Why Is There White Powder on My Raspberry Leaves?

A white or pale grey powdery coating on the upper surface of raspberry leaves — often appearing first on young shoots and the undersides of leaves before spreading — is powdery mildew, a fungal disease caused by Sphaerotheca species. Unlike most fungal diseases, powdery mildew thrives in dry, warm conditions rather than wet ones, which makes its appearance in summer heat waves particularly common. It is disfiguring but rarely fatal to an established raspberry row.

Why summer heat triggers powdery mildew

Powdery mildew spores germinate readily in warm, dry air — they need only moderate humidity on leaf surfaces, not rain or free water. In a hot dry July or August, leaves under drought stress are more susceptible. Dense, poorly ventilated rows where air circulation is restricted create warm stagnant pockets that encourage spore germination. Young, rapidly-growing tissue is the most vulnerable.

Impact on the crop

Powdery mildew on leaves alone rarely affects fruit quality significantly. If the infection reaches developing fruit, the berries may show a light powdery coating, but this is unusual and only occurs in severe outbreaks. More commonly the main effect is cosmetic damage to the leaves and mild stress on the plant. An established row affected late in the season after most of the harvest is in is rarely a serious problem.

Management and prevention

Thin the row to improve airflow — six to eight canes per metre is the target. Water consistently at the base of plants rather than overhead during dry spells; drought-stressed canes are more susceptible. Potassium bicarbonate sprays or dilute milk sprays (traditional organic method) applied to affected foliage reduce mildew pressure. Clear all old cane debris after autumn pruning to reduce overwintering inoculum.