Stepwise screening of candidate antagonists for biological control of Blumeria graminis f. sp. tritici

Antagonists for the biological control of Blumeria graminis f. sp. tritici were selected using a stepwise screening approach. Fungal colonizers of powdery mildew pustules were isolated from leaves of cereals and other plant species. Spore production, cold tolerance, drought tolerance and UV-B resistance as important characteristics for application of biocontrol candidates in the phyllosphere were tested in in vitro assays and preliminary risk assessments were conducted. Amongst 850 tested isolates 58% belonged to various taxonomical groups of Cladosporium. Only 3% belonged to species that have been reported in literature as antagonistic to powdery mildews. The stepwise screening approach allowed to reduce the number of candidate antagonists using screening criteria that can be tested reliably and cost-effectively in in vitro assays and by data mining from initially 1237 isolates down to 143 candidate antagonists belonging to 42 taxonomical groups. The potential of these isolates to reduce conidia production of B. graminis f. sp. tritici. in wheat was assessed in bioassays on potted winter wheat plants under controlled conditions. A set of ten superior isolates was subsequently tested in a series of trials on potted spring wheat plants under open field conditions. Isolates Tilletiopsis pallescens BC0441 and T. pallescens BC0850 significantly reduced the number of powdery mildew pustules per flag leaf by 30 to 62%. Slopes of the regression lines fitted to data on number of powdery mildew pustules during time were significantly reduced by the antagonists indicating that the powdery mildew epidemics were slowed down. Treatments with T. pallescens BC0441 and T. pallescens BC0850 also reduced leaf coverage with powdery mildew pustules in a small-scale field trial in spring wheat