Biological control of Botrytis cinerea on tomato plants by the use of epiphytic yeasts Candida guilliermondii strains 101 and US 7 and Candida oleophila strain I-182: II. a study on mode of action

The secretion of hydrolytic enzymes by yeasts was studied in batch cultures amended with different carbon sources (glucose, laminarin, and Botrytis cinerea cell wall preparation). Candida guilliermondii (strains 101 and US 7) and Candida oleophila (strain I-182) secreted detectable amounts of (β-1,3-exoglucanase and chitinase at environmentally relevant temperatures and in all carbon sources. Secretion was carbon source dependent with glucose inducing the production of the highest amount, followed by laminarin and B. cinerea cell wall preparation. In many cases, C. oleophila demonstrated the highest hydrolytic enzyme secretion. However, it was observed that the enzymes may constitute a relatively small part of the total protein secreted. The possibility that other products may be involved is discussed. Studies on population dynamics of all three yeasts on stems revealed a 10-fold increase in the number of viable yeast cells within the first 48 h after inoculation, followed by a persistent high level of viable cells for 15 days. In an assay, in which yeasts and B. cinerea spores were separated by Cellophane, yeasts significantly reduced germ tube length especially when Cellophane was overlaid on cut stems of tomato inoculated with yeasts. None of the isolates studied induced the production of diffusible antifungal compounds (phytoalexins) in a leaf bioassay. However, B. cinerea induced the production of such compounds, and significantly reduced B. cinerea spore germination but not germ tube elongation. All three yeast strains, both dead and living yeast cells, were shown to adhere to the B. cinerea hyphae, while an inactive control strain did not.