Fruit Crops: A Summary of Research 1998

Research Circular 299-99

Effects of Various Fungicide Programs on Powdery Mildew Control, Percent Berry Sugar, Yield, and Vine Vigor of 'Concord' Grapes in Ohio

Michael A. Ellis, Laurence V. Madden, L. Lee Wilson, and Gregory R. Johns

Introduction

Powdery mildew, caused by the fungus Uncinula necator (Schwein.) Burrill, is the most widespread and destructive disease of grapevines in the United States (6). In Ohio, the disease can cause serious losses of yield and fruit quality on susceptible cultivars. Although none of the grape cultivars currently grown in Ohio are completely resistant to powdery mildew, cultivars do vary in their degree of susceptibility (2, 11). Most cultivars of Vitis vinifera L. and many Vitis interspecific hybrid cultivars are highly susceptible to powdery mildew. In general, cultivars of Vitis labruscana L. H. Bailey tend to be more resistant to powdery mildew. The V. labruscana cultivar 'Concord' is one of the least susceptible and most widely planted cultivars in Ohio and throughout the Great Lakes region. Although 'Concord' grapes are relatively resistant, powdery mildew incidence and severity can be quite high in some growing seasons if the disease is not effectively controlled with fungicides. Although the disease can appear to be quite severe, effects of powdery mildew on yield and fruit quality of 'Concord' grapes is not well documented, and many Ohio growers feel that the disease is not a serious problem on 'Concord' grapes.

Control of grape powdery mildew largely depends on the use of effective fungicides. Depending upon susceptibility of the cultivar, three to 12 applications of fungicide may be required for powdery mildew control (9). Due to the relatively low value of 'Concord' grapes (as compared to V. vinifera wine grapes), many 'Concord' growers have expressed concerns about the cost of fungicides for disease control. In Ohio, most 'Concord' growers make three to five fungicide applications per season, starting when new shoots are 4- to 10-inches long, with repeated applications at 10- to 14-day intervals. Generally, the fungicide program is terminated two to three weeks after bloom.

Diseases of primary concern during this period are black rot caused by Guignardia bidwellii (Ellis) Viala and Ravaz, downy mildew caused by Plasmopara viticola (Berk. and Cont.) Berl. and deToni, Phomopsis cane and leaf spot caused by Phomopsis viticola (Sacc.) Sacc., and powdery mildew. Ideally, each fungicide application should contain fungicides with efficacy for controlling all of these diseases simultaneously. However, fungicides with efficacy for powdery mildew are often omitted. Traditionally, many growers apply a mid- to late-season copper fungicide if supplemental control of powdery or downy mildew is required. However, many growers avoid copper fungicides due to potential damage (phytotoxicity) to grape vines (3, 10). This presents an additional problem for growers of 'Concord' grapes in relation to late-season control of powdery mildew with fungicides. Sulfur is an effective and relatively inexpensive fungicide for powdery mildew control and is commonly used for late-season control on varieties that are not sulfur sensitive. Unfortunately, 'Concord' grapes are extremely sensitive to sulfur. If growers choose not to use copper fungicides, the only alternatives are ergosterol biosynthesis-inhibiting fungicides such as myclobutanil or fenarimol, JMS-stylet oil, or the newly registered azoxystrobin fungicide, Abound. All of these fungicides are highly effective for powdery mildew control; however, they are expensive and many 'Concord' growers are reluctant to use them for late-season control of powdery mildew. Whereas post-bloom (late season) development of powdery mildew is common on 'Concord' grapes, it is often disregarded by growers as having no perceived effects on yield or fruit quality (percent sugar).

The purposes of this study were to evaluate the efficacy of various fungicide programs for control of powdery mildew and determine the effects of powdery mildew on yield, vine vigor, and fruit quality (percent sugar) of 'Concord' grapes in Ohio.

Materials and Methods

Field trials were conducted during 1994, 1995, and 1996 in a block of 'Concord' grapes at The Ohio State University/Ohio Agricultural Research and Development Center's Grape Research Branch at Kingsville. Vines were planted in 1985 and were spaced with eight feet between vines and nine feet between rows. The vineyard consisted of 15 rows with nine vines per row. Vines were trained to a bilateral cordon system (no tie) and were pruned to a maximum of 60 buds per vine, regardless of pruning weights. The experimental vineyard was divided into three blocks of five rows each, and treatments were applied to the center three rows of each block. Each of the center three rows was considered as a sample for purposes of data analysis. In order to achieve large enough plots for airblast spraying, each block consisted of a single treatment. Fungicide treatments were applied to both sides of the three center rows using an FMC model 1229 FTM airblast spray in 100 gallons of water per acre at a pressure of 250 psi. In order to prevent drift, there were two nontreated rows between each treated block. Data was collected from each of the center three rows (replications). Powdery mildew disease incidence and severity for leaves and cluster stems were taken approximately two weeks prior to harvest during each year of the study. Foliar disease incidence was based on the number of leaves infected for 10 randomly selected leaves from each of nine vines per replication and treatment. Disease incidence for cluster stems was based on the number of infected cluster stems on 10 randomly selected clusters from each of nine vines per replication and treatment. Disease severity for leaves and cluster stems was based on the area of the same leaves and cluster stems covered by powdery mildew using the Horsfall-Barratt scale, and converting values to percent using Elanco conversion tables (1). Percent sugar content was measured at harvest from a 100-berry composite sample from each replication per treatment. Yield (total berry weight) was recorded for each vine at harvest. Pruning weights for each vine were recorded as a measure of vine vigor for each vine during the dormant season following each harvest.

