Young grape clusters are susceptible to attack from fungal diseases

Protect grape clusters from all major diseases at this time.

Young grape clusters are highly susceptible to all major diseases, including downy mildew, powdery mildew, black rot, Phomopsis and anthracnose. Luckily, weather conditions have been warm and mostly dry in recent weeks and activity of fungal pathogens has been suppressed. Dew also has been rare due to relatively warm nights. With a lack of prolonged leaf wetness, most diseases will be suppressed except for powdery mildew which thrives in the absence of rain and is promoted by high relative humidity and shading in the grape canopy. However, if there is a leaf wetting period for 6 hours or more in the next few weeks, infection periods for black rot, Phomopsis, anthracnose and downy mildew are expected. In our experience, a relatively dry spring and early summer will prolong the availability of inoculum for these diseases and they will sneak in infections when they can.

Black rot and Phomopsis lesions have been seen in the last three to five weeks and indicate that 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, whereas downy mildew symptoms have been seen at low levels in unsprayed Chancellor vines and are picking up on wild grapevines. Botrytis is unlikely to be active during dry, warm conditions. Michigan State University Extension continues to advise 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 the fruit of susceptible cultivars even if no powdery mildew has been seen on the leaves. In addition, 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. 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.

In general, aim to protect the 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.

Depending on the susceptibility of the grape cultivars that 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. The 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) plus a plant defense booster (Regalia- giant knotweed extract) is a good option. For grape cultivars 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. Oils and salts are best used as eradicants for exisiting powdery mildew lesions. Be careful using sulfur, oils, salts or phosphites during hot, dry conditions as these may cause leaf burning, especially on stressed vines. Also it may be advisable to avoid spraying grapevines during full bloom, as we have noticed yield reductions in juice grapes sprayed with fungicides at bloom.

Dr. Schilder’s work is funded in part by MSU’s AgBioResearch.

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