Localized brown or necrotic spots on green grapes are a common phenomenon that often concerns consumers and growers. These discolorations can range from minor cosmetic blemishes to indicators of serious plant disease, affecting the fruit’s marketability and health. Understanding the differences in these spots is the first step toward diagnosis and management. The primary origins of these markings fall into two major categories: infection by microscopic pathogens or damage from environmental and physical stress factors.
Pathogens Causing Brown Spots
The most destructive causes of brown spots are fungal diseases, which thrive in moist conditions and are often identified by the pattern of the lesion they create. Black Rot, caused by the fungus Guignardia bidwellii, is one of the most serious diseases, initially appearing as small, whitish spots on the green berry that rapidly develop a distinct, widening brown ring. This spot quickly expands until the entire berry turns brown, shriveling into a hard, black “mummy” that remains attached to the cluster. The fungus overwinters in these mummified fruit, providing a source of infection for the following season.
Anthracnose, or bird’s eye rot, is another significant fungal culprit, caused by Elsinoe ampelina, which is favored by warm, wet weather. On the green fruit, this disease creates small, circular, reddish spots that become sunken with dark, reddish-brown margins and ashy gray centers, giving the characteristic “bird’s eye” appearance. Phomopsis Cane and Leaf Spot, caused by Phomopsis viticola, can also infect the berries, often becoming visible near harvest when the fruit turns light brown and shrivels. Unlike black rot, the symptoms of Phomopsis on the fruit often include tiny, raised black pimple-like structures on the berry surface.
Environmental and Physical Stressors
Not all brown spots signal an infection; many result from external damage or physiological disorders. Sunburn is a frequent non-pathogenic cause, manifesting as a white, yellow, or brown blotch on the side of the grape cluster directly exposed to intense afternoon sunlight. This damage often appears dry and leathery and is most common on the south or west-facing sides of the vine. Sunburn is a physiological disorder resulting from excessive light, high temperature, and low humidity.
Physical injuries, such as those inflicted by hail or wind, also lead to necrotic brown spots. Hail damage creates distinct wounds, sometimes appearing as cuts or sunken areas where the skin was broken, which then turn brown as the tissue dies. These random, localized injuries lack the precise, concentric patterns of fungal diseases. Nutritional stress can also cause browning or necrosis, particularly bunch stem necrosis (BSN), which affects the stem connecting the berries to the vine. This physiological disorder causes the stem tissue to die and turn brown, restricting nutrient flow and leading to the dehydration and browning of the attached berries.
Safety and Edibility Assessment
A primary concern for anyone encountering brown-spotted grapes is whether they remain safe to consume. Grapes with spots caused by physical damage, like minor hail strikes or sunburn, are considered safe to eat, though the damaged area may be dry, tough, or have an unpleasant texture. The spot is merely a localized necrosis of the fruit skin, and the rest of the berry is typically unaffected by this superficial damage. If the brown spot is due to a fungal disease, the safety assessment changes based on the extent of the infection. While the fungi themselves are generally not considered human pathogens, the fruit should be discarded if the spot is widespread, the berry is soft, shriveled, or shows signs of mold growth. Fungi can sometimes produce mycotoxins, making it prudent to discard any fruit that exhibits deep or pervasive decay, a foul smell, or has gone completely mushy.
Prevention and Care Practices
Mitigating the risk of brown spots requires a combination of sanitation and careful environmental management. Cultural control practices are foundational for preventing fungal diseases, starting with the removal of overwintering sources of infection. This involves carefully pruning out and destroying any mummified berries, infected tendrils, or cankered wood during the dormant season. Improving air circulation within the canopy is equally important, achieved through proper pruning, training, and spacing of the vines. To combat environmental stressors, growers should manage the canopy to prevent direct sun exposure to the fruit. Strategic leaf removal should be done early in the season so that the fruit gradually acclimates to sunlight, avoiding sudden exposure that causes sunburn. For severe disease pressure, protective fungicide sprays are necessary. Addressing nutritional imbalances, such as applying foliar magnesium sprays, can also help reduce physiological disorders like bunch stem necrosis.