Establishment and application of bioassay method for screening fungicides against cucumber powdery mildew

Abstract

Sphaerotheca fuliginea Poll., a pathogen causing powdery mildew in cucumber, was used as a target for establishing a bioassay method for screening fungicides. Different cucumber varieties, seedling ages, spore culturing times, and inoculation concentrations were tested. The results showed that a highly susceptible variety “Xinchangchunmici” was a suitable cultivar for bioassay test, with the appropriate seedling age of 5―10 d, the spore culturing-time of 10―15 d and the spore concentration of 30―40 spores per field of vision (10 × 10 times under microscope) in the suspension. Cotyledon-spraying method, and leaf-disc method were compared for bioassay and Trypan Blue staining method was used for detecting thedevelopment of S. fuliginea on the leaf surface. Based on the bioassay system established above, the protective effect of chrysophanol on the development of S. fuliginea was evaluated using cotyledon-spraying method. The results showed that chrysophanol reduced the disease index effectively and the protective EC₅₀ value was 33.36 μg·mL^−1.

Key words： Sphaerotheca fuliginea Poll.; cucumber powdery mildew; bioassay system; chrysophanol