Eggplant Integrated Pest Management AN ECOLOGICAL GUIDE

___________________________________________________________________________________________Disease Ecology7.1 Plant diseases and pathogensDiseases are an important part of crop protection, but they are usually very difficult to understand in thefield. This is partly because the causal organisms are very small and cannot be seen moving around likeinsects. You can only recognize diseases by their symptoms which vary from dwarfing of the plant, colorchanges, leaf spots and necrosis to wilting, (root) malformations and rotting.Plant diseases are caused by living (biotic) organisms, called pathogens. Main pathogens of plants arefungi, bacteria, viruses, and nematodes. Some characteristics of pathogens are listed in the box below.Fungi, bacteria, viruses, and nematodes (and other organisms such as mycoplasmas) are often lumpedtogether under the term micro-organisms. Only very few micro-organisms may cause injury to the cropunder certain circumstances. Most of them are beneficial: they may be decomposers which play animportant role in the nutrient cycle. Several micro-organisms are true “natural enemies”. An example isthe virus NPV, which can control several pest insects of vegetables. Likewise, there are fungi that controlpest insects like aphids or caterpillars. Insect-pathogens are described in section 6.3.Some fungi can infect, attack or work against (antagonize) other fungi that cause plant diseases. Theyare called antagonists, the natural enemies of plant diseases, and also friends of the farmer! A wellknownantagonist is the fungus Trichoderma sp. which can reduce damping-off disease in nurseries (seesections 7.10.1 and 8.1.1).PATHOGENSFungi are plants that feed on other organisms, living or dead. There are many different types offungi: some are living in the soil breaking up dead plant parts, others feed on living plants andcause wilts and other diseases. Most fungi grow with tiny threads called mycelium and for theirreproduction they produce spores that serve as seeds. Sometimes a powdery mat that covers thediseased parts of a plant can be seen. This mat is composed of millions of spore-producingstructures of the fungus.Bacteria are very small organisms and can only be seen through a microscope. Few bacteriaaffect plants. Bacteria can cause rotting of plants, wilting, and leaf spots. Bacteria do not formspores like fungi. They often multiply through cell division (splitting themselves into two). Somebacteria can survive for a long time by surrounding themselves with a protective coating whichprevents them from drying out. Bacteria grow in wet conditions.Viruses are even smaller than bacteria. They can only be seen with a powerful electronmicroscope. Viruses exist in living cells and cannot live outside a plant or an insect vector. Virusdiseases may take a long time to recognize as often the only effect on the crop is a gradual loss ofvigor. Symptoms often depend on environmental conditions such as temperature. Plants are small,may be stunted and yields are lower. Sometimes the signs are more obvious when red or yellowstreaks appear on the leaves (mosaic). Still it is often difficult to distinguish a viral disease from amineral deficiency. Viruses can infect new plants through seeds or seed tubers, direct contactbetween plants or indirectly through vectors. The main vectors for plant viruses are sucking insectslike aphids, plant hoppers and whiteflies.Mycoplasmas, or mycoplasma like organisms (MLO) are tiny organisms that resemble viruses orbacteria but because their characteristics are slightly different, they have been placed into aseparate classification. MLOs that cause plants diseases live inside plant cells and disrupt normalfunctioning of plants. This results in symptoms like witches broom, stunting, little leaf, or bunchy topin a wide variety of crops. In eggplant, little leaf is an important disease caused by a mycoplasma.Nematodes are very little worms (about 1 mm long) which usually are present in large numbers inthe soil. Nematodes have life cycles like insects: they usually mate, lay eggs, and there are severallarval stages. Yet, they are lumped with pathogens and not with insects, presumably because oftheir symbiotic relationship with bacteria (see section 6.3.4). In addition, symptoms caused bynematodes are often hard to distinguish from other diseases. Some nematode species can causedamage by sucking plant roots. In some cases, roots may form galls due to nematode attack(rootknot nematodes). Some nematode species are damaging because they transmit viruses.Other nematodes may be beneficial because they attack pest insect species.118Eggplant Ecological Guide