The validity of in vitro screening methods in the search for fungal antagonists of Sclerotinia sclerotiorum causing wilt of sunflower

A multi-test screening system identified 63 fungal isolates with high in vitro biocontrol activity against Sclerotinia sclerotiorum. A bioassay method was developed using sunflower seedings growing in an unsterilized loam mixture. Twenty-six isolates were tested in a series of five bioassay tests and a significant correlation (P < 0.01) was found between sclerotial infection in vitro and the number of healthy plants in vivo. Conversely activity in an in vitro mycelial overgrowth test was not significantly correlated with activity in vivo. However some isolates showing only mycelial activity still exerted significant disease control in both the bioassays at Littlehampton and in three additional bioassays at Sittingbourne. Only one isolate not previously reported showed significant activity in both sets of bioassays and the lack of consistency in disease control activity by all other isolates, and biocontrol agents in general, was deemed a major barrier to their use.