Powdery Mildew on Blueberry Leaves — How to Treat It

A white or pale grey powdery coating on blueberry leaves — especially the upper surface — is powdery mildew, a fungal disease caused by Microsphaera vaccinii. It appears most commonly in late summer and early autumn, when warm days combine with cool nights and humidity. The coating is actually the fungal mycelium and spores living on the leaf surface, and while it rarely kills an established blueberry bush outright, a severe infection weakens the plant, reduces fruiting in the following year, and can cause early leaf drop.

What the fungus looks like and does

Unlike most fungal diseases, powdery mildew does not need wet conditions to infect — it prefers warm days and cool, humid nights with dry leaf surfaces. The white powder appears first on young leaves and shoot tips, and in bad years can coat entire branches. Affected leaves may yellow, distort slightly or drop early. The fungus weakens the leaves by drawing nutrients from the cells beneath the coating, and heavy infections in late summer can reduce the carbohydrate reserves the plant stores for next year's flowering and fruiting.

Cultural controls first

Powdery mildew thrives in dense, still canopies. The most effective long-term control is pruning to open up the bush and let air move freely through the branches. Remove crossing, crowded and inward-pointing shoots during dormant pruning each winter. Avoid high nitrogen feeding in late summer, which produces the soft, sappy growth the fungus colonises most easily. Water at the base rather than overhead, and water in the morning so any splash dries by evening. Rake up and remove fallen leaves in autumn — the fungal spores overwinter in dead plant material.

Organic and low-impact treatments

A dilute solution of baking soda — one teaspoon per litre of water with a drop of liquid soap — raises the pH on the leaf surface and inhibits the fungus. Apply to both leaf surfaces on a dry day, not in bright sunshine to avoid leaf scorch. Potassium bicarbonate products sold for garden use work on the same principle and are more reliable. Dilute neem oil is also effective when applied at the first sign of infection, before the mildew establishes across the canopy. Sulphur-based fungicides are effective but should not be used within four weeks of an oil spray.

When to be more concerned

A light coating of powdery mildew in September on an established bush that has already fruited normally is not an emergency. The plant will drop those leaves and the disease effectively resets. Take it more seriously if the infection covers large areas of the canopy in July or August, when the plant is still developing fruit and storing energy, or if the same bushes are badly affected year after year — that pattern points to a structural problem with the growing conditions that cultural changes can address.