PM is caused by the fungus Erysiphe polygoni and is a common minor disease of brassicas, mostly in late summer and autumn.
Powdery fungal growth occurs on upper and lower leaf surfaces. Leaves can become yellow and drop prematurely.
Sometimes causes damage in summer sown forage crops when going into winter.
PM survives winter as a fungus on infected plants, weeds or crop debris. Air-borne spores produced in spring can land on susceptible plants, causing new infections. Spores are also released after rain in autumn.
Prevention and management
Normally, no control is required. There are a number of fungicides available which are effective against PM, if it reaches high levels in high-value crops. The best prevention is to use PM resistant cultivars.