S. Bhardwaj

Biological control of white mould (S. sclerotiorum) by Trichoderma spp. isolates was tested on cucumber, lettuce, bean and tomato plants. Leaves were infected by means of mycelium discs placed on their adaxial (upper) side, whereas Trichoderma was applied as a suspension, either to the root zone or to the leaves. Plants were maintained in a greenhouse under conditions of high humidity at 20-25 degrees C. Disease control was achieved on the various hosts, but the level of control was affected by factors such as the Trichoderma isolate tested, the host species, the plant organ treated, the pathogen isolate, the nature of the biological control preparation and the time interval between biological control application and pathogen inoculation. An interaction between different factors was found throughout the experiments. Seven isolates of Trichoderma, including isolate T39 (of the commercial product TRICHODEX) were tested. Increased concentration of T39 conidia (from 1 to 5x105/ml) and application to the leaves and to the roots as a formulation, instead of as naked conidia, improved disease suppression on cucumber, tomato and lettuce. This happened in some of the isolates, probably because of their differing requirements. Application of the biological control agents 1-5 days before inoculation resulted in better disease suppression by certain isolates. Application to lower leaves resulted in disease control similar to that by soil application in cucumber and bean. Treatment of the abaxial side of the leaves of cucumber and bean resulted in suppression of the adaxial infection as well. Dead cells of T39 suppressed white mould as effectively as live cells. The results demonstrate variability in efficacy of biological control of S. sclerotiorum among the Trichoderma isolates and treated host species. Nevertheless, disease reduction of above 70% could be achieved. Apparently, the Trichoderma activity can be partly related to induced resistance. Given that there were spatial and temporal differences between Trichoderma application and pathogen inoculation, it can be concluded that the signal passes from roots or lower leaves to upper leaves and through the leaf tissue. A similar effect can be related to dead cells of Trichoderma.