If your ripe blackberries have distinct white spots, this is a common frustration for home gardeners. This discoloration is a sign of environmental stress affecting the fruit’s development. It is usually a localized issue rather than a full crop failure. These white spots frequently occur during periods of intense summer weather, indicating the plant is struggling to maintain a uniform ripening process.
Identifying White Drupelet Syndrome
The white spots you observe are most likely a condition known as White Drupelet Syndrome (WDS). Blackberries are aggregate fruits composed of many tiny individual spheres, each called a drupelet, which cluster together. WDS occurs when some of these individual drupelets fail to fully ripen and turn black alongside the others.
The affected drupelets appear hard, pale white, or tan against the dark, mature segments of the berry. This discoloration is localized to the unripened sections. The rest of the blackberry, including the unaffected drupelets, will be fully ripe and healthy. This condition is a physiological disorder, meaning it is a response to environmental factors, not a pathogen or disease.
The Environmental Triggers
The primary cause of White Drupelet Syndrome is exposure to intense environmental stressors, particularly high heat and solar radiation. When ambient temperatures rise consistently above 85 to 90 degrees Fahrenheit, the delicate cells within the developing drupelets can be damaged by ultraviolet (UV) light. This damage, sometimes called sunscald, prevents the development of the dark pigments that give the blackberry its characteristic color.
This effect is often compounded by a rapid drop in humidity, which allows more direct UV rays to penetrate the canopy and reach the fruit. Berries exposed to the direct afternoon sun are most susceptible. The white spots frequently appear only on the side of the fruit facing the heat source. Warm and windy weather also contributes by increasing the drying effects on the fruit.
A secondary cause can be the feeding activity of certain tiny pests, like eriophyid mites. These microscopic mites feed on the surface of the developing drupelets. Their feeding causes localized damage, which can either directly result in the unripened white spots or make the affected cells more vulnerable to sunscald. While mite damage looks similar to sunscald, it tends to result in a more random pattern of white or tan drupelets across the fruit.
Managing and Preventing White Drupelets
Preventing White Drupelet Syndrome focuses on mitigating the environmental stresses that trigger the condition. One effective strategy is to shield the plants from the intense afternoon sun. Home gardeners can use shade cloth, typically rated for 30% shade, draped over the plants during the hottest part of the summer. This significantly reduces UV exposure and fruit temperature.
Proper pruning provides natural protection by ensuring healthy leaf cover remains to shade the developing fruit. It is also helpful to orient blackberry rows in a north-south position if possible, as this minimizes exposure to the harsh midday sun. Adequate and deep irrigation is important, especially during heat waves, to help keep the plants cool and hydrated.
In cases where pest activity is suspected, specific controls can be implemented. Eriophyid mites are best managed during the dormant season before they become active. Applying a dormant oil or lime sulfur spray in late winter or early spring, just before the buds break, helps eliminate overwintering mites. If damage is observed during the growing season, an application of insecticidal soap may help control a developing population.
Are White-Spotted Blackberries Safe to Eat?
The good news for home growers is that blackberries affected by White Drupelet Syndrome are completely safe for consumption. This condition is an abiotic disorder, meaning it is not caused by disease, fungus, or bacterial contamination. The white or tan drupelets are simply portions of the fruit that did not fully mature and color due to environmental stress.
While the berries are safe, the texture and flavor of the affected drupelets may be different. The white sections are often slightly firmer and tasteless compared to the juicy, sweet black segments. You can eat the fruit whole, or simply pick off the few white drupelets before using the rest of the blackberry.