The epidemiology of powdery mildew on concord grapes

Vitis labruscana 'Concord' is a grape cultivar widely grown in the United S tates for processing into juice and other grape products. Concord grapes ar e sporadically but sometimes severely damaged by the grape powdery mildew p athogen, Uncinula necator. Although the foliage is often reported to be mod erately resistant to powdery mildew, severe fruit infection occurs in some years. We observed the seasonal development of powdery mildew on leaves, ra chises, and berries of unsprayed Concord grapevines. Inoculations of flower and fruit clusters revealed a brief period of berry susceptibility and a p rotracted period of rachis susceptibility. The rachis remained highly susce ptible to infection, and the severity of rachis infection increased through out the growing season until the rachis formed a periderm shortly before ha rvest. In contrast, berries were nearly immune to infection within 2 weeks after fruit set. Rachis and berry infections were detected before the disea se was observed on foliage, and the incidence of rachis and berry infection often exceeded disease incidence observed on foliage until after fruit acq uired substantial ontogenic resistance. Excellent control of fruit infectio n, and adequate control of leaf infection, was achieved by two fungicide ap plications targeted at the peak period of fruit susceptibility. Although Co ncord is thought to be moderately resistant to powdery mildew, the rachis i s highly susceptible, and may be the avenue by which prebloom infections ma ke their way onto the developing fruit. Late-season infection of the rachis neither spread to the fruit, nor did it cause fruit to drop prematurely, a nd may be of little economic consequence on fruit destined for processing. Although fruit of V. vinifera cultivars have been reported to remain suscep tible to infection until berry sugar levels reach 8 to 15%, Concord fruit b ecome nearly immune to infection nearly 6 weeks before this stage of develo pment. Because powdery mildew does not become conspicuous on foliage until late summer, it is generally regarded as a late-season problem on Concord g rapes, and previous management programs have reflected this belief. However , the greatest contribution to control of fruit infection is due to fungici des applied during the peak period of fruit susceptibility, from bloom unti l shortly after fruit set, long before the disease is observed on foliage.