COMPATIBILITY OF ESSENTIAL OILS WITH THE BIOCONTROL FUNGUS, BEAUVERIA BASSIANA

The tomato seedling damping-off pathogens Rhizoctonia and Pythium have the potential to cause severe loss in the greenhouse and field. Both seed application of Beauveria bassiana and soil amendment with bioactive monarda herbages are sustainable approaches that can play a role in suppressing damping-off of tomato seedlings. The objectives of this research were to determine the compatibility of essential oils and B. bassiana, and to determine the impact of the two when used together as a seed treatment in greenhouse experiments.

Different concentrations of five essential oils that were active against damping-off pathogens (cymene, carvacrol, thymol, borneol, and geraniol) were tested. Germ tube emergence was monitored every four hours after germination began. Germ tube lengths of B. bassiana conidia treated with each oil concentration were determined. Cymene had a stimulatory effect on germ tube development of B. bassiana at low concentrations but inhibited germ tube development at high concentrations. Thymol was the least inhibitory of all tested oils. All oils reduced germ tube length at the highest concentration at 24 h. In a separate study spore germination of B. bassiana treated with oils for 24 h was also determined. Germination percentage was decreased across concentration in all five oils treatment. Germ tube development with fresher spores (used in the second germination study) was similar to that for aged conidia (used in the first germination study) but at 24 h, fresher spores had a shorter average tube length.

In the second study, five bioactive monarda cultivars (‘Trinity Purple’, ‘Rose-scented’, ‘Violet Queen’, ‘Cerise’, and Monarda clinopodia) representing five chemotypes (cymene, geraniol, carvacrol, borneol, and thymol, respectively) were selected based on the GC-MS analyses. Colonization by B. bassiana of seedlings grown in media amended with the five cultivars was determined. All five cultivars had a slight negative impact on the ability of B. bassiana to colonize tomato. Treatment with Monarda clinopodia (thymol chemotype) resulted in very low germination of tomato seeds. Four other monarda cultivars had no negative impact on tomato seed germination and so have the potential to be used with B. bassiana as biological control agents.