Symptoms of grape diseases have shown up in the vineyards at the West Madison Agricultural Research Station (WMARS). Due to the recent drought conditions, disease pressure has remained low, but with cooler temperatures and possible rain events in the forecast, conditions may become more conducive for several important fungal pathogens.

Thanks to Emma Nelson for scouting the vineyards!

Phomopsis Cane & Leaf Spot

Symptoms of Phomopsis cane and leaf spot have appeared in several varieties at the WMARS vineyards. Small, yellow spots with black centers (Figure 1), and black lesions on canes (Figure 2) are diagnostic symptoms of Phomopsis infections. Inoculum comes from fruiting bodies that overwinter on canes or pruning stubs (Figure 3). The critical period to control Phomopsis is pre-bloom when spores are abundant. After bloom, the risk for Phomopsis infection is significantly reduced. Phomopsis symptoms can also show up on ripening fruits, and while these symptoms appear later in the season, the infections that cause Phomopsis fruit rot occur during the pre-bloom period. Fruit infections remain latent (i.e. not showing symptoms) until berries begin to ripen.

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Black Rot

Black rot lesions (Figures 4 and 5) have been observed occasionally at the WMARS vineyards. The best way to prevent black rot infections is to remove mummies that overwinter on the vine. Fungicide applications for black rot control during the growing season extend 4-5 weeks following bloom, as tissues remain susceptible.

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Downy Mildew

Symptoms of downy mildew have been observed in several varieties at the WMARS vineyards. Infections first appear as oil-soaked yellow spots (Figure 6) then develop into necrotic lesions. The downy mildew pathogen sporulates on the underside of leaves (whereas powdery mildew sporulates on the upper surface of leaves). Sporulation of downy mildew may not always be observed in the field.

Powdery Mildew

Foliar powdery mildew has rarely been observed in the WMARS vineyards so far this season. In vineyards with a history of powdery mildew, well-timed fungicide applications are critical for season long control.

For current fungicide recommendations consult the 2021-2022 Midwest Fruit Pest Management Guide. For organic growers, consult the 2016 Organic Production and IPM Guide for Grapes.

This article was posted in Disease, Grapes and tagged Black Rot, disease, Downy mildew, Grapes, leaf spot, Leslie Holland, Phomopsis, phomopsis cane, Powdery mildew.