Plasmopara viticola, the causal organism of grapevine downy mildew, poses a significant threat to Eastern U.S. grape production. In this study, we determined fungicide resistance profiles of vineyards across Maryland and Pennsylvania between 2019 and 2023. A total of 352 downy mildew samples were collected from 27 vineyards and 32 different cultivars and tested for resistance to various fungicides commonly used for downy mildew management at field application rates using whole leaf bioassays. DNA was also extracted and used to confirm known resistance mechanisms to azoxystrobin (G143A) and mandipropamid (G1105S) using Sanger sequencing and PCR-restriction fragment length polymorphism (PCR-RFLP). Resistance to azoxystrobin, mandipropamid, and phosphorous acids was found throughout all regions tested, with bioassays resulting in 69, 39, and 33% of isolates displaying resistance to respective chemicals. In addition, all isolates subsampled and sequenced displayed the G143A mutation regardless of resistance phenotype, indicating that bioassays could underestimate the frequency of resistance. Of the 51 isolates tested using PCR-RFLP to confirm the presence of the G1105S mutation, 46 displayed genotypes consistent with their observed phenotypes. This is the first report of resistance to phosphorous acid in North America, which was found to be widespread throughout regions tested and detected as early as 2020. Multichemical class resistance was also commonly detected, with 20% of isolates displaying resistance to all three chemicals simultaneously. This study highlights the escalating threat posed by P. viticola to grape production in the Northeast, as rising populations resistant to multiple chemical classes considerably diminish the fungicide options available to growers.

Late-season bunch rot can cause substantial yield loss in grapevines grown in humid regions. Fruit-zone leaf removal has been widely used to reduce bunch rot and pesticide applications through improvements in canopy microclimate and grape cluster morphology. In this study, we evaluated whether mechanical leaf removal can be a valid alternative to a labor-intensive manual application by comparing prebloom manual (PB-MA) and mechanical (PB-ME) leaf removal. We also evaluated the effects of the timing of mechanical application, prebloom (PB-ME) versus fruit set (FS-ME), on fruit traits and bunch rot, caused by Botrytis cinerea. Our trials were conducted on two Vitis vinifera 'Pinot noir' and 'Pinot gris' vineyards in the Northeastern United States over two seasons (2017 to 2018). Major findings were overall consistent between cultivars and years. Leaf removal provided reductions in fruit-zone canopy density regardless of method or timing. In general, PB-ME provided similar shifts in cluster morphological traits to PB-MA, including lower number of berries per cluster, cluster compactness, and cluster weight compared with control (no leaf removal) vines. At harvest, both prebloom leaf removal methods equally reduced Botrytis bunch rot severity, whereas Botrytis bunch rot incidence in Pinot noir was lowest for PB-ME in 1 year and PB-MA in the next year. When comparing timing of mechanical leaf removal, FS-ME provided Botrytis bunch rot reductions similar to PB-ME, without effects on cluster weight. Thus, under our growing conditions, FS-ME was considered the best mechanical leaf removal option to help manage Botrytis bunch rot without causing undesirable yield reductions.

Late-season bunch rot causes significant crop loss for grape growers in wet and humid climates. For 3 years (2016 to 2018), we integrated prebloom mechanized defoliation (MD) in the fruit zone and bloom gibberellin (GA) applications, either alone or in combination, into the bunch rot control program of Vignoles, a commercially valuable grape variety that is highly susceptible to bunch rot. We hypothesized that both treatments would decrease bunch rot through modification of cluster architecture or fruit zone microclimate compared with vines treated with the standard chemical control program. Grapevines were trained to two popular training systems, four-arm Kniffin (4AK) and high-wire bilateral cordon (HWC). Treatment responses varied between training systems. MD, alone or in combination with GA, reduced bunch rot incidence and severity every year on 4AK-trained vines, an effect attributed mainly to fruit zone improvements. Conversely, MD alone did not reduce bunch rot incidence on HWC-trained vines, despite significant improvements in cluster architecture (reduced number of berries per cluster and cluster compactness). GA applications were more effective than MD at reducing cluster compactness, regardless of training system. As a result, GA reduced bunch rot incidence and severity when applied alone or with MD on 4AK- and HWC-trained vines. All treatments positively improved fruit-soluble sugar concentration on both training systems, while positive effects on titratable acidity were more consistent across training systems with MD.

