Cabbage Integrated Pest Management : An Ecological Guide.

Disease EcologyNematodes can also be transmitters of pathogens. In case of vegetables, it is also likely that nematodescreate entry points for bacteria and fungi by making wounds in roots.· Humans, animals: spread of pathogens occurs in two ways: through the person, tools or animalsand through the objects that are transported. Persons and animals spread diseases by walking andworking in fields with infected plants, spreading sporessticking to the body but also causing small injuries toplants (e.g. during transplanting or field work) whichcan be entry points for pathogens. Longer distancedissemination by man is usually done by transportingdiseased planting materials or infected soil particles.7.4 How pathogens attack a plantA spore of a fungus or a piece of the mycelium (the “body” of the fungus) can penetrate a host plant. Itcan enter a plant through wounds in the plant tissue, through fine root hairs, through natural openings likestomata (the “breathing cells” of a plant) or it can actively penetrate the tissue of the plant. To do this,some fungi produce special chemicals (enzymes) that damage the plant tissue and allow the fungus toenter.Bacteria cannot actively penetrate plants and need woundsor natural openings to enter.A virus needs a wound to enter, either a mechanical woundor a wound created by an insect. Most nematode species,such as rootknot nematode, can actively penetrate plants.The differences in the ways of attacking a plant may be thereason that you sometimes see all plants in a field infectedwith a disease (for example leafspot can be present on allplants because it can actively penetrate the plant tissue)whereas another disease may only be visible on a few plants (for example softrot: it needs a wound toenter the plant).The infection process by some pathogens can be very quick. Damping-off in seedbeds for example, cankill seedlings in less than a day! That will usually be too short to even notice disease symptoms! Othersjust parasitize on a plant and do not cause the death of the plant - like leafspot on cabbage: it can reducethe yield but plants will survive.7.5 When can a pathogen attack a plant?A disease is the result of interactions between a pathogen, a host plant and the environment. Theseinteractions are shown in the disease triangle:pathogenenvironmenthost plantCabbage Ecological Guide - 2000140