Assessment of Potential for Biological Control of Botrytis cinerea by an Indigenous Trichoderma harzianum Isolate with a Novel Detached Leaf-droplet Inoculation Bioassay and Correlated Increase in Phytoalexin Production

A simple detached leaf-droplet inoculation bioassay was devised in order to investigate the potential for biological control of Botrytis cinerea by indigenous Trichoderma harzianum isolates. Following preliminary in vitro screening of the antagonistic capacity of four local isolates of Trichoderma harzianum (T1-T4) against B. cinerea, T3 spore suspensions were then tested for their ability to control infection development of B. cinerea on T3 pre-inoculated lettuce and broad bean leaves, using different nutrient concentrations in B. cinerea infection inocula. Again, T3 proved to significantly control infection development in all but one of the treatments, as long as T3 spores were introduced in a nutrient solution. This bioassay provides a rapid and simple method of assessing biocontrol efficacy under in vivo conditions. T3 was also tested for its ability to induce plant defense mechanisms in the form of phytoalexin production after inoculation of broad bean cotyledons with T3 spore suspension. Thin layer chromatography indicated that T3 lead to the induction of relatively high phytoalexin yields, suggesting that this could be another potential ‘indirect’ mechanism of protection from pathogens by T. harzianum strain T3.