Symptoms on the leaves

(1) In Spring – Primary infection results in the appearance of yellow oil spots about 10mm in size. These grow to about 50mm and merge to cover the leaf. With age they become necrotic developing a brown halo.
Oil spots in Spring

(2) After suitable warm humid nights a corresponding dense white “cottony down’ forms on the underside of the leaf.
Sporulation on underside of leaf

(3) These white areas turn reddish brown from the centre but remain active at the outer edge. Leaves shrivel and drop prematurely.
Downy areas brown with age

(4) In late summer and autumn affected leaves may display a mosaic patters of infection between the leaf vein. This signifies the sexual stage of downy mildew reproduction.
Mosaic pattern in autumn