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Scouting for grapevine diseases

Annemiek Schilder, Dept. Plant Pathology, Michigan State University
(This is a handout from Grape IPM Meeting, August 12, 2004, St. Joseph, Michigan)

Scouting for diseases is an important component of an integrated disease management program and can improve disease control efficacy and reduce unnecessary fungicide applications. Visual inspection of the foliage and fruit is the only method to scout for diseases of grapevines. Since most pathogen structures are invisible to the naked eye, we need to look for evidence of pathogen activity, i.e. disease symptoms, instead. This is best done on vines known to be susceptible to a specific disease ("indicator vines") or vineyard sites that are particularly conducive to disease development. Do not just observe the vines from the truck - get out and closely inspect the canopy.

Powdery mildew
The powdery mildew fungus overwinters in the bark of vines and infects leaves, shoots, and clusters. Since the first colonies will likely occur on leaves close to the trunk, this is an important place to look. Early colonies may not be very obvious and may consist of a single colony on a leaf. Also, inside the canopy, the humidity will be higher, which is more conducive to disease development. Berries and rachises can also get infected, often on the top of the cluster. Symptoms later in the season are a grayish white powdery layer on leaves, rachises, and clusters. Heavily infected berries will burst. A good time to start monitoring is at the 8-10" shoot stage. For cluster infections, start looking at buck-shot berry.

Downy mildew
The downy mildew fungus overwinters in leaves on the ground and the spores eventually get released into the soil. Early symptoms are "oil spots" (yellow, greasy-looking spots) on leaves or shoot tips covered with a white fluffy layer of spores. These shoots are often basal suckers that get infected as they grow through the soil or further up the vine. Infected leaves are usually on the outside of the canopy or even on the top of the canopy. Sometimes the leaves lowest to the ground are infected first. On cultivars that are highly susceptible to cluster infection (e.g., Chancellor), look at flower clusters at bloom or young fruit clusters. Any white sporulation indicates infection. A good time to start monitoring is at the 8-10" shoot stage. For cluster infections, start looking at bloom.

Black rot
The black rot fungus overwinters in infected fruit mummies on the ground or hanging in the vine. The more clusters are still hanging in the vine, the higher the risk of infection. Typical early symptoms on the leaves are small, more or less round, brown spots. These spots often have a ring of black pimples ) fruiting bodies in them. If spots occur really low on the vine, they are most likely paraquat damage and not black rot. Early symptoms on berries are creamy white spots with brown halos, which spread rapidly. Some berries are half brown, half green. Eventually they shrivel up and become dark purple to black (mummies). Symptoms can occur anywhere in the canopy and tend to be somewhat patchy. The time to start looking for leaf lesions is at about the 8-10" shoot stage, and for fruit infections at large pea-size berry.

Phomopsis
The Phomopsis fungus overwinters in 2-yr and older infected canes. First lesions on young shoots are tiny round to elongated, brown to black spots. On the leaves, tiny brown spots appear with yellow halos around them. If there are many lesions, the leaves start to crinkle. These spots can occur as early as the 4-5 leaf stage and are most common under old cane stubs. On the cluster rachis, small lesions can become visible before bloom. These lesions later expand and can consume the whole rachis, but not usually until after veraison. Berry infections (brown, shriveling berries) appear several weeks before harvest. The best time to start scouting is from 3-5" inches of shoot growth.

Botrytis bunch rot
The Botrytis fungus overwinters on infected canes and old fruit remnants on the ground. The spores can also come from dead leaves and flowers on other plants, so the spores tend to be widespread in the environment. The first symptoms may be blighted flowers or light-brown spreading lesions on leaves. A grayish tan layer of spores may be present on dead tissues. Green berries can also get infected from infected flower debris and turn brown and eventually shrivel up. More likely, the disease manifests itself at veraison or several weeks before harvest as a rapidly spreading rot of berries with a grayish-tan spore masses. This is particularly apparent in tightly-clustered varieties. The best time to start scouting for Botrytis is at bloom and then again at veraison.

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Funding for this web site provided by Project GREEEN, American Farmland Trust, EPA Region 5's Strategic Agricultural Initiative program, The National Foundation for IPM Education, the Center for Agricultural Partnerships and the MSU Integrated Pest Management Program in collaboration with MSU Extension and the Michigan Agricultural Experiment Station. Partially support from NC-IPM Center.

05/24/11 Contact: E. Haney