Effect ofTrichoderma harzianumonBotrytis cinereaPathogenicity

Germination and germ-tube elongation of conidia of the pathogen Botrytis cinerea on bean leaves were reduced in the presence of the biocontrol agent Trichoderma harzianum T39. A reduction of 20 to 50% in germ-tube biomass was observed 20 h after inoculation. This reduction in germination did not result in complete prevention of disease development on the leaves. One day after inoculation, disease severity on leaves infected by the pathogen with and without the biocontrol agent was similar (-10% necrotic area). Subsequently, the disease developed rapidly in the control leaves and caused almost complete necrosis, whereas in the presence of T harzianum T39 the necrotic area reached only -50% of the leaf surface. The production of pectin-degrading enzymes by B. cinerea was measured up to 4 days after inoculation. Up to 1.3 enzyme units of polygalacturonase (PG), 9 microequivalents of NaOH, which express the activity of pectin methyl esterase (PME), and up to 1.5 units of pectate lyase (PL) were detected on bean leaves. Under the same conditions, the biocontrol agent, T harzianum T39, did not produce any of these enzymes. On leaves infected with B. cinerea in the presence of the biocontrol agent, the activity of the pathogen's PG was reduced by 40 to 83%. This was reflected on an activity gel by the faintness of these PG isoenzymes and the delay in their appearance. An up to 100% reduction in PME activity and a -30% reduction in PL activity also were recorded. We suggest that T. harzianum T39 acts by reducing the enzyme activities of the pathogen. An indirect effect of enhancing the defense mechanism of the host plant is discussed.