Dr. Wayne Wilcox of Cornell University says Botrytis cinerea as a weak pathogen that prefers injured, senescent tissue, such as old blossom parts and ripening fruit. The more ripe the fruit, the more susceptible it becomes, an added consideration when rains delay harvest.

Botrytis thrives where there’s low evaporative water loss and prefers high humidity and poor air circulation.

Bunch rot (botrytis) overwinters on canes and vineyard debris, including cluster rachises and mummified berries, which is why vineyard sanitation is important. During humid periods in the growing season, especially in the spring, spores are produced from overwintering sources and dispersed by wind. Spore germination and infection require rain and the right temperatures. The optimum temperature range for the disease is 59° to 77°F, though it survives in a wide range of ­temperatures, from 2° to 86°F.

He noted that infection can occur during bloom and postbloom on the floral organs and cap scars, and ­infections usually remain latent or dormant until postveraison and preharvest. “The probability of the latent infections becoming active increases as the berries continue to senesce or ripen,” Wilcox said. The disease can also infect developing berries through wounds caused by insects, mechanical damage, rain cracking, or powdery mildew damage. “There are many ways the ­disease can come in.”

Moreover, botrytis can infect healthy ripening berries after veraison and through harvest. Nesting within a cluster is a classic disease pattern, and one that can cause significant crop loss.

In various studies, Wilcox has found that cluster structure greatly influences disease severity. Loose clusters will have less disease, and tight-clustered varieties, when thinned to have a loose structure, will have less disease than those not thinned.

Latent infections are a concern because after initial establishment, all it takes are favorable conditions for growers to have a big problem, he said. To highlight this, he inoculated one berry with the disease in an unthinned, tight-clustered bunch of grapes. “By harvest, the cluster was toast, with about 50 percent of the berries diseased,” he reported.

Leaf removal

Wilcox, like other plant pathologists, has seen the positive effects on disease control from removing leaves. Leaf removal helps control both botrytis and powdery mildew, enhances air movement through the canopy, and improves spray penetration, he said. The earlier leafing is done, the better the disease control. He recommended removing leaves about four weeks postbloom, with care taken to avoid sunburn by leafing only on the east side.

Regarding the timing of fungicide sprays, Wilcox has studied the question for 15 years, with variable results. Spraying early can protect against latent infections and blossom trash colonization, while spraying later (postveraison) protects from new infections and secondary spread. “It really depends on the ­climate that year.”

Botrytis-specific

But choosing the right fungicide is key. “Most standard fungicides are relatively ineffective against botrytis,” he said. Likewise, most botrytis-specific fungicides have limited activity against unrelated pathogens.

Vanguard (cyprodinil) and Scala (pyrimethanil), both in the anilinopyrimidine family, have been very consistent performers in New York trials, according to Wilcox. The AP fungicides are readily absorbed and provide protective and internal activity, but are at high risk for resistance to develop. Elevate (fenhexamid) has also been a consistent workhorse under New York conditions as was Rovral (iprodione). However, Rovral is no longer used much because of ­resistance problems.

Other effective fungicides include Flint (trifloxystrobin), Pristine (pyraclostrobin + boscalid) and Endura (boscalid). Endura also provides excellent powdery mildew control, he said.

Wilcox stressed that growers must rotate chemistries in any fungicide program to avoid the development of disease resistance.