White or bleached spots on ripe raspberries are often termed White Drupelet Disorder. This phenomenon involves the discoloration of the fruit’s individual segments, known as drupelets. A raspberry is composed of dozens of these small, fleshy drupelets, and when a portion fails to develop color correctly, it results in a mottled appearance. While the affected fruit is safe to eat, the spots indicate a physiological or external issue that impacts the quality and aesthetic appeal of the harvest.
Primary Causes of Raspberry Whitening
The most frequent reason for localized bleaching is sun scald, a non-infectious physiological disorder. This occurs when intense sunlight combined with high temperatures physically damages the delicate cells of the drupelets. Ultraviolet radiation causes localized bleaching or drying, preventing the exposed portion of the fruit from developing its typical red color. This damage is most likely to occur during heat waves, as the fruit surface temperature can rise significantly higher than the ambient air temperature.
Another primary cause involves feeding activity from certain insects, notably the Tarnished Plant Bug (Lygus lineolaris). This pest uses piercing-sucking mouthparts to feed on developing green fruit, injecting a toxic substance. The tissue surrounding the feeding site fails to mature properly, resulting in a cluster of pale, shriveled, or desiccated drupelets. Other sucking insects, like certain species of stinkbugs, can cause similar damage, often making the resulting berry feel “crumbly.”
Distinguishing Between Environmental and Pest Damage
Observing the pattern of the white spots is the most reliable way to determine the cause of the disorder. Sun scald damage is typically uniform and appears only on the side of the berry directly exposed to the sun, often the south or southwest face. This environmental damage usually results in a larger, bleached area where the drupelets look dry and whitened.
In contrast, insect-related white spots are scattered randomly across the raspberry, reflecting the pest’s feeding location. The damage from a Tarnished Plant Bug appears as smaller, more distinct patches of pale or shriveled drupelets, creating a random, mottled pattern over the berry. Gardeners can also diagnose a bug problem by gently shaking the flower or fruit clusters over a white surface to check for the presence of the small, active, greenish-brown nymphs.
Prevention and Management Strategies
If sun scald is the identified problem, managing the berry’s exposure to intense afternoon sun is the best preventative step. Providing partial shade during the hottest part of the day can significantly reduce the incidence of the disorder. This is often achieved by installing a temporary shade cloth over the row during peak summer months.
Maintaining consistent soil moisture is also helpful, as water stress makes plants more susceptible to heat damage. Optimizing pruning to ensure a healthy canopy of leaves remains to shade the developing fruit can offer natural protection.
To manage the Tarnished Plant Bug, cultural control measures are recommended. Removing weeds within and around the raspberry patch is helpful, as they provide an early food source and habitat for the pests. If chemical intervention becomes necessary, treatments should target the nymphs rather than the more mobile adults. Products containing active ingredients like pyrethrins or insecticidal soaps can be used, ensuring they are rated for use on edible crops and following all label instructions.