Article written by Madeline Wimmer, UMN Extension Fruit Production Educator.
Images: Black rot is a disease caused by the fungal pathogen Guignardia bidwellii, and can affect grape leaves (left image), but is most devastating when it infects grape clusters (right image).
Black rot is a fungal disease caused by the pathogen Guignardia bidwellii. Infections can affect grape leaves and shoots during early parts of the season, which can lead to fruit infections later on.
Life cycle
The pathogen overwinters in fungal fruiting bodies (located on mummified fruit, tendrils, and canes) that release spores during rain events in the spring. Black rot infections tend to occur faster with prolonged leaf wetness when temperatures are between 65-80 F. Shoots, petioles, rachises, leaves, and fruits are all susceptible to infection to some degree. Leaf infections start as light cream or light yellow spots that eventually turn reddish-brown (necrotic) and are circular in appearance.
Images: Two different infected clusters with berries in various stages of black rot development.
When berries are infected, a light spot may form and either part of the berry, or the whole berry will turn purple. Berries sometimes dimple or have visible ripples and eventually shrivel as the season progresses.
Infections do not always impact the whole cluster. Symptoms tend to become more apparent on berries as they enlarge and pass through their second growth stage, although grape berries develop resistance to infections about 4-8 weeks after bloom, depending on the grape cultivated variety. This is why strategic, early season management is so important for black rot control, and controlling other grape diseases that spread during the spring.
Impacts
Black rot can negatively impact grape yields when clusters are infected, and lead to increased disease pressure the following year. Clusters with mummy berries are generally excluded from winemaking, however, there is little research published that assesses the impacts on wine aroma, flavor, or fermentation process.
Management
Because the fungal pathogen that causes black rot overwinters on infected clusters, removing mummified fruits from the vineyard can be an important management tactic for vineyards with a known outbreak. During the growing season, vineyards that are smaller, or have less extensive infections can also remove infected leaves and berries to reduce the number of current season spores (conidia).
Continuing a stricter fungicide program during the infection season and the following year will also be helpful to preventing future outbreaks. Generally, spray programs developed to manage Phomopsis will also manage black rot— one done during pre-bloom, and two more applied post bloom (season dependent). During years with frequent rain occurrences like we’ve had this year, long lasting spray coverage can be challenging, and outbreaks are more likely to occur.
While certain fungicides have some kickback action, they are not curative, and do not eliminate the problem of existing inoculum in the vineyard. Black rot management improves greatly when infected tissues are removed in addition to chemical management. For more information related to managing black rot in grapes, refer to the Midwest Fruit Pest Management guide grape section, starting on page 155.
References and further reading:
Black rot (Cornell Cooperative Extension)
Banning black rot and Phomopsis from young grape clusters (MSU Extension)
Comments
Post a Comment