Michelle M. Moyer

Modeling Dormant Bud Cold Hardiness and Budbreak in Twenty-Three Vitis Genotypes Reveals Variation by Region of Origin

Cold injury is a key environmental challenge in many grape-producing regions, especially those at high latitudes. Although grapevines acclimate to cold temperatures in fall and deacclimate when warm temperatures return in spring, cold hardiness varies with species, cultivar, phenology, ambient weather, photoperiod, and plant organ, which hampers implementation of effective mitigation practices. Using long-term data sets of lethal temperatures and spring phenology for primary buds of Vitis vinifera and Vitis labruscana, we parameterized and evaluated a discrete-dynamic model that simulates cold hardiness from early fall through budbreak of 23 genotypes. The model uses mean daily temperature as the sole input variable to drive daily changes in hardiness. Genotype-specific parameters, such as initial and maximum hardiness, temperature thresholds, acclimation and deacclimation rates, and chilling and heating requirements, were optimized through an iterative process. The model predicted cold hardiness with 0.89 ≤ r2 ≤ 0.99, depending on genotype. Because it simulates hardiness at budbreak, the model can also be used to predict the time of budbreak. Optimized model parameters revealed a north/inland-south/coastal gradient for genotype origin in terms of initial and maximum cold hardiness, and time of budbreak. Budbreak occurred earlier in hardier genotypes, consistent with more rapid deacclimation of genotypes originating from colder climates, paradoxically making these genotypes more vulnerable to spring frost in warmer environments. The current model of grapevine bud cold hardiness has uses in both climate modeling and risk assessment.

Effects of Acute Low-Temperature Events on Development of Erysiphe necator and Susceptibility of Vitis vinifera

Growth and development of Erysiphe necator (syn. Uncinula necator) has been extensively studied under controlled conditions, primarily with a focus on development of grapevine powdery mildew within the optimal temperature range and the lethal effects of high temperatures. However, little is known of the effect of cold temperatures (above freezing but <8 degrees C) on pathogen development or host resistance. Pretreatment of susceptible Vitis vinifera leaf tissue by exposure to cold temperatures (2 to 8 degrees C for 2 to 8 h) reduced infection efficiency and colony expansion when tissues were subsequently inoculated. Furthermore, nascent colonies exposed to similar cold events exhibited hyphal mortality, reduced expansion, and increased latent periods. Historical weather data and an analysis of the radiational cooling of leaf tissues in the field indicated that early-season cold events capable of inducing the foregoing responses occur commonly and frequently across many if not most viticultural regions worldwide. These phenomena may partially explain (i) the unexpectedly slow development of powdery mildew during the first month after budbreak in some regions and (ii) the sudden increase in epidemic development once seasonal temperatures increase above the threshold for acute cold events.

Release of Erysiphe necator Ascospores and Impact of Early Season Disease Pressure on Vitis vinifera Fruit Infection

Populations of Erysiphe necator cleistothecia can dehisce and release ascospores over an extended period ranging from fall through late spring and are an important source of primary inoculum for grapevine powdery mildew epidemics. The temporal distribution of ascospore maturity was monitored and measured as ascospore release from field-stored samples in a controlled environment assay. Assays were conducted over multiple seasons, in multiple locations, using multiple source populations. The influence of primary inoculum quantity and how that influences subsequent fruit disease severity was assessed. Cumulative ascospore release was positively correlated with accumulated wetting events and heat units; however, in most situations, >50% of the season-total ascospores were released before the date of local budbreak of Vitis vinifera. Both abbreviated and season-long fungicide programs suppressed mildew on fruit across a 100-fold gradient of inoculum dose when seasonal weather was relatively unfavorable for epidemic development. Suppression was degraded with progressive 10-fold increases of ascosporic inoculum dose when seasonal weather was more conducive to epidemic development. Combined, these findings suggest that differences between severe and mild years in grape powdery mildew can relate to the amount of primary inoculum present in the vineyard, that the levels of primary inoculum can be influenced by pre-budbreak weather conditions, and that effectiveness of spray programs at controlling primary infection events is related to the favorability of in-season weather conditions.

Effect of Early Fruit-Zone Leaf Removal on Canopy Development and Fruit Quality in Riesling and Sauvignon blanc

Canopy management is vital for quality winegrape production. During the 2012 and 2013 growing seasons, the timing of fruit-zone leaf removal (FZLR) was evaluated in two commercial vineyard blocks (Vitis vinifera Riesling and Sauvignon blanc) located north of Prosser, WA. Three different timings of manual FZLR were evaluated with a no-removal control. Leaf removal consisted of complete removal of all leaves and lateral shoots in the fruit zone on both sides of the canopy at prebloom, bloom, and four weeks postbloom. Each vine received the same treatment in both years. No negative implications were observed in total fruit set in either year. When leaf removal was performed, regardless of timing, the fruit zone of the canopy had less lateral shoot development and canopy refill than the control. Leaf removal also improved spray coverage in the fruit zone in Riesling, but the effect was related to the timing of leaf removal relative to the timing of the spray. In 2013, prebloom leaf removal significantly reduced Botrytis bunch rot severity in Sauvignon blanc (p = 0.01) below that of the control and four weeks postbloom leaf removal treatments. In 2013, prebloom leaf removal in Riesling increased terpene concentrations in the harvested juice (p = 0.03). In 2012, postbloom leaf removal in Riesling reduced concentrations of acids below those of the prebloom treatment (p = 0.04) in juice.