Fruit-zone leaf removal is typically applied in cool and humid regions to improve grape and wine quality, while reducing disease pressure. When fruit-zone leaf removal is applied early in the season, before bloom [early leaf removal (ELR)], it also reduces fruit-set, cluster compactness, and susceptibility to bunch rot, a complex disease that involves fungi ( Botrytis cinerea , Aspergillus sp., Penicillium sp.) and bacteria ( Acetobacter sp.). Over 2 years (2015–16), we tested whether ELR applied mechanically [mechanical defoliation at stage E-L 18 (MD-I)] would mimic the effects of a hand removal [hand defoliation of the first six basal leaves and laterals at stage E-L 18 (Coombe, 1995) (HD-I)] with respect to ‘Riesling’ ( Vitis vinifera ) production parameters, canopy density and cluster sunlight exposure, fruit composition, and bunch rot control. We also compared the effects of mechanical defoliation applied either at prebloom (MD-I) or at fruit-set [mechanical defoliation at stage E-L 27 (MD-II)]. In both years, fruit-zone leaf removal, regardless of method and timing, reduced yield, cluster weight, and berries per cluster, while maintaining fruit composition and bud fruitfulness as compared with nondefoliated vines (control, C). In 2015, HD-I vines had a lower percentage of clusters infected by bunch rot as compared with the C and MD-II vines. However, severity of bunch rot was low in all treatments, and there was not significant treatment effect on bunch rot severity in either year. ELR consistently shortened cluster length, offsetting much of the intended cluster loosening effect induced by a lower number of berries per cluster—that would have reduced bunch susceptibility to late seasons rots. Despite removing only half the leaf area of HD-I, MD-I successfully mimicked the canopy improving effects of HD-I in terms of fewer interior clusters and leaves, fewer cluster-shading layers, and greater light available to clusters and leaves as compared with C vines.

Black rot on grapevine is a fungal disease caused by Phyllosticta ampelicida (syn. Guignardia bidwellii ) affecting grape leaves as well as clusters. A novel black rot decision support system termed VitiMeteo Black rot was assembled based on existing sub-models and incorporated into the established VitiMeteo forecast and decision support platform. Based on local weather data and a 5-day weather forecast, VitiMeteo Black rot simulates the relative susceptibility of grape clusters, the occurrence and severity of infection events as well as the duration of incubation periods. Data sets obtained in extended international (14 case studies; eight monitoring locations; 11 cultivars; seven countries in Europe and North America) field monitoring campaigns in 2012 and 2013 were used to evaluate the model predictions of newly expressed symptoms on leaves. In the case of the Vitis vinifera cultivars, on average 26.3 disease assessments took place per season. On average, 9.9 predictions were classified as true positive, 8.0 as true negative, 5.2 as false positive and 3.2 as false negative. Model precision, sensitivity and accuracy were on average 64, 77 and 67 %. Potential reasons for false positive and false negative predictions are discussed. VitiMeteo Black rot is freely available for several locations in Germany, Luxembourg and Austria on the internet via the VitiMeteo platform and might be expanded to other regions in the future.

Pennsylvania wine grape growers were surveyed to obtain information on factors affecting varietal selection, challenges to production, and their perceptions of canopy management practices. Our survey revealed that participants perceived site as a key factor in varietal selection decisions and winter injury as the greatest challenge for their economic sustainability. Other issues limiting production and profitability were disease control, frost injury, and labor cost and availability. Participants recognized the importance of canopy management practices for reaching optimum wine quality but had concerns over the shortage and cost of labor to implement them. Mechanization of canopy management likely would increase adoption.

Over six seasons from 2007 to 2012, the effects of the timing of cluster zone leaf removal and bloom gibberellin applications on Botrytis bunch rot, cluster morphology, yield, and juice composition were evaluated on Vitis vinifera Chardonnay grapevines. All experimental plots received Botrytis-specific fungicide applications at preclosure and veraison. Leaf removal at veraison, postfruit-set, or trace bloom reduced Botrytis severity by an average of 14, 47, or 71%, respectively, compared to no leaf removal. Leaf removal at trace bloom (LRTB) reduced Botrytis incidence and severity in five and four seasons, respectively, and was as effective as (2007 to 2010), or more effective than (2011 and 2012) two additional fungicide applications (at bloom and at preharvest), suggesting potential to reduce fungicide inputs. Gibberellin was less effective than LRTB, but reduced the incidence of Botrytis in 2008 at 5 mg/L and the severity of Botrytis in 2010 at 25 mg/L. Botrytis development increases with number of berries per cluster and berries per centimeter of cluster (compactness). Berries per cluster was reduced by LRTB in 2007, 2010, and 2011, and by 10 mg/L gibberellin in 2007 and 25 mg/L in 2011. Berries per centimeter was reduced by LRTB and gibberellin in 2007, 2008, and 2011. Leaf removal at postfruit-set produced the most noteworthy effects on juice composition, reducing titratable acidity in 2007, 2011, and 2012. Yields were reduced by LRTB in 2008, but no significant yield effects were observed in subsequent seasons. The efficacy of LRTB for reducing Botrytis bunch rot was highest in years with the most compact clusters, suggesting that the value of this treatment increases with increasing potential for bunch rot development.

