Black dots on grapes usually indicate a fungal pathogen thriving in warm, humid conditions. These spots are physical manifestations of a plant disease, not cosmetic blemishes. Quick identification is important because these infections can spread rapidly, leading to significant crop loss and weakening the entire plant. Understanding the specific appearance of the spots is the first step toward effective management.
Key Fungal Diseases Causing Black Dots
The most frequent cause of black dots on grape berries is Black Rot, caused by the fungus Guignardia bidwellii. This disease first appears as small, light-brown spots that quickly enlarge and spread across the fruit. Infected grapes eventually shrivel into hard, black, raisin-like structures called mummies, which are covered in tiny black fruiting bodies called pycnidia. Black Rot causes severe crop damage, often destroying the entire cluster when the fruit is susceptible, typically from bloom to four weeks post-bloom.
Anthracnose, also known as “bird’s eye rot,” is caused by Elsinoe ampelina. On the fruit, this disease creates distinctive, sunken spots that start as small, reddish circles. As the lesion expands, the center turns grayish-white, while the edge retains a reddish-brown to black margin, creating the characteristic bird’s eye appearance. These lesions can cause the fruit skin to crack, making the berries shrivel and mummify before reaching full size.
Phomopsis Cane and Leaf Spot, caused by Phomopsis viticola, is a third possibility, though fruit symptoms are often less distinct. The disease generally appears late in the season, after the berries begin to ripen. It causes light brown spots that enlarge and become dark brown, often starting where the berry attaches to the pedicel. Phomopsis can also lead to mummification, resulting in dark, shriveled berries.
Differentiating Symptoms on Foliage and Fruit
A thorough inspection of the entire vine is necessary to diagnose the specific fungal infection, as each disease presents unique symptoms beyond the berry. Black Rot lesions on leaves are circular, reddish-brown spots with a darker border. Small black pycnidia often form in a ring just inside the border of the spot, and this leaf symptom usually precedes fruit infection by one to two weeks.
Anthracnose can be identified on leaves by spots with grayish centers and dark, irregular margins that often dry out and fall away, creating a “shot-hole” effect. On the canes and shoots, Anthracnose forms deep, sunken, purplish-brown lesions with gray centers and raised edges, which can cause the shoot to crack and die back. Phomopsis primarily affects the basal nodes of young shoots, forming elongated, dark brown to black specks or streaks near the base of the cane.
The timing of infection provides a strong diagnostic clue. Black Rot infects the fruit early in the season, leading rapidly to mummification. Phomopsis, however, is often latent in the fruit until later in the season, with visible fruit rot appearing closer to harvest time, often after veraison (the onset of ripening). Recognizing these differences in location, shape, and timing is key to determining the correct pathogen.
Immediate Management and Control Measures
Once a fungal infection is identified, immediate sanitation measures are necessary to reduce the spore load on the vine. This requires the prompt removal of all infected plant material, including shriveled, mummified berries hanging on the vine or lying on the ground. Infected canes and shoots should be pruned out and destroyed, as the fungi overwinter in this dead wood, serving as the primary source of infection for the following season.
Improving air circulation around the vine is a practical, non-chemical control method that limits the conditions fungi need to thrive. Pruning the canopy to remove excess foliage and ensuring proper trellis management allows the leaves and fruit to dry quickly after rain or morning dew. For active disease management, home gardeners should apply a fungicide program, with applications occurring from when new shoots are about one to five inches long through four weeks after bloom.
A copper-based fungicide, such as Bordeaux mixture or a fixed copper product, is a common and effective broad-spectrum choice for home use, targeting all three diseases. These protective sprays must be applied before a rain event to prevent spores from germinating and infecting the new growth. During periods of heavy or frequent rain, the application interval may need to be shortened from the standard 10-14 days to seven days to maintain a continuous protective barrier on the susceptible tissues.
Consumption Safety and Long-Term Prevention
Grapes with black dots caused by Black Rot, Anthracnose, or Phomopsis are safe for human consumption, as these fungi do not produce toxins harmful to people. However, the affected berries are unappetizing due to their cracked skin, sunken lesions, or shriveled texture. It is best practice to discard any fruit showing significant signs of rot or fungal growth, as their quality and taste are compromised.
Long-term prevention focuses on cultural practices that break the disease cycle and reduce the need for constant fungicide application. A preventative step is a dormant spray of lime sulfur or Bordeaux mixture, applied in late winter or early spring just before bud break. This application targets the overwintering fungal structures on the dormant wood, significantly reducing the initial source of spores for the growing season.
Choosing grape varieties with known resistance to these common fungal pathogens is the most effective long-term strategy for minimizing disease pressure. Proper vine training, such as using a high-wire trellis, also helps by increasing air flow and keeping the developing fruit off the ground. Consistent annual pruning and cleaning up all plant debris each fall are fundamental to maintaining a healthy environment and preventing recurrence.