Mitigating the Economic Impact of Grapevine Red Blotch: Optimizing Disease Management Strategies in U.S. Vineyards

Grapevine red blotch disease (GRBD) is a recently-recognized viral disease found across some of the major grapegrowing regions in the United States. Vineyard managers were surveyed to (i) estimate the economic impact of GRBD on Vitis vinifera cv. Cabernet Sauvignon in Napa and Sonoma Counties in California and on V. vinifera cv. Merlot in eastern Washington and Long Island in New York, and (ii) to identify cost-minimizing management strategies under various disease incidence rates, price penalties for suboptimal fruit composition, timing of disease onset relative to vineyard age, and costs of control. The economic cost of GRBD was estimated to range from

Adjusting Product Timing during the Powdery Mildew Critical Window to Improve Disease Management

2213/ha in eastern Washington, when disease onset occurs at a low initial infection level and there is a low price penalty, to

Vitis spp. Rootstocks Are Poor Hosts for Meloidogyne hapla, a Nematode Commonly Found in Washington Winegrape Vineyards

68,548/ha in Napa County, when initial infection rates and quality penalties are both high. Our results further suggest that roguing symptomatic vines and replanting with clean vines derived from virus-tested stocks minimize losses if GRBD incidence is low to moderate (below 30%), while a full vineyard replacement should be pursued if disease incidence is higher, generally above 30%. These findings should help vineyard managers in the four examined viticultural regions adopt optimal GRBD management strategies.

Dual Fumigant and Herbicide Use Optimizes Replanting Preparation in a Virus and Nematode-Affected Vineyard

Summary Goals: This project was designed to answer questions about the timing of different cultural (leaf removal) and chemical (fungicide rotation) disease management practices and how they influence the overall efficacy of a powdery mildew management program. Key Findings: The timing of specific chemical modes of action plays an important role in the overall efficacy of powdery mildew fungicide programs. In this study, programs in which the evaluated products were applied once during the critical window performed as well as programs in which the same products were applied twice (during the critical window and at bunch closure). This finding indicates that disease control depends on application of sprays during the critical window rather than later in the season. As expected, leaf removal significantly improved spray coverage in the fruit zone; however, this improved coverage did not last the entire season. Canopy refill, which occurred as summer laterals developed, reduced fruit-zone coverage later in the season. Leaf removal also did not result in any positive additive effects on overall disease control. Impact and Significance: These findings can be used to aid growers in the selection and rotation of products used for powdery mildew management programs to maximize the efficacy of disease control and minimize the risk of fungicide resistance. While early fruit zone leaf removal did not improve overall disease control in this high-pressure scenario, leaf removal resulted in improved spray coverage, which may enhance disease control when low-volume spray applications or contact-active chemistries are emphasized in a commercial program.

Cold Hardiness of Vitis vinifera Roots

The majority of winegrape (Vitis vinifera) vineyards in Washington are planted with own-rooted grapevines, as opposed to grapevines grafted onto rootstock varieties. The plant-parasitic nematode Meloidogyne hapla (common name: northern root-knot nematode) is commonly found in Washington winegrape vineyards, and own-rooted grapevines are susceptible to this nematode. Before rootstocks are used to manage M. hapla or other horticultural characteristics in Washington, their host status for M. hapla should be defined. In greenhouse experiments, 10 commercially available rootstock varieties were evaluated for their M. hapla host status. Additionally, the reproductive potential of different M. hapla populations collected from Oregon and Washington, and of another root-knot nematode, M. chitwoodi, on rootstock varieties and own-rooted V. vinifera Chardonnay was evaluated. The rootstocks Salt Creek, Freedom, Harmony, St. George, Riparia Gloire, 101-14 Mgt, 3309C, 110R, 420A, and Matador were poor hosts for M. hapla. Populations of M. hapla varied in reproductive potential and virulence on own-rooted Chardonnay. An M. hapla population collected from a V. vinifera vineyard in Paterson, WA had 33 to 78% greater reproduction than the other M. hapla populations. An M. hapla population collected from a V. vinifera vineyard in Alderdale, WA was consistently more virulent than the other M. hapla populations. Own-rooted Chardonnay and the rootstock Matador were poor hosts for M. chitwoodi. This is the first report of the host status of several grapevine rootstocks for M. hapla.

Development of an Advisory System for Grapevine Powdery Mildew in Eastern North America: A Reassessment of Epidemic Progress

Summary Goal: An exploratory study evaluated best-management techniques for replanting a commercial vineyard suffering from both a high incidence of Grapevine leafroll associated virus-3 and high densities of northern root-knot nematode (Meloidogyne hapla). The grower had two goals: to reduce the density of plant-parasitic nematodes, which can delay establishment of young vines, using a pre-plant fumigant, and to reduce survival of virus-infected vine tissue by applying an herbicide to the foliage prior to vine removal. Key Findings: Fall-applied metam sodium reduced densities of M. hapla in the following growing season. Its herbicidal properties also resulted in high root mortality. In cases where vines survived, only a few shoots (and suckers) emerged, displaying typical morphology. Fall-applied foliar glyphosate significantly reduced shoot and sucker regrowth, but did not kill root tissue. In the few instances where shoots and suckers grew the following spring, that tissue expressed typical symptoms of glyphosate phytotoxicity. While low rates of abnormal growth would eliminate much of the aerial reservoir of virus, the presence of live roots indicates that rogue vines and living root debris could contribute to infection of new, virus-free planting stock. Combining fumigant and foliar glyphosate was additive: nematode control, root mortality, and a lack of healthy shoot/crown sucker regrowth. Impact and Significance: This preliminary work showed that the combination of fall fumigation and foliar glyphosate application reduced nematode populations and vine survival. In situations where plant-parasitic nematodes are the sole issue facing replanting, fumigation alone suffices. Where virus alone is the impetus for replanting, fall-applied foliar glyphosate may provide sufficient overall vine mortality, with a caveat of low root mortality and its attendant risk of root-feeding vectors spreading the virus to new planting stock.