Why Are My Grapevine Leaves Pale and Mottled?

A grapevine with leaves that have lost their deep green colour and developed a pale, washed-out, or silvery mottled appearance — often with fine webbing on the underside — is almost certainly suffering from red spider mite (Tetranychus urticae). This tiny sap-sucking pest is particularly prevalent on greenhouse-grown vines and on outdoor vines during hot, dry summers. Population explosions can happen remarkably quickly, and a heavy infestation weakens the vine significantly.

Recognising spider mite damage

Mites are tiny (about 0.5 mm) and difficult to see with the naked eye, but their damage is distinctive. They pierce leaf cells on the underside of the leaf and extract the contents, leaving the upper surface with thousands of tiny pale or white feeding dots that merge into a general mottled, bronzed, or silvery appearance. A hand lens or magnifying glass reveals the mites themselves — they are oval, greenish-yellow with dark spots in summer, and orange-red in autumn. Fine silk webbing on the underside confirms their presence.

Hot, dry conditions driving outbreaks

Spider mites are warm-season pests that reproduce explosively when temperatures are high and humidity is low. A single female can produce hundreds of offspring in a matter of weeks in ideal conditions. Greenhouse vines in summer are particularly susceptible because temperatures are higher and humidity tends to be lower than outdoors. Adequate ventilation and regular misting of the canopy underside are the most effective cultural controls.

Encouraging natural predators

The predatory mite Phytoseiulus persimilis is commercially available and highly effective at controlling red spider mite in greenhouses. Introduce it as soon as mites are detected, before populations become very large. Outdoors, lacewings, ladybirds, and predatory beetles naturally keep mite populations in check in a garden with good biodiversity. Avoiding broad-spectrum insecticides preserves these natural enemies.

Physical and chemical controls

Washing the underside of heavily infested leaves with water under moderate pressure dislodges mites and their eggs. Repeat every few days. Horticultural oil sprays (such as plant-based oils) suffocate mites and eggs when applied thoroughly to the underside of leaves. Organic-approved acaricides based on fatty acids are also effective. Rotate products to prevent resistance — mites develop resistance to acaricides very rapidly with repeated use of the same chemistry.

Dormant lime-sulphur wash

A dormant spray of lime-sulphur applied to the bare vine in late winter kills overwintering mite eggs in the bark and crevices of the old wood. This single treatment in winter significantly reduces the population entering the following season. It must be applied while the vine is fully dormant and before bud burst.