Fungicide treatments were designed to evaluate a four-spray program of mancozeb plus an ergosterol biosynthesis-inhibiting fungicide (either myclobutanil or fenarimol) applied on a 10- to 14-day schedule from approximately six-inch new shoot growth through two weeks after bloom. This represents the currently recommended fungicide program for 'Concord' grapes in Ohio (2). This four-spray program (standard program) was the foundation for all fungicide treatments during all years of testing. Fungicides, rates, and application dates are presented in Tables 1, 2, and 3. In 1994, additional treatments consisted of one additional post-bloom application of metalaxyl plus copper 70W or two additional post-bloom applications of JMS-Stylet oil (Table 1). In 1994, an untreated control was not included in the trial. In 1995, treatments consisted of the standard program alone, the standard program plus two additional post-bloom applications of JMS-Stylet oil, and an untreated control (Table 2). In 1996, treatments consisted of the standard program alone, and the standard program plus one additional application of JMS-Stylet oil, and an untreated control (Table 3).

For data analysis, the standard error of the difference (SED) between means was calculated for each variable (e.g., disease incidence). An approximate least significant difference (LSD) was calculated based on the SED.

Results and Discussion

In 1994, powdery mildew incidence was 100% on leaves and cluster stems for the standard program and one additional application of metalaxyl plus copper. Although there was no significant difference in disease incidence between these treatments, the additional application of metalaxyl plus copper resulted in significantly less disease severity on both leaves and cluster stems (Table 1). Two additional applications of JMS-Stylet oil resulted in significantly less powdery mildew incidence and severity on leaves and cluster stems than any other treatment. JMS-Stylet oil reduced disease severity from 38% and 42% to 1% and 1.6% for leaves and cluster stems, respectively. JMS-Stylet oil has been reported to be highly effective for control of powdery mildew of grape (4, 5, 7, 8). Although disease incidence and severity was high on vines in the standard program and was significantly reduced by two additional applications of JMS-Stylet oil, there were no significant differences in percent sugar of berries, total yield, and vine pruning weights between any of the treatments (Table 1).

In 1995 and 1996, a nontreated control was included for comparison. In both years, there were no significant differences in disease incidence on leaves or cluster stems between the standard program and the untreated control. However, disease severity for both leaves and cluster stems was significantly less for the standard program than the untreated control during both years. Two additional applications of JMS-Stylet oil in 1995 resulted in significantly less disease incidence and severity on leaves than the standard program. On cluster stems, two additional applications of Stylet oil resulted in significantly less disease incidence than the standard program; however, there were no significant differences between the treatments in disease severity (Table 2). In 1996, one additional application of JMS-Stylet oil resulted in significantly less disease incidence and severity on both leaves and cluster stems than the standard program (Table 3). As in 1994, there were no significant differences in percent sugar content of berries, total yield, and pruning weight per vine between any of the treatments in 1995 or 1996.

The results of this study indicate that the standard four-spray program for grape disease control is not highly effective for providing season-long control of powdery mildew on 'Concord' grapes. It should be noted that in all years of testing, no powdery mildew was observed in any of the test plots at the time when the standard program was terminated (approximately two weeks after bloom). Therefore, all visible powdery mildew in the plots developed relatively late in the season. Although disease incidence and severity were high in the standard program and unsprayed plots, the disease appeared to have no effect on fruit sugar content, yield, or vine vigor over the three-year period that tests were conducted. In all years of testing, late-season applications of JMS-Stylet oil resulted in significantly less disease incidence and severity (Tables 1-3). In some years, especially 1994 and 1996, these differences were quite large regardless of degree of significance; however, they did not result in significant differences in fruit quality, yield, or vine vigor.

Based on the results of this study, it appears difficult to economically justify late-season fungicide applications for control of powdery mildew on 'Concord' grapes. The authors are not suggesting that growers ignore powdery mildew on 'Concord' grapes. However, the authors believe that additional information is needed to demonstrate the need for, and benefits of, additional fungicide applications for powdery mildew control beyond the traditional four-spray program on 'Concord' grapes.

Acknowledgments

The authors wish to thank the Ohio Grape Industries program for partial funding of this research.

Literature Cited

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