The discovery of white spots or discolored patches on your blackberries can be concerning, especially when you are looking forward to a perfect harvest. While these spots, known as White Drupelet Disorder (WDD), might look alarming, they are rarely caused by a serious disease. Understanding the various factors that cause this condition is the first step toward preventing it and ensuring a bountiful crop in the future.
What is White Drupelet Disorder?
White Drupelet Disorder is a physiological condition that affects the individual segments, or drupelets, of blackberries and raspberries. This disorder causes some drupelets to turn a tan or white color, leaving the berry with an unappealing, mottled appearance. While the affected fruit remains edible, commercial growers often reject berries with WDD due to consumer expectations for unblemished fruit.
The primary cause of WDD is environmental stress. High temperatures, direct sunlight, low humidity, and strong winds can all contribute to this condition. Early in the season, when the plant canopy is still developing, newly exposed fruit is especially vulnerable to direct UV-B radiation.
Environmental causes
- Sunscald: Intense, direct sunlight during hot, dry periods is the most common cause of white drupelets. This acts like a sunburn on the fruit, especially affecting the side facing the sun. Temperatures over 90°F can cause the berry to "blanch" or "cook" in the sun.
- Low Humidity and Wind: Hot, dry air and strong winds can remove humidity from the canopy, reducing the protective scattering effect of water vapor on UV radiation. This makes the fruit more susceptible to sun damage.
- Rapid Temperature Changes: A sudden shift from mild weather to very hot conditions can shock the plants and lead to the development of WDD. Some research also suggests a connection between rapid temperature changes and the reversion of color in harvested berries (red cell regression).
Pest-related damage
In some cases, pests can cause symptoms similar to White Drupelet Disorder by damaging the individual drupelets. Here is how to tell the difference:
- Stink Bug Damage: Stink bugs feed on the fruit by inserting their mouthparts into the berry. This feeding activity causes localized, random patches of discolored drupelets rather than the more uniform patches associated with sunscald. The affected drupelets may turn white, brown, or remain unripe. In addition, the berry might taste like a stink bug smells if the insect releases its defensive chemical during feeding.
- Redberry Mite: While less common, these tiny mites can also cause uneven ripening and color changes. They are most often associated with berries that have distinct boundaries between red and black drupelets, a condition sometimes called redberry mite syndrome. Unlike sunscald, which affects the exposed side, redberry mite damage can appear anywhere on the berry.
Comparison Table: WDD, Pest Damage, and Mold
| Characteristic | White Drupelet Disorder (Sunscald) | Pest Damage (e.g., Stink Bug) | Mold (e.g., Botrytis) |
|---|---|---|---|
| Appearance | White or tan patches on sun-exposed side of drupelets; often a solid block or uniform area. | Random, scattered white, tan, or unripe drupelets; puncture marks sometimes visible. | Fuzzy, white or gray patches that grow over the surface of the fruit; often starts in a soft spot. |
| Affected Area | Primarily on the side of the fruit facing the sun. | Isolated to one or a few drupelets; pattern appears random or spotty. | Spreads across multiple drupelets and can engulf the entire berry and spread to others. |
| Effect on Edibility | Edible and harmless, though texture and taste may be slightly altered. | Edible unless a strong "buggy" flavor is present; texture may be slightly off. | The berry should be discarded, as mold can be harmful. |
| Cause | Intense UV radiation and heat, often combined with low humidity. | Feeding activity by pests, which inject enzymes into the fruit. | Fungal infection, often occurring in warm, wet, and humid conditions. |
Preventing white drupelets
While white drupelets are not a sign of disease, preventative measures can help ensure a more uniform and aesthetically pleasing harvest. Here are some strategies to implement:
- Provide Shade: For home gardeners, using a shade cloth can protect berries from the most intense afternoon sun. For commercial operations, a north-south trellis orientation can help shade the fruit. It is best to apply shade only after pollination has occurred to avoid impacting bee activity.
- Improve Canopy Health: Ensure your plants have a robust, healthy foliage canopy. Healthy leaves provide natural shade for the developing fruit, reducing its exposure to direct sun. Proper fertilization and watering are key to maintaining a full canopy.
- Irrigation for Cooling: In extremely hot and dry climates, some growers use overhead irrigation for brief periods during the day to cool the plants and increase humidity. This technique should be used with caution, as it can increase disease risk if done incorrectly, particularly late in the day.
- Control Pests: Regular scouting for pests like stink bugs can help gardeners take action before significant damage occurs. Prompt and appropriate treatments can prevent pest-related discoloration.
- Manage Variety Selection: Certain varieties are more susceptible to WDD than others. Researching and choosing more resistant cultivars can reduce the incidence of white drupelets.
- Harvest Promptly: Picking ripe berries frequently helps reduce the overall heat load on the plant and prevents fruit from being overexposed to harsh conditions.
What to do with affected blackberries
When you find white drupelets on your blackberries, the first step is to correctly identify the cause using the comparison table above. If it is clearly a case of WDD from sunscald, the berries are still safe to eat. They may have a slightly different texture or flavor, but they are perfectly suitable for use in pies, jams, and other cooked dishes where visual perfection is less important. If the berries show signs of mold, they should be discarded.
Conclusion
Finding white patches on your blackberries is a common issue for many growers, but it is typically not a cause for serious concern. In most cases, it is the result of White Drupelet Disorder caused by sun exposure, heat, and other environmental factors. While pest damage or mold can sometimes be the culprit, a quick inspection can help you differentiate the problem. By implementing preventative measures like providing shade, maintaining a healthy canopy, and controlling pests, you can minimize the occurrence of white drupelets and enjoy a more perfect-looking harvest. For further reading, authoritative sources like agricultural extension services (e.g., North Carolina State University) provide additional research and tips for managing this and other berry issues.
