natural-products-in-plant-pest-management

Suppressive Effects of Compost Tea on Phytopathogens 249Mycoparasitism is parasitism of a pathogenic fungus by another fungus.These events require specific interactions between the parasite and fungalhost. The process involves direct contact between the fungi, resulting in deathof the plant pathogen and nutrient absorption by the parasite. Trichodermaspp. parasitize fungal plant pathogens. The parasite extends hyphal branchestoward the target host, coils around and attaches to it with appressorium-likebodies, and punctures its mycelium (Chet et al., 1981; Goldman et al., 1994).Mycoparasites produce cell-wall-degrading enzymes, which allow them tobore holes into other fungi and extract nutrients for their own growth. Butmany so-called mycoparasites also produce antibiotics, which may firstweaken the fungi they parasitize. So the digestion of host cell walls is accomplishedby a battery of excreted enzymes, including proteases, chitinases andglucanases. These enzymes often have antifungal activity individually andare synergistic in mixtures or with antibiotics (Di Pietro et al., 1993; Loritoet al., 1993, 1994; Handelsman and Stabb, 1996). By manipulating their activitythrough the construction of ‘overproducing’ mutants, enzyme-negativemutants or even transgenic plants expressing the enzyme, a role for theirproduction in biocontrol has been implied (Whipps, 2001).The best documented examples of hyperparasitism involve the mycoparasiticTrichoderma spp. on R. solani. Highly competitive as a saprophyte,R. solani, can utilize cellulose and colonize fresh bark but cannot colonize thelow-in-cellulose mature bark compost. However, isolates of Trichoderma thatfunction as biological agents for R. solani are capable of colonizing maturecompost. Biological control does not occur in fresh, undecomposted organicmatter because both fungi grow as saprophytes and R. solani remains capableof causing disease. In mature compost, on the other hand, sclerotia of R. solaniare killed by the hyperparasites and biological control prevails (Hoitink andFahy, 1986; Chung and Hoitink, 1990). One way to ensure specific suppressionis to inoculate the compost with the appropriate beneficial microorganisms,fungus like Trichoderma sp., Talaromyces flavus, non-pathogenic Fusariumoxysporum, or bacterial like Bacillus subtilis and Streptomyces griseoviride,which leads to increased levels of disease suppressiveness.Induced resistanceThe natural resistance of plants to pathogens is based on the combined effectsof preformed barriers and induced mechanisms. In both cases, plants usephysical and antimicrobial defences against the invaders. In contrast to constitutiveresistance, induced resistance relies on recognition of an invaderand subsequent signal transduction events leading to the activation ofdefences (Mauch-Mani and Métraux, 1998). Plants possess active defencemechanisms against pathogen attack; some biotic and abiotic stimuli increasetheir tolerance to infection by a pathogen, by activation of these active defencemechanisms. This phenomenon is known as induced resistance. Inducedresistance was defined by Kloepper et al. (1992) as ‘the process of active resistancedependent on the host plant’s physical or chemical barriers, activated