Grapevine Leaf Spots

Disease Identification

• Grapevines are susceptible to the phomopsis leaf and cane spot disease caused by the Phomopsis viticola fungus. The disease is common in all the grape growing regions. The infection was previously believed to be part of the dead-arm disease of grapes. It was during the mid-1970s that the disease was classified as a separate disorder. If not controlled in time, the disease can caused significant loss in yield.

Symptoms

• The disease starts with the appearance of light green, 1/16-inch diameter spots on foliage and new shoots. The internodes on new shoots develop small, black spots that gradually enlarge to create irregularly shaped patches. In later stages, affected shoots start to split and the lesions spread to the canes and the fruit. Cane lesions are the primary overwintering site for fungal spores. Affected foliage starts to distort and die.

Damage

• The damage caused to the foliage reduces the level of photosynthesis in plants and affects overall plant health. Lesions on canes weaken the grapevine and lower resistance to winter injury. When the fungus spreads to the fruit, fruit yield is affected and there is premature fruit drop. Fungal spores persist in the infected live or dead canes for more than a season and infect plants again the next season. Often seemingly healthy shoots without any outward symptoms of the disorder produce the disease causing spores or pycnidia in the dormant plant state.

Management

• Plant the grapevine in a well ventilated and sunny area to reduce incidence of disease. Thin the plant regularly to let light into the inner areas. Prune and remove all infected canes during the dormant season and leave only the healthy, stronger canes. Apply preventative fungicides early in the season. Recommended fungicides include azoxystrobin, kresoxim-methyl, mancozeb or ziram.