Evaluation of fungal antagonists to control black mold disease under field conditions and to induce the accumulation of antifungal compounds in onion following seed and set treatment

• We examined the antagonist fungi for their control of black mold disease in onion.
• We used two onion locations, naturally infested with pathogen, Aspergillus niger.
• Two bulb production materials, onion seeds and set, were studied.
• Antagonist fungi controlled the black mold disease under field conditions.
• Antifungal compounds against the pathogen in bulbs were induced by antagonist fungi.

Three isolates, AS3 (non-aflotoxigenic Aspergillus flavus Link), TRIC7 and TRIC8 (Trichoderma harzianum Rifai), from onion (Allium cepa L.) growing soils were recently found to control black mold disease caused by Aspergillus niger (An) van Tieghem and to increase the accumulation of antifungal compounds in pot-grown onion sets. Their ability to increase bulb diameter and total soluble solids in marketable bulbs, to control black mold and to induce the production of antifungal compounds were tested in sets and marketable bulbs raised from treated seeds and sets, respectively, in naturally An-infested field soils at two locations. These isolates significantly controlled the disease at both locations, but they did not have any enhancing effect on set or bulb diameter and soluble solids in marketable bulbs. AS3 and TRIC8 in particular led to defense reactions with accumulation of antifungal compounds in sets and marketable bulbs in both locations. Different compounds were also identified in the fractions with highly antifungal effects.