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Evaluation of fungicides for control of powdery mildew and Phomopsis cane and leaf spot of 'Concord' grape, 2000

This trial was conducted in a mature vineyard at the Lake Erie Regional Grape Research and Extension Center, North East, PA. Vines were trained to a single curtain, high wire cordon system. Treatments were applied to 6-vine plots in a randomized complete block design with 4 replications. Each block of treatments was separated by at least one untreated buffer row. Fungicides were applied with a Friend covered-boom plot sprayer at 200 psi, applying 100 gal/A. Applications were made on 11 May (4.6 in. shoot), 22 May (8.7 in. shoot), 7 Jun (pre-bloom), and 19 Jun (post-bloom). Rainfall for May, Jun, Jul, Aug, and Sep was 5.26, 5.04, 5.63, 2.12, and 5.69 in. respectively. Bud burst occurred on 3 May, full bloom on 13 Jun, and harvest on 6 October. Powdery mildew incidence and severity were determined from 25 randomly selected leaves and clusters per plot on 15 August, 29 September (clusters only), and 2 Oct (leaves only). Phomopsis incidence and severity were determined on 2 August from the 5 most basal leaves and internodes of each of 5 primary shoots per plot. Phomopsis incidence was determined on 29 September from 25 randomly selected rachises per plot and on 9 October from the 5 most basal internodes of 10 primary shoots per plot. Severity was rated using the Barratt-Horsfall scale and was converted to % area infected using Elanco conversion tables.

With the abundance of rainfall this season, pressure from powdery mildew and Phomopsis was considered moderately high. We did have small amounts of downy mildew and black rot in the Concord experimental block this year, but not enough for a fair test. Due to frequent rainfall, sometimes heavy, spray intervals were shortened when possible. However, the interval between the second and third application was unavoidably stretched to 16 days during which 0.86 inches of rain fell. This may have affected the performance of materials that are not particularly rain-fast.

Results

Powdery mildew: Despite wet conditions and frequent primary infection periods throughout May and June, incidence of powdery mildew on leaves and cluster rachises on unsprayed Concord vines increased slowly. Fungicide applications ended with a single post-bloom spray on 19 June. However, after mid-July, incidence of leaf and rachis infection rose sharply. By late season, disease incidence on leaves and cluster rachises was at 100% for all treatments.

1. Leaf infection - At the first rating (4 weeks after the last spray), only plots sprayed with the Penncozeb/Nova/Sovran (standard) program and the 4 and 6 lb/A rates of Cuprofix had significantly lower incidence of leaf infection than the check. From the mid-season rating (8 weeks after the last spray) until harvest, there were no significant differences between the check and any of the chemical treatments with respect to severity of leaf infection.

2. Rachis infection - Rachises remain susceptible throughout the season. At mid-July, all chemical treatments had statistically similar incidence of powdery mildew on rachises and significantly fewer infected rachises than the check. However, by mid-August, the Penncozeb/Nova/Sovran (standard) and Sovran (applied alone) treatments were emerging as the superior programs for control of rachis infections. By late season, only these two treatments had significantly less surface area of rachises covered with powdery mildew than the unsprayed check, but provided only 23-26% control.

3. Berry infection - All treatments were statistically similar in controlling berry infections through the critical period of berry susceptibility (immediate pre-bloom period to about 2-3 weeks after fruit set) and had significantly less powdery mildew on berries at harvest, than the unsprayed check.

4. Rates - With respect to Cuprofix, there were no significant differences in levels of powdery mildew on leaves, rachises, or berries between treatments sprayed with the 2, 4, or 6 lb/A rate. In fact, plots sprayed with lower rates (2 and 4 lb/A) had numerically less severe rachis and leaf infection, by the final rating, than the higher 6 lb/A rate. As for TM41501, the higher 13 oz/A rate generally provided better control of powdery mildew than the 9 oz/A rate, but the differences were not significant.

Phomopsis: On May 9-10 a wetting period and warm temperatures may have provided conditions for an early Phomopsis infection period. Most vineyards did not require protection at that time as inoculum levels should have generally been low going into this year. However, our first application was made on 11 May as clusters are exposed at this time and there is the potential for rachis and leaf infection to occur. Subsequent weather conditions were conducive to disease development and may have generated 12 or more infection periods from bud break to the end of June.

1. Leaf infection - Leaves were not rated in the Cuprofix plots as many of the most basal leaves (leaves most likely to be infected with Phomopsis) were either missing or were damaged by copper phytotoxicity. None of the other treatments had significantly fewer leaves infected than the check. However, the incidence of leaf infection was significantly lowered when Penncozeb was used instead of the 9 oz/A rate of TM41501 during early season growth.

2. Rachis infection - Programs consisting of early applications of Penncozeb, Sovran applied alone, or the 6 lb/A rate of Cuprofix provided good to fair control and had significantly fewer rachises infected with Phomopsis than the check. The good results on rachises from the two programs with Penncozeb applications at 4.6 and 8.7 in. shoot, suggest that this timing was important in 2000 for control of Phomopsis rachis infections. Programs composed solely of TM41501, Nova, or the lower rates of Cuprofix provided fair to poor control and did not have significantly fewer rachises infected than the check.

3. Internode infection - TM41501, Nova, and Cuprofix were poor at controlling incidence of Phomopsis on internodes. Sovran alone, although providing only 60% control, was the only treatment with significantly fewer internodes infected than the check.

Phytotoxicity: Cuprofix (Bordeaux) was phytotoxic to Concord leaves, shoots, and rachises. Symptoms of phytotoxicity were first noticed around the time of the fourth application, which was on 19 June. It was most severe on vines to which the highest rate (6 lb/A) was applied, being least severe on vines treated with the lowest rate (2 lb/A). The most severe symptoms included abscission and/or severe necrosis of the most basal 1-4 leaves (leaves having received 4 applications). Symptoms on leaves farther along shoots ranged from severe marginal necrosis and chlorosis, to stunting, cupping, and dark spotting. The canopies of vines sprayed with Cuprofix, in general, were reduced in comparison to unsprayed vines, the most severe reductions occurring on vines sprayed with the highest rate. Also, there was dark spotting and scabbing of green shoots, leaf petioles, and cluster rachises. There was no phytotoxicity observed with any of the other treatments.

18 Jul 2000

2 Aug 2000

15 Aug 2000

Final Rating 2000

1 Dates for spray timings: 1 = 11 May; 2 = 22 May; 3 = 7 Jun; 4 = 19 Jun
2 Dates for disease evaluations: Rachises and berries = 29 Sep; Leaves = 2 Oct; Internodes = 9 Oct.
3 Actual data are shown. Data were transformed by arcsin transformation before statistical analysis
4 Means followed by the same letters within columns are not significantly different according to Tukey-Kramer (P < 0.05).
5 No significant differences according to Tukey-Kramer (P < .05).
6 Bracketed [ ] values represent % control relative to the untreated check.