Why Do My Raspberry Leaves Look Bronzed and Dry in Summer?

Raspberry leaves that develop a mottled bronze or silvery appearance during hot, dry summer weather — often accompanied by fine white webbing on the undersides of leaves — are infested with two-spotted spider mite (Tetranychus urticae). These tiny sap-sucking mites are nearly invisible to the naked eye but produce characteristic bronzing as they puncture leaf cells in large numbers. In severe outbreaks leaves dry out and drop early, reducing the photosynthetic capacity of the plant in the weeks leading up to harvest.

How to confirm spider mite

Inspect the undersides of bronzed leaves with a hand lens or magnifying glass — you will see tiny moving dots (the mites), pale translucent cast skins and the fine silk webbing that distinguishes spider mites from other causes of leaf bronzing. Tapping a suspect leaf over white paper and looking for moving specks is another quick diagnostic. The distinctive two dots (dark markings on the body) may be visible under magnification.

Why dry heat triggers outbreaks

Spider mites reproduce very rapidly in hot, dry conditions — populations can explode from barely visible to severe within two weeks during a heat wave. The same conditions often reduce populations of predatory mites that naturally control them. Dusty, dry leaf surfaces are also preferred habitats. Plants under drought stress are more susceptible. Regular overhead watering during dry spells actually suppresses mite populations.

Control options

Water misting on the undersides of leaves reduces mite populations physically — mites struggle in humid conditions. Organic insecticidal soap sprays applied to the undersides of leaves at the first sign of infestation give direct control. Predatory mites (Phytoseiulus persimilis) are available as biocontrol and are very effective in warm conditions. Avoid broad-spectrum insecticides which kill the natural predators and often worsen mite problems long-term.