Eggplant Integrated Pest Management AN ECOLOGICAL GUIDE

___________________________________________________________________________________________Disease Ecology7.10 Antagonists: the Natural Enemies of pathogensNot only insects, but also plant pathogens have natural enemies. These are usually also fungi, bacteria,nematodes or viruses which can kill plant pathogens, reduce populations, or compete for nutrients orattachment to a host plant. Such micro-organisms are called antagonists. Sometimes, the term“biofungicide” is used for antagonists if they are used to control a disease.Antagonists of pathogens are not yet well understood. However, the research that has been done hasgiven promising results, and the study of antagonists has become a rapidly expanding field in plantpathology. The most “famous” antagonist in vegetable production is probably Trichoderma (see below)but others may be interesting as well. In Philippines for example, a fungus called Bioact strain 251, wasisolated from the soil which controls nematodes. Spore solutions of this fungus are now commerciallyavailable as “Bioact” (FAO Dalat report (V.Justo), 1998).Antagonists: how do they work…? Some examples:The fungus Gliocladium virens reduces a number of soil-borne diseases in three ways: it producesa toxin (gliotoxin) that kill plant pathogens, it also parasitizes them in addition to competing fornutrients.The biocontrol capacity of the fungus Trichoderma harzianum, recommended for control of severalsoil-borne pathogens, competes in the soil for nutrients with pathogens. Trichoderma fungioutcompete pathogens for nutrients and rhizosphere dominance (=area for a fungus to growaround the plant roots), thereby preventing or reducing the impact of pathogens.Others may compete for the entry place to the host plant, such as pathogenic and non-pathogenicFusarium sp. When a non-pathogenic organism blocks the entry, the pathogen cannot infect theplant.(Copping, 1998)Antagonists have been applied to the above-ground parts of plants, to the soil (and roots), and to seeds.Under constant conditions, such as in greenhouses, antagonists can completely protect plants frompathogens. In the field, disease control is likely to be more variable due to the varying environmentalconditions (mainly temperature, moisture, nutrient availability and pH).Proper methods for the multiplication of antagonists as well as ways to formulate them need to be furtherstudied. However, some examples of successful field use of an antagonist are described below.7.10.1 Trichoderma speciesAn example of an antagonist that is widely available in Southeast Asia is Trichoderma sp. Trichodermasp. can suppress soil-borne plant pathogens, including those causing damping-off (Pythium sp.), root rot(Rhizoctonia solani), stem rot (Sclerotium rolfsii), and wilt (Verticillium dahlia) in vegetables. Trichodermafungi often promote plant growth, maybe due to their role as decomposers. They may also aid inpromoting soil fertility. In addition, Trichoderma sp. stimulates tissue development, for example, inpruned trees, through the enhancement of natural auxin release. Specific formulations containingTrichoderma are available to treat pruning wounds of fruit trees.Some Trichoderma species are:• Trichoderma harzianum – suitable for warm, tropical climates• Trichoderma parceramosum – suitable for warm, tropical climates• Trichoderma polysporum – suitable for cool climates• Trichoderma viride – suitable for cool climates and acid soils• Trichoderma hamatum – tolerant to excessive moisture• Trichoderma pseudokoningii – tolerant to excessive moistureTrichoderma harzianum and some others occur widely in nature. Isolates of e.g. Trichoderma harzianumwere selected for commercialization because of its ability to compete with plant pathogenic fungi. Thebeneficial fungi outcompete the pathogens for nutrients and for a place to grow around roots or inpruning wounds, thereby preventing or reducing the impact of pathogens.127Eggplant Ecological Guide