Factors affecting the control of Pythium ultimum damping‐off of sugar beet by Pythium oligandrum

Application of Pythium oligandrum to a soil‐based compost as a mycelial suspension (5 × 10 2 CFU g −1 of dry compost) and oospore alginate pellets (10 5 oospores/g of dry compost) controlled pre‐ and postemergence damping‐off of sugar beet caused by Pythium ultimum to a level similar to metalaxyl seed treatment. Oospore seed treatments and aqueous suspensions of oospores applied to compost failed to control disease. Problems in the use of P. oligandrum oospore inocula for the control of damping‐off were highlighted. It was shown that treatment of oospores with cellulase (20 g L −1 ) increased germination approximately three‐fold in comparison to untreated spores. Untreated and cellulase pretreated oospores were subsequently evaluated as seed treatments for their ability to control damping‐off of sugar beet. The highest rate of pretreated oospores (10 4 oospores/seed) gave levels of emergence and establishment in infested compost that were not significantly different from the uninfested controls, whereas seed treatment with untreated oospores gave no significant reduction in disease. In a trial carried out in a controlled environment to assess the effect of pH (4.5–8.0), P. oligandrum (10 4 cellulase pretreated oospores/seed) was shown to control pre‐ and postemergence damping‐off of sugar beet at pH 7.0 and 7.5 only.