A Survey for Grapevine Viruses in Washington State ‘Concord’ (Vitis × labruscana L.) Vineyards

Within the United States, Washington is the number one producer of ‘Concord’ grapes and the number two producer of wine grapes. Previous studies in wine grapes revealed the presence of twelve different viruses in Washington vineyards. Despite what is known about viruses in wine grapes in Washington, virtually no studies have focused on viruses in Concord vineyards. To further understand the epidemiology of viruses in Washington State, a survey of Concord grapes was conducted in 2010 and 2011. Five hundred Concord vines were sampled and tested for the presence of virus, and five different viruses were found among 92 virus-positive samples. The most common virus found was Grapevine leafroll-associated virus 3 (51 samples), followed by Grapevine fanleaf virus (24 samples), Grapevine leafroll-associated virus 4 (nine samples), Grapevine leafroll-associated virus 2 (four samples), and Grapevine leafroll-associated virus 9 (four samples). The presence of viruses in Concord vineyards is of great concern for growers due to likely movement of virus diseases from Concord grapes to wine grapes that might result in economic losses to the grape industry. Accepted for publication 19 May 2012. Published 5 August 2013.

The Epidemiology of Powdery Mildew on Concord Grapes

Vitis labruscana ‘Concord’ is a grape cultivar widely grown in the United States for processing into juice and other grape products. Concord grapes are sporadically but sometimes severely damaged by the grape powdery mildew pathogen, Uncinula necator. Although the foliage is often reported to be moderately resistant to powdery mildew, severe fruit infection occurs in some years. We observed the seasonal development of powdery mildew on leaves, rachises, and berries of unsprayed Concord grapevines. Inoculations of flower and fruit clusters revealed a brief period of berry susceptibility and a protracted period of rachis susceptibility. The rachis remained highly susceptible to infection, and the severity of rachis infection increased throughout the growing season until the rachis formed a periderm shortly before harvest. In contrast, berries were nearly immune to infection within 2 weeks after fruit set. Rachis and berry infections were detected before the disease was observed on foliage, and the incidence of rachis and berry infection often exceeded disease incidence observed on foliage until after fruit acquired substantial ontogenic resistance. Excellent control of fruit infection, and adequate control of leaf infection, was achieved by two fungicide applications targeted at the peak period of fruit susceptibility. Although Concord is thought to be moderately resistant to powdery mildew, the rachis is highly susceptible, and may be the avenue by which prebloom infections make 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, and may be of little economic consequence on fruit destined for processing. Although fruit of V. vinifera cultivars have been reported to remain susceptible to infection until berry sugar levels reach 8 to 15%, Concord fruit become nearly immune to infection nearly 6 weeks before this stage of development. Because powdery mildew does not become conspicuous on foliage until late summer, it is generally regarded as a late-season problem on Concord grapes, and previous management programs have reflected this belief. However, the greatest contribution to control of fruit infection is due to fungicides applied during the peak period of fruit susceptibility, from bloom until shortly after fruit set, long before the disease is observed on foliage.

Effects of Powdery Mildew on Vine Growth, Yield, and Quality of Concord Grapes

Vitis labruscana ‘Concord’ is a widely planted grape cultivar grown in the United States for processing into juice and other products. Concord fruit are sporadically but sometimes severely damaged by the grape powdery mildew pathogen, Uncinula necator. The effects of powdery mildew on vine growth, yield, and quality of Concord grapes at three levels of cropping intensity commonly found in commercial grape production were determined in vineyard studies. Top-wire cordon-trained Concord vines were balance pruned, pruned to retain 80 nodes, or minimally pruned. Replicated plots of the foregoing were then either protected from powdery mildew by regular fungicide applications, or were inoculated and left unsprayed. Over a 4-year period, the effects of foliar infection on vine growth, yield, and juice quality of unsprayed vines were compared with vines that received a conventional protection program of four fungicide applications. Failure to control powdery mildew resulted in a chronic reduction in wood maturity measured as the number of nodes on canes that developed periderm. The reduction in nodes did not reduce yield, possibly due to compensation in shoots produced from the remaining nodes. Powdery mildew did not affect bud survival or vigor, measured as the number of shoots produced per node on retained canes. The most significant effects of powdery mildew were on berry sugar levels and juice color and acidity, which on the unsprayed vines were sometimes reduced below minimally acceptable thresholds for processed grapes. Significant reductions due to powdery mildew in these parameters occurred in all three pruning treatments, but were most pronounced at higher cropping levels.