The downy mildew can be confused with gray mold, sclerotina and blackleg, or alternaria brassicae.
Downy mildew (cabbage plants) [Peronospora parasitica]
Cabbage plants, oilseed rape, and all other plants of the cabbage family.
Pattern of damage
Yellowish to reddish-brown irregularly angular, slightly sunken spots can be found on the upper side of leaves (Picture 1). The underside of the leaves show a dirty-white fungal lawn (Picture 2). The cotyledons seem stunted and die prematurely through yellowing when the infection is severe. In oilseed rape infected pods have pale-gray spots showing a loose, grayish fungal lawn in high humidity. With increasing infection the entire pod turns pale-gray and ripens prematurely.
Cultivation of more tolerant varieties. Promoting the germinating plant and the development of young plants. Proper preparation of the seedbed and adjusted nutrients.
This disease cannot reproduce without a suitable host plant. It, can survive as mycelium on harvest residue of oilseed rape or for a short period of time as conidiospores. Germination, infection, and sporulation are promoted by high humidity and temperatures around 20 °C. The fungus infects the plant through the stoma on the underside of the leaves. Conidia carriers grow from the openings, a fungal lawn becomes apparent. The conidia growing there are propagated via wind and rain splash.