Cultural Control of Phomopsis Cane and Leaf Spot

The overall goal of this project is to evaluate the potential for control of Phomopsis cane and leaf spot caused by Phomopsis viticola using cultural methods to reduce initial inoculum. Phomopsis is cane and leaf spot is a regular problem in grape production in certain areas of California and can be a widespread problem when disease is accentuated by spring rains. A standard control measure in many areas was a dormant spray of dinoseb, however registration was lost last year. The only other dormant spray available is sodium arsenite, an extremely toxic compound that many wineries and packing sheds will not allow the growers to use. The first and second objectives were to determine the relationship of the intensity of overwintering infections and disease severity and the potential of reducing disease by reducing the number of pycnidia through careful pruning. The source of primary inoculum which initiates the epidemic is from mature pycnidia which form on second year wood. Two vineyards in the Russian River Valley and two in the Lodi area where Phomopsis is a consistent and severe problem were surveyed for the incidence of infected second year wood both before and after pruning. Three treatments were established in one vineyard. First, all of the diseased spurs were left, next 1/2 of the disease spurred were removed, and third, all of the diseased spurs were removed. This was replicated 4 times in a 5 vine by 5 vine blocks to minimize interplot interference. At two weeks prebloom, bloom, and two weeks post bloom, the number of leaves with lesions and the number of lesions per leaf were examined. No disease was observed in any of the plots. Although we could readily isolate Phomopsis from diseased spurs in the laboratory, we concluded that the drought had reduced the inoculum to such low levels that disease was no longer present. These experiments were repeated the next year with the same results. Therefor, as mentioned by phone and letter, we are not continuing the request for funding at this time, but will continue to monitor for the disease.