While walking through your vineyard, you might notice yellow spotting on leaves (Figure 1) and we’ve begun to see it in the vineyards at the West Madison Agricultural Research Station. This yellow spotting is commonly observed on newly expanding leaves. These yellow spots will develop a black spot in the center, a diagnostic symptom of Phomopsis leaf spot (Phomopsis viticola). Phomopsis cane and leaf spot is an early season disease. The pathogen overwinters in the vines, and sporulates and spreads with early season wetting events.

If the weather following bud break was rainy, and vines stayed wet for a prolonged period you may see Phomopsis cane and leaf spot infections in your vineyard. In Wisconsin, we are also prone to fruit and rachis infections as we maintain high humidity throughout most of the growing season. Even though fruit symptoms are not visible until much later in the season, the infection period happens early in the season. Interspecific grape hybrids (like those grown in Wisconsin!) vary in susceptibility and Vitis riparia and V. labrusca cultivars are susceptible.

Symptoms of infection in the early season show up as lesions on leaves and shoots. Lesions on shoots typically develop on basal internodes and appear black and elongated. Cracks may develop in these lesions and consequently weaken the shoot. Phomopsis can also cause fruit infections, however, symptoms do not appear until pre-harvest. Phomopsis fruit infections may be confused with other fruit-infecting fungi such as the black rot pathogen. Despite this similarity, black rot symptoms typically show up on green fruits before veraison, and Phomopsis develops after veraison.

Fungal infections that occur this season can lead to infections in future seasons if not properly managed. The fungus can continue to produce spores years after infection occurs in the overwintering fungal fruiting bodies. Diseased tissue is usually bleached or greyish with small, black fruiting bodies of the fungus (Figure 2). Even after wood dies the fungus can continue to produce spores and release them in humid or wet weather. Spores are rain-splashed onto newly developing tissues in the spring (shoot tissues become relatively resistant as they mature). The spores do not travel far, and infections may appear localized on some vines where there is abundant inoculum.

In the north central region, most inoculum (i.e., spores) is dispersed between bud break and bloom. Secondary infections following this period of spore dispersal is rare even if rainy or wet conditions persist. Therefore, management efforts for Phomopsis should focus on the period from bud break to bloom in the vineyard. Lesions develop on the leaves and shoots 3-4 weeks following infections; usually 1 month following infection for the rachis but symptoms are most distinct during the pre-harvest period. Fruit infections are asymptomatic (latent) until veraison. Infections that occur early during cluster development are often the most damaging.

Management of Phomopsis should begin before bud break by removing infected canes (Figure 2) during dormant pruning and sanitation practices to reduce overall inoculum in the vineyard. Significant Phomopsis outbreaks are often a result of high inoculum levels in the vineyard and conducive (i.e., wet) weather conditions. Sanitation practices can improve the efficacy of your fungicide sprays by reducing fungal inoculum. Early season fungicide options such as mancozeb, captan, and ziram are good control options for Phomopsis. Check the 2021-2022 Midwest Fruit Pest Management Guide or the 2022 Organic Production and IPM Guide for Grapes for fungicide efficacy information. To determine predicted Phomopsis spore release events in your area check out the NEWA prediction models for grape disease infection events.

Mention of a fungicide product is not an endorsement. The label is the law.

This article was posted in Disease, Grapes and tagged Courtney Cameron, disease, Grapes, Leslie Holland, Phomopsis.