Why Are My Grapes Bleached and Sunburned?

Grape berries that develop white, papery, bleached patches on the sun-facing side of the bunch — while the remainder of the berry appears normal — are suffering from sunscald. Unlike most grape problems, sunscald is not caused by a pathogen or nutrient issue but by the direct physical effect of intense sunlight and heat on fruit that has not been given a chance to acclimatise to exposure. Understanding when and why it occurs helps you prevent it without sacrificing the solar exposure that drives sweetness.

What sunscald looks like

Affected berries develop a pale tan or white bleached area on the side facing the sun. The skin over the affected area becomes paper-thin and may collapse inward, creating a sunken, wrinkled patch. The flesh beneath dries out. Sunscald patches are an entry point for secondary botrytis infection, particularly in humid conditions. The damage appears within a day or two of a heat event or sudden exposure.

Why sudden exposure causes burning

Berries that have developed within a shaded canopy have thin skins, high water content, and no accumulated UV-protective pigments. When their canopy cover is suddenly removed — by late aggressive leaf stripping in August — they are exposed to full sun without any tolerance. The surface temperature of the berry rapidly exceeds the temperature of the ambient air (surfaces in direct sun can be 10–15°C hotter than the air), causing cell death in the exposed skin.

Gradual canopy opening as prevention

Leaf removal for fruit zone exposure is most effective and least damaging when done in stages from late June onwards, starting with the oldest leaves and removing two to three at a time per bunch rather than stripping everything at once. Berries that develop with progressive, gradual exposure build up pigment and thicker skin without suffering the sudden-exposure shock that causes sunscald. Removing leaves on the morning-sun side first and only later the afternoon-sun side further reduces risk.

Timing of leaf removal in cool climates

In the UK and northern Europe, sunscald risk is lower than in warmer regions because air temperatures rarely reach the levels that cause extreme berry surface heating. Leaf removal in these climates can usually be more aggressive without sunscald risk. The rare exception is a wall-trained vine facing south-west that may experience very high reflected heat from a brick or stone wall during a hot August — light shading cloth on the hottest part of the day provides insurance.

Greenhouse management

Greenhouse-grown grapes are at higher sunscald risk than outdoor vines because glass amplifies heat and the restricted air movement reduces cooling. Ventilate aggressively in midsummer, consider summer whitewash (diluted white paint or purpose-made shading paint) on the glass, and avoid stripping leaves suddenly on hot days. Ventilation is the single most effective measure against high temperature damage under glass.