Phomopsis viticola, is a fungus that overwinters in last year’s cane growth and rachises. Fruiting bodies of the fungus (pycnidia) produce many spores in the spring that are splashed to developing shoots during rain events. It can impact canes, shoots, leaves, rachises and berries, causing leaves, berries and clusters to drop to the ground prior to maturity.
Small light green lesions on leaves that turn black with yellow edges. Heavy infection causes the leaf to crinkle and fall off. Typically only the lower leaves and cane nearest the cordon are infected. Sunken black lessions also become obvious on the shoots, petiole and rachis. Berries below the damaged rachis can shrivel. Near harvest, infected berries develop brown discolorations and are easily detached from their pedicels.
The disease thrives on cool, wet conditions with optimal infection occuring after six hours of leaf wetness during temperatures between 60° – 68°F. Berry infection can occur throughout the growing season; however, most fruit rot infections occur early in the season (pre-bloom to two to four weeks after bloom). Once inside green tissues of the berry, the fungus becomes inactive until late in the season when the fruit matures. Thus, fruit rot that develops at harvest may be due to infections that occurred during bloom.
Early season prevention sprays are critical. During dormancy, infected cane can become bleached white and the many black fruiting bodies become visable. Remove infected cane during annual pruning. If there was a phomopsis problem the previous year, a dormant (prebud) lime/sulfur spray is recommended for optimal control.