Eggplant Integrated Pest Management AN ECOLOGICAL GUIDE

___________________________________________________________________________________________Disease Ecologyanimals spread diseases by walking and working in fields with infected plants, spreading sporessticking to the body but also causing small injuries to plants (e.g. during transplanting or field work)which can be entry points for pathogens. Longer distance dissemination by man is usually done bytransporting diseased 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 openingslike stomata (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 wounds or natural openings to enter.A virus needs a wound to enter, either a mechanical wound or a wound created by an insect. Mostnematode species, such as rootknot nematode, can actively penetrate plants.The differences in the ways of attacking a plant may be the reason that you sometimes see all plants in afield infected with a disease (for example early blight can be present on all plants because it can activelypenetrate the plant tissue) whereas another disease may only be visible on a few plants (for examplebacterial wilt: it needs a wound to enter 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 but may reduce yield or quality.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 plantThe disease triangle shows that a plant will get infected with a disease when:• the variety of that plant is susceptible to the disease, AND• the disease is present and virulent (able to infect the plant), AND• the environment (e.g. humidity, temperature) is favorable for the disease to develop.Disease management is focused on changing or influencing one of the three elements of the triangle toprevent the disease from attacking the plant. A few examples:Changing the host plant can be: not growing a host plant, e.g. by crop rotation, or using a resistantvariety.Changing the presence of the pathogen can be: removing leaves with the spores of the disease from thefield before planting a new crop so that the disease cannot infect the new plants from the leaves thatwere left in the field after harvest (sanitation).Changing the environment can be: using furrow irrigation rather than overhead irrigation so that theleaves will not get wet. Humidity stimulates spore formation (e.g. early blight) and spread of the disease.121Eggplant Ecological Guide