Biocontrol agents efficiently inhibit sporulation of Botrytis aclada on necrotic leaf tips but spread to adjacent living tissue is not prevented

Ulocladium atrum (isolates 385 and 302) consistently inhibited Botrytis aclada sporulation on dead onion leaf pieces under constant moist conditions and with an interrupted wetness period of 9 h. Clonostachys rosea (isolate 201) was as effective as U. atrum under constant moist conditions, but was ineffective if exposed to a drying period. No sporulation of B. aclada was observed 8 and 12 days after inoculation in the presence of U. atrum 302. C. rosea 201 significantly reduced B. aclada sporulation 8 days, but not 12 days after inoculation. When U. atrum 302 or C. rosea 201 was applied 1 day prior to B. aclada the antagonistic effect was higher compared to when the antagonists were applied on the same day. C. rosea 201 and U. atrum 302 did not obstruct the growth of B. aclada from necrotic onion leaf tips into living tissue, when artificially induced necrotic leaf tips were infested with B. aclada 24 h prior to antagonists. Three days after antagonist application, no symptoms could be observed on the healthy leaf tissue, nor was there sporulation on the necrotic leaf tip. However, B. aclada was immunologically detected 2 cm below the inoculation site. We conclude that under constant moist conditions the antagonists C. rosea 201 and U. atrum 302 cannot stop the progress of B. aclada from necrotic into fresh leaf tissue.