What Causes Brown Spots on Grapes and How to Fix Them

Finding brown spots on grapes can be concerning, raising questions about the cause and safety of the fruit. These blemishes are a common symptom that can point to several distinct problems, from fungal diseases to environmental stress. Understanding the potential causes is the first step toward determining the correct course of action.

Common Causes of Brown Spots on Grapes

Fungal diseases are a primary driver of brown spots on grapes. Black rot is one of the most widespread, beginning as small, whitish spots on immature grapes that quickly expand. These lesions then sink and turn dark brown to black, eventually causing the entire fruit to shrivel into a hard, black mummy. The fungus overwinters in these mummified berries and old cane lesions, releasing spores during damp spring weather.

Another fungal issue, anthracnose, is often called “bird’s-eye rot” due to its characteristic appearance. It creates small, sunken spots on the berries that have a dark, reddish-brown border and a lighter, gray or tan center. This disease thrives in warm, wet conditions and can also affect leaves and shoots. Downy mildew can also impact the fruit, causing grapes to turn a dull brown, soften, and fall from the cluster, especially during periods of high humidity.

Physical and environmental factors can also blemish the fruit. Sunscald is a common environmental issue that occurs on the side of the grape most exposed to direct, intense sunlight, resulting in a brownish, toughened patch of skin. Mechanical damage from hail, high winds causing grapes to rub against each other, or careless handling can create wounds that turn into brown, corky scars as the skin heals.

Pest activity is another source of brown blemishes. Insects like the grape berry moth lay eggs on the fruit, and the hatching larvae burrow inside. Their feeding activity causes localized discoloration and decay, which appears as a soft brown spot. This damage also creates entry points for secondary fungal infections, such as Botrytis bunch rot, which can spread to adjacent berries.

Are Grapes with Brown Spots Safe to Eat

The safety of grapes with brown spots depends on the cause. If the spots are from environmental factors like sunscald or minor mechanical injury, the grapes are generally safe for consumption. These spots are superficial, affecting only the skin’s appearance. Simply cutting away the small, affected area will leave the rest of the grape edible, provided it is still firm and has a normal smell.

Conversely, exercise caution when the brown spots are caused by fungal infections. For a minor infection where only a single grape in a bunch shows a small, firm brown lesion, it may be acceptable to discard that individual grape and consume the rest after careful inspection. It is best to discard grapes that show signs of widespread fungal growth. If spots are large, soft, or accompanied by visible mold, a mushy texture, or an off-putting odor, the fruit should not be eaten, as the rot is extensive and may affect safety.

How to Treat Affected Grapevines

Once a fungal disease has been identified on a grapevine, immediate action is necessary to manage its spread. The first step involves cultural controls, which focus on sanitation. Carefully prune and remove any affected parts of the plant, including spotted grape clusters, leaves, and canes. By removing the visibly infected tissues, you limit the source of spores that can be spread by rain and wind. This material should be destroyed by burning or burying it, as composting can allow fungal spores to survive and reinfect the vineyard.

For persistent or severe fungal infections, the application of fungicides may be required. It is important to first accurately identify the specific disease, as different fungicides are effective against different pathogens. For specific recommendations on products and application timing, consulting a local agricultural extension office is highly advisable, as they can provide guidance tailored to your region and the particular disease.

Preventing Brown Spots on Grapes

Long-term prevention of brown spots hinges on creating an environment that is less favorable for disease. One strategy is to promote good air circulation through proper pruning and training during the dormant season. By selectively removing canes, you open up the vine, allowing sunlight to penetrate and air to move freely, which helps leaves and fruit dry quickly after rain or morning dew.

Vineyard sanitation is another preventative measure that targets the overwintering stages of fungal diseases. After the final harvest and leaf drop, a thorough cleanup is necessary. Rake up and destroy all fallen leaves and any mummified fruit left on the vine or on the ground. These materials serve as the primary reservoir for pathogens.

The way you water your grapevines can also influence disease pressure. Overhead watering that wets the foliage and fruit clusters creates moist conditions for fungal spore germination. Instead, use drip irrigation or a soaker hose to deliver water directly to the base of the plant, keeping the canopy dry.

Finally, a proactive strategy is the selection of grape varieties that have inherent resistance to common diseases. Many modern grape cultivars have been bred for their natural ability to fend off specific pathogens. When planting a new vineyard, choosing varieties known to be resistant to diseases prevalent in your area can greatly reduce the need for future intervention.