Pest Management Fact Sheet #5106

Black Rot of Grape

Bruce A. Watt, Extension Plant Pathologist

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Introduction

The fungus Guignardia bidwellii causes black rot of grape. It is the most common and serious disease of grape in Maine and during years when the weather is favorable losses can range up to 80% of the crop.

Symptoms

The fungus infects leaves, fruit, canes, shoots, and tendrils. Infections are usually first noticed on the leaves where they appear as small spots which may enlarge to 1/4 inch in diameter. The spots are commonly light tan in the center circled by a dark tan band (Fig. 1). As the infection ages, tiny dark spore producing structures (pycnidia) form within the spots. Spots formed by infection of the shoots are larger and darker and these also produce pycnidia.

Symptoms of infected fruit begin as small brownish spots that quickly expand to involve the entire fruit in a matter of days. As the infection continues, the grape shrivels from a soft brown rotted fruit to a small, black, hard mummy (Fig. 2). A few, many, or most of the fruit may be infected in each cluster. Once again, pycnidia will form in these infected fruit.

Figure 1

Figure 2

Environmental Conditions

The spores that begin the disease cycle are released in the spring. The spores are of two types. Ascospores (sexually produced spores) are produced in pseudothecia that develop in infected, over-wintered leaves and fruit. These spores are ejected after bud-break in the spring after about 1/100 inch or more of rain has fallen. Conidia (asexual spores) from pycnidia in the over-wintered fruit and cane lesions (and later from new infections) may be released two to three weeks earlier following 1/10 inch or more of rain. After the spores land on tissues susceptible to infection they require sufficient time in the presence of free water in order to germinate and infect the tissue. Optimally, six hours at 80oF is sufficient for infection whereas at 50oF 24 hours is necessary (Fig.3).

Figure 3.  Hours of leaf wetness required for infection.

Spore dispersal continues through the middle of the summer and then declines until by the end of the summer no new infections occur. Through this period only new growth is susceptible to infection except for the fruit, which can be infected until the onset of color.

Management

Non-chemical

1. Fall clean up may be the most important component of a control strategy because this can remove most of the inoculum (spores) source from the planting. Rake up all leaves and fallen mummies. Be especially careful to remove mummies that remain attached to the vine. When pruning, preferentially prune out infected canes and be careful to remove infected tendrils.
2. Plant in locations that will provide plenty of sun and maximum air circulation and orient rows parallel to the prevailing winds (generally west-east). Provide for proper vine spacing when planting, try to maintain an open canopy and maintain good weed control. These practices will allow for rapid drying of the plants.
3. Select resistant varieties when planting.

Chemical

There are two strategies for chemical control of black rot. The first strategy involves the use of protectant fungicides that must be present during infection periods to prevent infections from occurring. The second strategy relies on the ability of certain fungicides to eradicate early infections after they have occurred. All fungicides have protectant action and those commonly used for black rot include Captan (marginally effective), copper+lime and Bordeaux mix (may cause phytotoxicity), mancozeb, ferbam, sterol inhibitors (SI), and the newer strobilurins. Also, some recent work from Cornell has shown the use of surfactants alone may help to control black rot. The SIs are known for their post-infection activity. A good general protectant schedule would be to spray at 1/2-1 inch shoot growth, immediately pre-bloom, immediately post bloom, and mid-season sprays depending on the weather until the fruit starts to color.

When Using Pesticides

ALWAYS FOLLOW LABEL DIRECTIONS!

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