Harvest bunch rot of wine grape, caused primarily by Botrytis cinerea, is a perennial problem limiting the productivity of eastern vineyards, especially on cultivars with compact clusters. The aim of the present study was to evaluate the effectiveness of gibberellic acid (GA) sprays at reducing the compactness of Chardonnay and Vignoles clusters and minimizing bunch rot. Applications of GA reduced the number of berries per centimeter and the incidence and severity of bunch rots in Vignoles and, to a lesser extent, in Chardonnay over three consecutive years; however, the magnitude of GA effects often depended on the timing and rate of application. Bloom GA applications were more effective (P < 0.001) at reducing compactness and bunch rots than prebloom applications. Significantly, negative effects of GA applications on yield were negligible based on data from 4 years of trials on single vines and 2 years of data on 24-vine plots of Vignoles, provided the rates did not exceed 25 ppm. Regression analysis showed that berries per centimeter accounted for between 89 and 94% of variation in the incidence of Botrytis rot on Vignoles. On Chardonnay, compactness accounted for 53% of the variation in incidence, and the estimated compactness level at which no bunch rot would occur was 4.40 ± 1.05 (mean ± standard error) berries per centimeter. The relationship between cluster compactness and spray coverage of berries was also investigated in two separate experiments. Spray coverage of individual berries decreased linearly as cluster compactness increased within the range tested (3 to 18 berries per centimeter). Cluster compactness accounted for two-thirds of the variation in individual berry coverage, and coverage was reduced by 40 to 50% for clusters with about 18 berries per centimeter. These results strongly support the use of GA in integrated management of bunch rot on Vignoles and Chardonnay in eastern U.S. vineyards.

The impact of cluster compactness and debris retention on harvest bunch rot of Vitis interspecific hybrid ‘Vignoles’ was investigated between 2001 and 2005 near Lake Erie, in Pennsylvania. Cluster compactness was characterized subjectively based on the OIV code 204 standard and objectively by determining the number of berries per centimeter of rachis. In 2001, 2002, and 2004, the median number of berries per centimeter for loose clusters was 6.3, 7.0, and 6.4 compared with 10.2, 12.7, and 12.4 for the compact clusters, respectively. Kolmogorov-Smirnoff and the Mann-Whitney U two-samples tests confirmed that the distribution of the berries per centimeter was significantly (90 ≤ χ2≤ 184.3; P < 0.0001) different between the two subjective compactness categories. Cluster compactness was strongly correlated with bunch rot incidence (χ2= 73.1 and 62.2 for 2001 and 2002, respectively; P < 0.0001), whereby disease incidence was higher in compact than in loose clusters. Logistic regression analysis indicated that every additional berry per centimeter unit of compactness almost doubled the likelihood of a cluster becoming infected with bunch rot (odds ratio = 1.828, 95% confidence interval [CI] = 1.392 to 2.399 in 2001 and odds ratio = 1.705, 95% CI = 1.394 to 2.085 in 2002). In 2004, bunch rot severity in compact clusters was nearly four times that of loose clusters. Linear regression analysis revealed that berries per centimeter accounted for >89% of the variation in bunch rot severity (R2= 0.893, P < 0.0001, n = 30) and >74% in cluster weight (R2= 0.745, P < 0.0001, n = 30). Accumulations of dehiscent floral debris contributed to greater bunch rot severity, and the effect was more pronounced in compact clusters than in loose clusters. Removal of basal leaves at trace bloom reduced berries per centimeter by 13% in 2004 and >25% in 2005, with corresponding reductions in bunch rot severity of 60% in 2004 and 62.5 to 82% in 2005. These results indicate that berries per centimeter is a good indicator of cluster compactness in Vignoles, and that practices that reduce cluster tightness would be effective in an integrated program for control of bunch rot on this cultivar.