Black rot is one of the most damaging fungal diseases of grapes in Ohio, especially during warm and humid weather. Black rot can result in 5-80% loss if it is not controlled. All cultivated grape varieties and all above ground tissue are susceptible to black rot. Symptoms are most often seen on young leaves and fruit first. The fungus that causes black rot is Guignardia bidwellii and it overwinters as pycnidia in mummified berries still attached to the vine, or on the ground and within cane lesions.
Signs and Symptoms
Lesion with pycnidia
Canker with pycnidia
Black Rot on berries
Circular or irregularly shaped spots (lesions) first appear on the upper surface of leaves (image on the right, Photo courtesy- The Ohio State University). Spots are two to ten millimeters in diameter and brownish red in color with dark brown edges. Within the lesions, small black fungal structures called pycnidia can be seen with the naked eye (can be seen in the image on the right). Leaf lesions are not confined within the veins on leaves.
Shoot infections appear as elongated oval shaped black lesions (cankers) that are 1mm to 2 cm long (image on the right, Photo courtesy- Virginia Grape Disease Updates). Similar to leaves, shoot cankers contain small black fungal structures called pycnidia. Cankers can also develop on pedicels and tendrils.
Fruit symptoms are not usually seen until the berries are about halfway developed. Initially, spots are round, slightly sunken and soft to the touch (image on the right, Photo courtesy- Michigan State University). Spots expand quickly rotting the entire fruit. Early black rot symptoms can be confused with sour rot. Diseased berries shrivel into hard blue-black, wrinkled mummies, and remain firmly attached to the pedicel. Not all of the fruit on a cluster will become diseased. Similar to leaves and shoot cankers, mummies contain small black fungal structures called pycnidia.
Black Rot Disease Management
Currently, there are no cultivated grape varieties with resistance to black rot fungus.
Plant grapes in sunny areas that allow for good air movement. Maintain a healthy plant by following good growing practice, more information can be found at Midwest Grape Production Guide.
Sanitation is a critical component of an integrated program to manage black rot disease of grape, especially for organic and home garden growers.
- Clusters with mummies should be removed and destroyed.
- Diseased tendrils from the wires should be removed and destroyed.
- Select fruiting canes without cankers.
- Within reason, remove leaves with black rot spots in the spring.
- Rake and destroy plant litter on the ground.
The timing of fungicides for black rot control is critical. The most important period for controlling fruit infections is from immediate pre-bloom through 3-4 weeks after bloom.
For commercial wine grape growers, there are many effective fungicides for controlling black rot. Commercial growers should consult the most current issue of the Midwest Fruit Pest Management Guide for up to date fungicide recommendations.
Homeowner fungicides that are easily obtained at a local farm or garden specialty stores and are effective at controlling black rot include Bonide Mancozeb Flowable (37% mancozeb) and Spectracide Immunox Multi-purpose Fungicide Spray Concentrate (1.55% myclobutanil). Homeowner products that contain copper, sulfur or oils are NOT effective at controlling black rot of grapes.
There are currently no biocontrol products available that can effectively manage black rot.