Grape is attacked by a number of diseases. However, anthracnose, downy mildew, powdery mildew, and bunch rot/ berry rot caused by certain fungi or group of fungi are most important. Their control lies in modifying vine microclimate; timely annual pruning of dead arms/vines, and also removal of abnormal un-harvested bunches from the vines immediately after harvest is over, and timely and judicious application of fungicides, after annual pruning and during the season.
Anthracnose (Elsinoe ampelina, anamorph Sphaceloma ampelinum)
It causes heavy reduction in yield and quality. Spots on leaves are gray with darker margins. Severely infected leaves become distorted and curl down from the margins. Later, the center of the lesion may drop. Fruit infections appear as circular spot with light-gray centers and reddish-brown borders, quite similar to bird’s eye, hence often referred to as ‘Bird’s eye rot’. Stem lesions are similar in color, elongated, sunken, with slightly raised borders.
During annual pruning, remove the infected vines drastically; apply Bordeaux paste at cut ends and spray 0.5% Bordeaux mixture or 0.3% mancozeb or 0.3% copper oxychloride or 0.2% copper hydroxide or 0.1% carbandezim. Burn the infected /pruned vines, left over berries, if any, and leaves.
During the season, spray the vines with any of the fungicides twice or thrice. Carbendazim application should be used once only and not to be repeated. Avoid spraying during flowering.
Downy mildew (Plasmopara viticola)
It is a major disease of grape and could be more damaging under prolonged humid spell. Small, pale yellow circular lesions appear first on the upper surface of the leaf. On the underside of the lesion downy growth of the fungus appear as whitish-grayish growth. The fungus produces sporangia, which liberate zoospores into a film of water and spread the infection during prolonged rains or dew. Later, the leaf tissue dies in the area of the lesion. Often numerous lesions coalesce, causing large areas of the leaf to turn dark brown or black. Badly damaged leaves fall prematurely.
Apply fungicides Bordeaux Mixture/ Copper hydroxide/ copper oxychloride/ Ridomil MZ 0.25% or Strobilurin fungicides such as Azoxystrobin or Krezoxim methyl with first appearance of symptoms or in the event of humid and wet weather as preventive spray or once before bloom and once after bloom. Avoid repetition of spray of Ridomil MZ and or Strobilurin fungicides. In no case strobilurin should be used more than once in the season.
Powdery mildew (Uncinula necator, anamorph Oidium tuckeri)
The disease is characterized by production of white powdery or dusty growth on the upper surfaces of the leaves and other green parts of the vine. Severely affected leaves turn brown and fall. Later in the season it attacks berry clusters and young berries just after flowering. As the berries enlarge fungus growth disappears but Infected grapes become rough skinned, turn hard, remain undersized and may drop.
Spray wettable sulphur 0.2% or 0.025-0.05% dinocap with first appearance of symptoms. Avoid sulphur sprays or dusts when day temperature is >350 C. Triazole fungicides such as penconazole 0.05% or hexaconazole 0.1% can also be used but avoid their repeated spray. Use of Strobilurin has the special advantage of managing downy mildew and anthracnose as well besides powdery mildew. But apply strobilurin only once in the season in order to check resistance development
Fruit rot/ Berry rot
A number of fungi are responsible. While a few of them may be noticed while fruits are unripe, majority of them may be observed when the fruits are approaching maturity or even matured.
Anthracnose (Elsinoe ampelina):
It causes typical Bird’s eye rot while the fruits have not matured.
Botrytis bunch rot (Botrytis cinerea):
Initially fruits appear soft and watery, subsequently brown mold may be noticed on white grapes and reddish mold on shriveled purple grapes. Later the bunch gets covered with gray mycelial growth of the fungus, which is predominant in humid weather.
Aspergillus rot (Aspergillus niger):
Wasp or mechanical injury may lead to black moldy growth on injured and adjoining fruits
Rhizopus rot (Rhizopus arrhizus & R. stolonifer):
Affected bunches get sparsely covered with strands of spongiophores of the two fungi.
Berry rot may also occur due to yeasts. Most of the rots are managed by following systematic spray schedule in the vineyards.