Wisconsin Grape Harvest: Fruit and Cluster Rots
As harvest is underway in Wisconsin vineyards many may be observing (or smelling) some rotted fruits on the vine. This article provides a brief overview of some of the fruit and cluster rots you may observe this season. Some rot-causing fungi infect early in the season during bloom while other fungi infect wounds made by mechanical damage such as rain (i.e., berry splitting) or are vectored by various flying insects.

Black Rot. Last year I received many reports of black rot in Wisconsin grapes. It is one of the most devasting diseases of grapes in humid climates. The pathogen (Guignardia bidwellii) favors humid, wet weather and warm temperatures. Several cold-climate grape varieties are severely to highly susceptible to this disease including Valiant, Marquette, Frontenac, Frontenac gris, and Petite Pearl. Fortunately, this disease can be managed with timely fungicide applications. You may have noticed foliar infections (ex. tan lesions on leaves) early in the season as spore release for this pathogen begins as early as budbreak and continues through midsummer, or even through harvest in some seasons. Fruit infections first appear as tan spots on green fruits then quickly expand until the entire berry is rotted. If you are seeing significant black rot in your fruits as you harvest this season, one or more of the following may be a factor:
- Missed critical fungicide application period for controlling black rot. Black rot fungal spores disperse early in the growing seasons, therefore preventative fungicide sprays beginning at budbreak and continuing through 1-2 weeks post-bloom (if you continue to observe black rot, a second post-bloom application may be warranted). Consult NEWA for predicted black rot risk periods in your area. With the warm day and night temperatures we experienced this summer in the vineyards at the West Madison Ag. Research Station, the progression of bloom was rapid, and we struggled to perfectly time our critical applications for i.) immediate pre-bloom, ii.) bloom, and iii.) 1-2 weeks post-bloom.
- Fungicide applications were made at critical periods but due to conducive conditions (wet/humid + warm temperatures) additional sprays were needed to reduce further spread. A helpful way to see if weather conditions are conducive for black rot is to consult NEWA grape disease models.
- Winter sanitation practices were not performed prior to bud break. Sanitation practices that remove the fungus from the vineyard (i.e., remove mummified fruits, prune out infected canes) will reduce the overall source of inoculum for black rot. This is critical and will aid in the success of your fungicide spray program.
- Chemistries used did not target the black rot pathogen. Excellent control of black rot can be achieved with the multisite fungicides mancozeb and ziram early in the season. Fungicides in FRAC Group 3 are also excellent control options for black rot during the early season critical application periods. In organic vineyards, there are limited options. While copper is effective it must be re-applied frequently to achieve good control, and some cold-climate varieties are sensitive (i.e., phytotoxicity) to copper applications.
- Many cold-climate varieties are very vigorous vines with quick and densely growing shoots. If summer thinning practices were not performed this could create a microclimate of warmth and humidity that the fungus can thrive in. Appropriate shoot thinning will allow more air circulation in the canopy and enhance the penetration of fungicide sprays in the canopy.
- High disease pressure due to pathogen presence in your vineyard + highly susceptible grape varieties + conducive environmental conditions.

Phomopsis Fruit Rot. We rarely observed Phomopsis fruit rot infections in the West Madison vineyards this season. I will attribute this to our sanitation program prior to bud break (i.e., removal of canes with Phomopsis fruiting bodies). Dormant applications of lime sulfur are also known to reduce Phomopsis overwintering inoculum in canes. Like black rot infections, Phomopsis spores disperse between bud break and bloom. Usually, the first fungicide spray of the season will target Phomopsis spore dispersal. These spores can infect leaves, shoots, and flowers. Infections that occur in flowers remain inactive (latent) in developing fruits until fruit ripen. This is unlike black rot; symptoms of black rot show up before veraison and symptoms of Phomopsis fruit rot show up during veraison. Fungicide sprays are critical early in the season for Phomopsis fruit rot prevention; once fruit rot symptoms appear fungicides will not manage established infections.

Botrytis Bunch Rot. Sometimes referred to as Gray Mold, Botrytis cinerea, likes humid and stagnant air. It appears as a gray, velvet-like fungal growth on fruits. Utilizing practices such as shoot thinning that allow for good air circulation in the canopy are important for Botrytis control. Berry injury significantly increases susceptibility to Botrytis infections. Injury can result from rain (i.e., berry splitting), insect damage, and even scars on fruits. Botrytis most commonly targets ripening fruit. Tight clustered varieties tend to deal with Botrytis infections more often than those with loose clusters.

Sour Rot. Over the last few years in Wisconsin sour rot has proven to be a challenge as fruits begin to ripen. Sour rot involves a combination of 4 elements: wounded grapes + yeast + acetic acid bacteria + fruit flies. Berry touching and warmer temperatures are conducive to sour rot development. We have observed sour rot in some varieties at the West Madison vineyards. We have also observed flying insects (ex. wasps) on sour rot-infected fruits. If you can smell sour rot in your grapes, this may indicate that the infection is advanced and control options may be limited. Control practices involve managing both the pathogens and fruit flies.
I have not received many reports of Ripe Rot or Bitter Rot in Wisconsin grapes. These fungal diseases infect mature fruits and form black fruiting bodies on fruits. Some fungicides used to control other grape diseases are also effective against ripe and bitter rots.
To prevent fruit and cluster rots from spreading to healthy fruits as harvest operations are ongoing consider the following:
- Remove infected fruits and/or severely infected clusters. This is not always feasible on the commercial scale but in smaller vineyards this may be a valuable option.
- Modify harvest dates to minimize possible losses. If leaving fruits on the vine longer will result in increased spread and/or if conducive environmental conditions are predicted which will worsen the situation, changing harvest dates may be considered.
- Minimize injury to fruits as this can provide an entry point for pathogen infection.
- Chemical control is often very limited at this point in the season due to pre-harvest intervals (PHIs). There are some fungicides with short PHIs (i.e., 0 or 1 day) that may offer control depending on the disease you need to manage. Consult the Midwest Fruit Pest Management Guide or the Organic Production and IPM Guide for Grape for recommendations.
****Mention of a product is not an endorsement.***
This article was posted in Disease, Grapes and tagged Black Rot, botrytis bunch rot, Fruit Rot, grape harvest, Grapes, Leslie Holland, Phomopsis, sour rot.