Downy mildew is common in commercial cruciferous crops on Long Island during fall, but has occurred rarely and sporadically during spring. Photographs below are kohlrabi that developed symptoms while in the seedling tray. Cotyledons have tiny black spots and true leaves have dark irregular-shaped symptoms. White spores of the pathogen were observed primarily on the underside of symptomatic cotyledons, especially after they were incubated under high humidity for at least a day. There is also a white spray residue on these plants.
It is recommended to discard trays with affected plants when downy mildew begins to develop so early in crop production. Spores are dispersed in air currants, thus other cruciferous seedlings in the greenhouse should be inspected for symptoms.
See also: Downy mildew on cabbage
