- Fruits: grapes
- Pest Management: integrated pest management
Lack of knowledge regarding the susceptibility of cold-climate wine grape cultivars may be leading to the overuse of fungicides and under-utilization of plant host resistance to combat disease in the northern U.S. To provide new insight on diseases of cold-climate cultivars and to update management recommendations, disease was evaluated in three vineyards containing eight cultivars that were not sprayed with fungicides in 2015 and 2016. Disease incidence and/or severity of downy mildew (Plasmopara viticola), powdery mildew (Erysiphe necator), and black rot (Guignardia bidwellii) were measured from bud break until 2 weeks after harvest. Cold-climate cultivars ranged widely in susceptibility to different diseases, and while several cultivars were relatively resistant to two diseases, no cultivar was highly resistant to all three diseases. Additionally, a difference between foliar and fruit susceptibility for all three diseases was noted in several cultivars. These data provide a foundation for developing low-spray and certified organic disease management strategies for cold-climate wine grapes based on susceptibility to disease.
Wine grape cultivation has historically focused on the European wine grape, Vitis vinifera. While V. vinifera cultivars produce notable wines, they lack the cold tolerance needed to survive winter temperatures below -5˚C to -10˚C. This lack of cold tolerance in wine grapes has limited large-scale wine production to regions where V. vinifera reliably survives winter temperatures. Over the past two decades however, both public and private grape breeders have released several “cold-climate cultivars” that are crosses of cold-hardy native American grape species (e.g., Vitis riparia, Vitis aestivalis, Vitis cinerea, Vitis labrusca) with the European wine grape. The cold-climate cultivars can tolerate winter temperatures of -16 ˚C to -37 ˚C and produce high quality wine. The introduction of these hybrids has resulted in a rapid expansion of the wine grape industry in the Upper Midwest, Great Plains, and Northeast U.S., areas largely devoid of wine grape cultivation previously (Tuck and Gartner 2013). As of 2011 the industry accounted for nearly 400 hectares and contributed over $400 million and 12,000 jobs annually to the economies of states in those regions, with further economic impact predicted as newly planted vineyards mature (Tuck and Gartner 2013). A survey in 2012 revealed that over 60% of vineyards in these regions had been established in the previous 15 years, and 82% of respondents planned on continuing the expansion of their businesses (Tuck and Gartner 2013). Growers surveyed also rated disease as one of the most significant threats to their businesses (Tuck and Gartner 2013). Downy mildew (Plasmopara viticola), powdery mildew (Erysiphe necator), and black rot (Guignardia bidwellii) are particularly widespread diseases that can reduce yield and winter hardiness. The season-long use of fungicides to manage these diseases is expensive, and carries risks to the health of applicators, consumers, and the environment. A more recent survey reported a 63% increase in cold-climate grape acreage since 2011, and that further expansion is expected among existing businesses in coming years (Gartner 2016). This continuing growth trend underscores the need for more knowledge on diseases and host resistance in cold-climate cultivars.
In the Upper Midwest wild grape species such as Vitis riparia are endemic, and high humidity and summer rains favor disease development; therefore, economically challenging grape pathogens are likely to be present in most regions where commercial vineyards are established. Although previous efforts have been made to rate and categorize disease susceptibility of cold-climate cultivars (Bordelon et al. 2016), we are aware of no such studies performed in vineyards in which cultivar blocks were replicated, randomized, and left not sprayed with fungicides.
The objectives of this research were to determine: (i) the relative susceptibility of commercially important cold-climate cultivars to downy mildew, powdery mildew, and black rot; (ii) whether foliage and fruit on a given cultivar differ in disease susceptibility, as this distinction may have significant implications for development of reduced spray programs; and (iii) determine the validity of using potted vines exposed to field inoculum and artificial humidification as a useful comparison to field vineyard trials. Experiments were conducted at two locations over two growing seasons with cultivars that collectively account for approximately 58% of cold-climate wine grape acreage (Gartner 2016). Based on the results of this study, the use of host resistance to reduce fungicide inputs in cold-climate wine grape production is discussed.