Black Rot

The causal fungus (Guignardia bidwellii) overwinters in mummified fruit on the vine or on the ground. Spring rains trigger the release of airborne ascospores and/or rain splashed conidia from the mummies. Primary infections occur on green tissues if temperatures and duration of leaf wetness are conducive (Table 5). Recent research indicates that the majority of ascospores from mummies on the ground are discharged within a time period from one inch shoot growth to 2 to 3 weeks after bloom. If mummies are allowed to hang on the vines, they can discharge ascospores and conidia throughout the growing season.

Black Rot Disease Cycle (Figure 58)

Figure 58: Black rot disease cycle. We wish to thank the New York State Agriculture Experiment Station for use of this figure. Figure taken from Grape IPM Disease Identification Sheet No. 4.

In conventional production systems, black rot is controlled primarily through the use of effective fungicides combined with various cultural practices. Black rot may be particularly important in organic production systems because the organically approved fungicides (copper and sulfur) are not very effective for black rot control. Therefore, growers should develop a thorough understanding of the black rot disease cycle and the cultural practices used to control it.

Lesions on canes from the previous season can also produce conidia for a period of at least one month starting at budbreak. Cane lesions are probably most important in mechanically pruned or hedged vineyards that have an abundance of canes in the canopy. All green tissues of the vine are susceptible to infection. Leaves are susceptible for about one week after they reach full size. Brown circular lesions develop on infected leaves about 9 to 11 days after infection (Figure 59). Within a few days, black spherical fruiting bodies (pycnidia) form within the lesions (Figure 60). Each one of these pycnidia can produce a second type of spore (conidia). These conidia are spread by rain splash and can cause secondary infections of leaves and fruit throughout the growing season. It is important to emphasize that a single ascospore can cause a primary infection (leaf lesion). Within each leaf lesion (primary infection), many pycnidia form. Each pycnidium can produce hundreds of thousands of conidia, each of which can cause another infection (secondary infection) later in the season. Therefore, it is extremely important to control the early season primary infections caused by ascospores. Remember, infection by one ascospore can result in the development of millions of secondary conidia in the vineyard.

Figure 59: Black rot lesions on grape leaf.

Figure 60: Close-up of black rot leaf lesion showing fungal fruit bodies (pycnidia).

Figure 61: Black rot on berries.

Figure 62: Close-up of black rot mummy.

The fruit infection phase of the disease can result in serious economic loss. Berries are susceptible to the infection from bloom until shortly after bloom. Older literature reports that berries become resistant when they reach 5 to 8 percent sugar. Recent research in New York indicates that berries become resistant to black rot much earlier (2 to 3 weeks after bloom). Therefore, the most critical time to control black rot fruit infections with fungicide is from immediately prior to bloom through 2 to 4 weeks after bloom. An infected berry first appears light brown in color. Soon the entire berry turns dark brown, and then black pycnidia develop on its surface. Infected berries eventually turn into shriveled, hard, black mummies (Figure 61, 62). These mummies also serve as a source of secondary inoculum later in the growing season and are the primary means by which the fungus overwinters.

Table 5. Grape Black Rot. Leaf wetness duration-temperature combinations necessary for grape foliar infection by black rot.

*Data represent a compilation from several experiments with the cultivars Concord, Catawba, Aurora and Baco noir.

The bottom line for black rot control

Sanitation is critical to successful black rot control. Mummies are the most important overwintering source of the black rot fungus. If all mummies and infected canes are removed from the vineyard, there is no source of primary inoculum in the spring and, thus, the disease is controlled. Any practice that removes mummies and other infected material from the vineyard will be beneficial to the disease management program. If all mummies cannot be removed from the vineyard, it is extremely important that they are not left hanging in the trellis. As mentioned previously, mummies on the ground appear to discharge their ascospores early in the season, while those hanging in the trellis may discharge ascospores and conidia throughout the growing season. Sulfur is not effective for black rot control. Copper fungicides are not highly effective, but will provide some level of control. The most critical period to control black rot with fungicide is from immediate prebloom through 2 to 4 weeks after bloom.