The causes of brown spots on grapes are insect infestations, certain fungal infections, bacterial diseases, and measles. In the case of grapes, bacterial diseases are second only to fungal infections.
Grapes turn brown on the vine because they are either dehydrated or suffering from an infection. The infection may arise due to either bacteria or fungal growth. If the grape is dehydrated, then it is probably because the weather is especially hot. Shade can assist the grape and restore its health.
Decaying berries and grapes on vines are more susceptible to infections. This fungal disease of the vine can also affect berries, especially flowering ones, and cause them to dry out and look like raisins. Infected berries wilt and look more like raisins than grapes.
These areas soften, sink and rot after a few days, and what is left of the fruit dries up and turns into a tiny, hard, raisin-like fruit, a mummy covered with fungal fruiting bodies. As they spread, these lesions turn brown and sprout black fungal fruiting bodies that look like peppercorns. As the reddish brown color matures, the center turns brown and small black pimply fruiting bodies called pycnidia appear in the center.
Where Diseases Affect Grape Plants
Affected leaves develop reddish-brown round spots (lesions) on the upper surface of the leaf. The first signs of gray mold or gray mold are spots on the leaves, which turn brown and begin to rot. Gray rot or gray rot is most detrimental to berries when the fruits become covered with gray rot and begin to rot.
Infected berries turn dark brown and are covered with numerous black pycnidia on the surface. Infection of fruits occurs from the middle of flowering until the berries begin to color. If infections become numerous, protection against fruit rot at the end of the growing season becomes very difficult.
While black rot is the most common, other grape diseases such as bitter rot, ripe fruit rot and anthracnose can also cause fruit to turn black and wilt. If bunches of grapes rot before they have time to ripen, it is likely that the plant is suffering from a common and widespread fungal disease known as black rot. Watch out for wet weather Black rot affects both cultivated and wild vines, causing fruit symptoms when the berries are still green and have not yet reached full size. The fungus Guignardia bidwellii overwinters in reeds, tendrils and leaves on the vine and on the ground.
An Overview of Black Rot
Black rot (Guignardia bidwellii (Ellis)) is a potentially destructive fungal disease that attacks the leaves, buds, berries and stems of grapes. Grape black rot is a fungal disease caused by the ascomycetes Guignardia bidwellii, which infects grapevines in hot, humid weather.
Ascomycetes overwinter in many areas of the vine, especially mummy and affected areas, and are also able to overwinter on vineyard soils. Black rot is caused by the fungus Guignardia bidwelli, overwintering on infected fruit and shoots on the ground or left on vines.
Fungus spores form inside diseased fruits and infect leaves, flowers, and young fruits during spring rains. The spores overwinter in diseased leaves on the ground and are dispersed in spring with rain or irrigation water. During rain, microscopic spores (ascospores) are ejected from numerous black fruiting bodies (perithecia) and are transferred by air currents to young, growing leaves. The leaves, and later the stems, eventually wither in the middle of the growing season, and the berries eventually fall to the ground.
Visual Signs of Infected Leaves and Vines
Infected vines look depressed, leaves turn red or yellow in summer, berries wilt, and dead, dry leaves fall from the vine. Diseases that cause grapes to wilt on the vine usually start with other symptoms, including leaf scorch or dried shoots. Spots and lesions appear on young leaves before symptoms appear on berries. In many cases, the fruits are half grown before they begin to show signs of infection – the same small brown spots on the leaves begin to appear on the grapes.
Whenever you notice signs of illness, remove infected vines, leaves, or fruit clusters before the infection spreads to other healthy parts. In the meantime, remove and dispose of any diseased grapes still on the vine as well as fallen ones, and keep the area around your vines clean.
All mummified bunches should be removed from the trellis at the time of pruning and 2-6 weeks after the cap has fallen and all affected bunches should be cut off before they allow black rot to spread, so much the better (spores to spread the disease are dispersed by rain). mostly inside the canopy, so the risk of new infections should be minimal if these clusters are simply dropped to the ground).
Grapes Can Get Measles Too
Berries mummified on the ground or still clinging to vines become a major source of infection the following spring. The roots become deformed, and fungal infections appear there, blocking the supply of nutrients and water to the vines.
Measles infects grapes through areas of plant damage and rot. Two grape diseases in this category, Petri disease and Tinder (black measles), are caused by fungal pathogens. Downy mildew is a dreaded fungal infection caused by Plasmodium vinifera. Downy mildew, caused by the fungus Plasmopara viticola, is a serious grape disease that, if left untreated, can cause severe yield losses.
Erysiphe necator White causes anthracnose or bird’s eye. Gray powdery rot covers green tissues. It targets wine grape vines and the disease has been known to degrade the quality of wine. Grape anthracnose affects all green parts of the plant: vines, leaves, stems, tendrils and fruits. The fungus Elsinoe ampelina causes grape anthracnose, which is the formation of brown or black spots on fruits, brown or black spots on fruits.
While there can be many reasons for vine leaf tan, only black rot lesions will have black pycnidia within the tanned field. Symptoms include black or brown spots 400 mm to 600 mm wide on the foliage.
