Fruit Update -
SW Michigan Fruit Update -
SW Michigan
Phomopsis is Competing for Your Grapes.
Annemiek Schilder
Department of Botany and Plant Pathology
Michigan State University
Phomopsis
cane and leaf spot is caused by the fungus Phomopsis viticola. It is a disease of
increasing concern in Michigan. It affects both juice and wine grapes. Some examples of
susceptible cultivars are Niagara, Concord, Vignoles, Chancellor, Chardonnay, and Cabernet
Sauvignon.
Phomopsis viticola overwinters in diseased canes and pruning stubs. Spores ooze out from fruiting bodies on the canes during wet weather and are dispersed by raindrops. In a 2000 study in SW Michigan, most of the spores were released during rain events in May and June. No spores were detected after bunch closing (mid July). The reason that late-season infections are uncommon may be that fungus spores are absent at that time. Last spring was exceptionally wet. The study will be repeated to confirm the results.
Only young actively growing tissue is infected. While cane and leaf lesions are common, it is infections of the rachis and berries that cause economic losses through fruit loss. The rachis can be infected from the time the flower cluster first becomes exposed (about 6-8" shoot growth). Fruit infections mostly take place between bloom and bunch closing. But research in Ohio has shown that the rachis and berry are actually susceptible throughout their development.
The first
Phomopsis symptoms are usually small brown spots with yellow halos on expanding leaves.
The leaves may have a wrinkled appearance around the spots. Later, brown or black lesions
on shoots, petioles, and rachises appear. As lesions enlarge, infected tissues become
brittle and break easily. Berry infection is usually first noticed several weeks before
harvest. But the infections may have taken place much earlier. On fruit, the fungus causes
latent infections. The fungus doesn't actively grow in the fruit until it starts to ripen.
It may take 3 to 4 weeks after infection for symptoms to become visible. The fungus can
infect the berry directly or through the berry stem. The berries shrivel, turn brown and
rubbery (not hard like black rot), or may just shrivel up without turning brown because
the berry stem is killed by Phomopsis. Infected or shriveled berries drop when touched or
when the vine is shaken during harvesting.
Since the fungus overwinters in diseased canes, rigorous pruning can help to reduce overwintering inoculum. Be sure to remove the pruned material from the vineyard and destroy it by burning or burying. Fungicide protection of the rachis and developing berries is crucial for control of this disease. In addition, protection of the canes can reduce cane infection and carry-over of disease inoculum from year to year. Effective protectant fungicides are mancozeb and captan. The use of these materials is restricted on juice grapes. Preliminary findings from recent fungicide trials indicate that the strobilurin fungicides (Abound, Flint and Sovran) and ziram are also effective against Phomopsis. Based on available information, a sensible for approach for control of this disease would be use to mancozeb up to bloom, then switch to strobilurins or ziram from bloom to bunch closing. Further studies will be done to optimize the timing of fungicide applications for Phomopsis control.
