quickly and adapt to most environments. Many of these insects have specialized themselves in living on certain crops. With monocultures on large plots this may result in extensive pests. There are some insects that occur on various crops.
2 types exist:
- with incomplete metamorphose (egg -nymphe-insect)
- with complete metamorphose (egg-larva-cocoon-nymph-insect)
The difference is that a nymphe looks like the insect, whereas a larva does not look like the insect (eg butterfly)
mites:generate honeydew -->formation of fungi can occur hereon
dry weather: mite wet weather: fungi daily checkups need to be done for both
An aphid is a parasite that appears in the garden from March. They are small(2-3mm),oval, black, gray or green insects, and belong to the order of the lice (Phthiraptera). They carry on their abdomen, two tubular wax glands from which they secrete sugars in the form of a sticky substance which is called honeydew. Ants love it and stay close to aphids. Best known is the green peach aphid and the black bean aphid. Especially in hot weather, the aphids multiply rapidly as they reproduce Parthenocarp and vivipare (without fertilization and viviparous) in the summer. They spawn on the underside of the leaf. Lice are sucking insects that feed on plant saps. We can observe this by the curling and wilting of the leaves and shoots on which they sit (youngest leaves). The damage manifests itself by:
- Growth inhibition and growth arrest
- Formation of Dematiaceae, a black fungus on the honeydew
- Transmission of viruses
Weak or imbalanced plants are first affected. We will thus preventively make sure of
- A healthy and strong plant (balanced diet and certainly not too much nitrogen)
- Plant strengthening substances such as nettle manure, seaweed extracts, fertilization with rock flour, sowing of nasturtium catch crops
When infected, we can spray with:
- A solution of brown soap or an ethanol-soap mixture
- Organic insecticides such as pyrethrum, or derris, or kwassia (bitter wood)
- The use of natural enemies such as Coccinellidae, parasitic wasps and Cecidomyiidae in greenhouses
Beetles usually disappear after a certain period of time
These are small (3 mm) beetles with long back legs that can jump high. They are best recognized by their metallic sheen. They eat out holes in the lower leaves of germinating plants. They are especially active in dry, crusty soils with cool and dry weather. One can expel them by pouring cold water over the plants during full sun. To catch them, one can use a glue board. A shelf is foreseen at the bottom with glue or wood tar. In the middle we place soft bristles that are bristled over the crop. The beetles will jump up and get stuck in the glue.
This is the larva of the Crane fly, it is around 5 cm long and gray in color. Leatherjackets occur on wet soils overgrown with weeds or where green parts (grass) are dug into the soil. They eat the roots and stems of virtually all crops and can cause major damage. Prevention is done by keeping the ground free of weeds and by not digging grasses under. Leatherjackets can be caught by searching the soil surface on a hot day. We can also use chickens to do this work.
These are copper colored, worm-shaped larvae of the skipjack. They are a typically a problem occuring on newly cleared land, and often occur with tuberous crops. They stay 3 to 5 years under the soilsurface and to grow up to 2 cm large. They mainly eat half decomposed vegetable matter and have a preference for root and tuber crops. They eat the plants roots and drill into the stems, the plants wither from this. Prevention can be done by keeping the soil free of weeds. Wireworms can be caught by cutting a potato in half and digging it in 5 cm deep into the soil with the cut part facing downwards.
This is the larva of a brown moth (Agrotis segetum, Agrotis ipsilon, Agrotis exclamationis). They are about 4 cm large. While resting or in danger they roll up in a spiral. They like warm, dry and light soils where they can hide at daytime. At night they hunt also above the soil and eat young shoots off, precisely at the separation of soil and air. Prevention can be done by already digging in weeds and greens in the winter. Since the larvae are not fond of moisture, the plants can be watered well the first weeks after planting. A good method is a caterpillar bait based on mixing 25% hardwood sawdust, 25% wheat brans, 50% syrup or molasses and water. Caterpillars eat from it, and get stuck in the stuff. In the morning, they are dead. At temperatures above 15°C, we can also work with Bacillus thuringiensis. Administer in the evening, the effect is only visible after a few days. Caterpillars also hate sage, if this herb is planted, they are also expelled.
 Spider mites
mites Spider mites are not insects. They belong to the order of the spiders, having 8 feet. The one that does damage to crops is the urticae red spider mite, and mostly spawns under glass. These spider mites suck juice from the plants causing white dots to appear on top of the leaves. Later-on, the entire leaf becomes brown. Spider mites prefer to live in a dry climate. Regular spraying with water makes the threat of infection smaller. Spider mites can also be fought with soapy ethanol. In the greenhouse , we can also use natural enemies such as predatory mites and assassin bugs.
Mice and rats All rats and mice do damage to crops. Only the shrew, which is an insect eater, is useful in the garden. Mice can expelled by planting mint. You can catch them with traps or by laying down poisoned bait.
Moles The mole is a useful insect eater, but because of the eating of the useful earthworms and by creating corridors in seed beds, it is an unwanted organism in the garden. They do not gnaw (as sometimes thought) on the plant roots. To fend them off, we can plant crops with a more or less mole-expelling effect such as Fritillaria imperialis (place 1 every 8 meters completely around the garden) and paper spurge (about 10 plants per are), also a clove of garlic in the corridors, would scare off the mole. The most efficient to expell them are mole traps or ultrasound devices.
Rabbits These rodents also do much damage to crops. Near forests, it is best to protect your garden with a dense mesh of 1m high that is also (20 cm deep) dug into the soil. To protect young trees, we can cover them with lime manure. This is made from 3 parts pig manure, 1 part cow manure, and 1 part lime. The whole is mixed well with water, so that a semi-liquid mixture is attained.
The gastropods belongs to the order of the molluscs. They only feel good in wet conditions and are especially active at night. The snails (Gastropods with shell) do not do much damage, only the slugs are voracious. They do not eat, but grate holes in the leaves. Especially the soft parts of plants and young seedlings suffer. Preventive measures are to eliminate the moist places from which they operate, eg by removing weeds, by keeping the grass strips short, by cleaning up waste piles. Wood ash or sharp sand strips of a few cm wide stops gastropods. Gastropods hate some plants such as nasturtiums, sage, hyssop, thyme and mustard. Also, the use of rye as a green manure seems to be expell gastropods. A gastropod trap can be made with a jam pot that we dig into the soil, equal to the soil's edge. We pour in beer up to about 1/3 of the pot; gastropods love it but will drown in it.
 C. Non-parasitic causes (physio = disease gene)
 Soil Influences
- Lack or excess nutrients
- Too much or too little water
- Poor structure
- Adverse pH
- To high salinity
 Weather Influences
- Unaduquate temperature
- Shortage of light
- Rain, hail
- Unaduquate humidity
- Wind and storm
 Air pollution
- Acid rain
This article is made using information from the coursebook: Cultivation techniques </br> Course from: 1st year of Herbalism</br> Course year: 2005 - 2006</br> Education center: Syntra, Asse-establisment</br> Teacher: Leo Van Crombrugge