Jun 4, 2015
Protect grape clusters from diseases during early fruit development

Young fruit clusters are highly susceptible to all major diseases, including downy mildew, powdery mildew, black rot, Phomopsisand anthracnose. If prolonged cool, wet weather prevails during bloom, Botrytis can also gain a foothold in clusters of susceptible cultivars by promoting fungal growth on senescent flower parts. However, if conditions are dry and warm during bloom, it is unlikely bloom will be an important time for Botrytis infection. Black rot and Phomopsis lesions have been seen in the last couple of weeks and indicate the pathogens are active. Grape anthracnose symptoms are also visible on shoots, leaves and cluster stems of susceptible varieties. Powdery mildew has not yet been reported, but with frequent wetting of grapevine bark, the fungus is likely releasing ascospores on a continuing basis. Downy mildew symptoms have not been seen either, but this disease is expected to get an early start this year as well due to frequent rains and suitable temperatures over the past month. Michigan State University Extension advises careful scouting for disease symptoms on a weekly basis.

It is possible for fruit clusters to be infected by powdery mildew without seeing any foliar infections first, so protect fruit of susceptible cultivars even if no powdery mildew has been seen on the leaves. Remember, downy mildew infections of flower clusters may occur before leaf infections as well, particularly in the cultivar Chancellor, whose clusters are highly susceptible to downy mildew. In 2009, we first observed downy mildew on Chancellor clusters in Fennville, Michigan, during the first week of June and in 2010 during the second week of June. Growers are strongly advised to protect flower and fruit clusters from infection by all grape pathogens using effective fungicides. The risk of infection is especially high if we have multiple rain events and moderate temperatures, resulting in prolonged wetting of foliage and developing fruit. Only the powdery mildew fungus does not need wetness, but it thrives in shady areas under high relative humidity.

In general, aim to protect clusters from the major diseases from immediate pre-bloom until four to five weeks after bloom. As the berries develop, they become naturally resistant to most diseases and the need for fungicide protection diminishes. This happens quite rapidly for downy mildew (two to three weeks after bloom), whereas for powdery mildew it is about four weeks after bloom. Concord grapes become resistant to black rot at four to five weeks after bloom, but some wine grape varieties may remain susceptible to black rot for up to eight weeks after bloom. Be aware that the cluster stem (rachis) and berry stems can remain susceptible longer than the berries in most cases. The only disease in which berries remain susceptible throughout their development is Phomopsis, but the risk of infection diminishes after bunch closure because spore numbers drop off then. In the case of Botrytis, berries actually become more susceptible closer to harvest, especially in tight-clustered varieties. However, Botrytis may gain a foothold in clusters at the end of bloom by growing on senescing flower parts during cool, wet weather and then hangs out until becoming active again after veraison.

Depending on the susceptibility of the grape cultivars you are growing, broad-spectrum fungicides or fungicide combinations are most appropriate at this time of the growing season to get the broadest control. A good option would be a tank-mix of a sterol inhibitor (e.g., Rally, Tebuzol) plus a protectant (e.g., Manzate, Ziram). In addition, broad-spectrum fungicides such as Pristine, Abound and Sovran or pre-mixes such as Revus Top and Quadris Top are useful at this time. Pre-mixes Luna Experience and Inspire Super are good options for wine grapes where efficacy against Botrytis is also desired, however, they do not work against downy mildew, so a downy mildew fungicide may need to be added (e.g., Phostrol, Presidio, Revus, Forum, etc.).

For organic vineyards, a tank-mix of a protectant biocontrol agent (e.g., Double Nickel 55, Serenade) plus a plant defense booster (Regalia – giant knotweed extract) is the best option. For grapes that can tolerate these products, sulfur is a good powdery mildew fungicide and copper will protect vines against downy mildew in addition to suppressing other fungal diseases.

By Annemiek Schilder, Michigan State University Extension, Department of Plant, Soil and Microbial Sciences

75 Applewood Drive, Suite A
P.O. Box 128
Sparta, MI 49345
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