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Evaluation of fungicides for control of downy mildew and powdery mildew of 'Chancellor' grape, 2000

The objective of this trial was to examine the efficacy of three strobilurin fungicides, Kresoxim methyl (Sovran), Trifloxystrobin (Flint), and BAS 500, for the control of powdery and downy mildew of grape (cv. 'Chancellor'). Each of these fungicides was applied alone (solo) in a season long program, and as a tank mix with Mancozeb (Penncozeb) and Ziram. These treatments were compared to standard treatment programs of Penncozeb (mancozeb)/Nova (myclobutanil)/Ziram, and an unsprayed check.

This trial was conducted in a ten-year-old 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, and treatments were applied to 4-vine plots in a randomized complete block design with 4 replications. Fungicides were applied with a Friend covered-boom plot sprayer at 200 psi, applying 100 gal/A. Rainfall for May, Jun, Jul, Aug, and Sep was 5.26, 5.04, 5.63, 2.12, and 5.69 in. respectively. 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 4 May and full bloom on 16 Jun. Disease incidence on 25 Jul was determined from 50 randomly selected leaves and clusters per plot. Incidence and severity were determined on 24 Aug and 20 Sep from 25 randomly selected leaves and clusters per plot. Severity was rated using the Barratt-Horsfall scale and was converted to % area infected using Elanco conversion tables.

Results

Powdery Mildew: Disease pressure was relatively high, due to adequate rainfall that resulted in frequent primary infection periods. A pre-bloom and two post-bloom applications of Sovran, Flint (applied at the rate for downy mildew suppression, which is twice the rate for powdery mildew control), and BAS 500, were as effective as the season long programs of these materials respectively, suggesting that sprays 1 (3.6 inch shoots) and 2 (8.9 inch shoots) had a relatively small effect on powdery mildew development in this season. The Flint and BAS 500 treatments provided significant control of leaf, rachis, and berry infections. Sovran's performance was somewhat weaker. The Sovran treatments did not significantly control rachis infections and fell just inside of (season long (solo) treatment) or outside of (pre-bloom through 2nd post-bloom) significant leaf infection control. However, both Sovran treatments provided significant control of berry infections. Two pre-bloom plus two post-bloom applications of Nova (standard), failed to significantly reduce leaf, rachis, or berry infection over that of the untreated check, but did provide a level of control that was statistically equivalent to all other fungicide treatments with regard to berry infection and all treatments except the Flint and BAS500 solo programs with regard to leaf infection. Extending the standard treatment program by one additional application of Nova (VITIS treatment), extended protection beyond 3 weeks post-bloom to 4-5 weeks post-bloom with a resulting 5 % increase in the level of control on berries. This supports research by other investigators that berries of susceptible French hybrid varieties (like 'Chancellor') may be susceptible to powdery mildew until 4-5 weeks post bloom.

Downy Mildew: Disease pressure built up slowly, remaining low throughout most of the block. By August, an assessment of disease in all unsprayed buffer plots within the experimental block made it apparent that disease pressure was unevenly distributed and that some plots were more challenged by downy mildew than others. As a result, variability between plots of the same treatment (particularly with respect to downy mildew on clusters) was high. To further confound matters, most of the plots of the unsprayed check, both treatments containing Flint, and the BAS 500 solo treatment, were in areas of relatively low disease pressure. On the other hand, most of the plots of the treatments containing Sovran, particularly the solo treatment, were in areas of relatively high disease pressure, and were likely challenged the most. In support of this, more severe disease developed in the Sovran solo treatment than the unsprayed check.. Better control was observed when Sovran or Flint were combined with reduced rates of Penncozeb and Ziram than when these two strobilurins were applied alone season long. BAS 500 alone and combined with Penncozeb and Ziram provided complete control on clusters.

20 Sept.		Downy Mildew					Powdery Mildew
		Leaves		Clusters			Leaves	Clusters
								Berries	Rachis
		%	%Area	%	%Area		%Area	%Area	%Area
Treatment and rate/A	Timing1	Infected	Infected	Infected	Infected		Infected	Infected	Infected
Sovran 50WG 5 oz	1-5	76.0 cd2	1.85 cd2	61.03	11.413		72.61ab2	1.43 a2	82.41 c2
Penncozeb 75 DF 4 lb	1, 2
Penncozeb 75 DF 3 lb + Sovran 50WG 5 oz	3, 4
Sovran 50WG 5 oz + Ziram 76 DF 3 lb	5	50.0 abc	1.17 abc	24.0	3.48		73.67 abc	1.19 a	77.96 c
Flint 50 WDG 4 oz	1-5	76.0 cd	1.92 cd	28.0	3.99		59.80 a	0.30 a	53.43 b
Penncozeb 75 DF 4 l b	1, 2
Penncozeb 75 DF 3 lb + Flint 50 WDG 4 oz	3, 4
Flint 50 WDG 4 oz + Ziram 76 DF 3 lb	5	33.0 ab	0.77 ab	6.0	0.14		65.67 ab	0.40 a	50.16 ab
Standard treatment A Penncozeb 75 DF 4 lb	1
Penncozeb 75 DF 4 lb + Nova 40 W 4 oz	2, 3
Nova 40 W 4 oz + Ziram 76 DF 4 lb	4, 5	51.0 abc	1.22 abc	32.0	4.81		81.89 bc	4.28 ab	95.21 c
Standard treatment B Penncozeb 75 DF 4 lb	1
Penncozeb 75 DF 4 lb + Nova 40 W 4 oz	2, 3
Nova 40 W 4 oz + Ziram 76 DF 4 lb	4-6 4	59.0 bcd	1.38 bc	33.0	3.71		81.96 bc	3.41 ab	95.79 c
BAS 500 20WG 12oz	1- 5	15.0 a	0.35 a	0.0	0.0		58.65 a	0.19 a	38.58 ab
Penncozeb 75 DF 4 lb	1, 2
Penncozeb 75 DF 3 lb + BAS 500 20WG 12oz	3, 4
BAS 500 20WG 12oz + Ziram 76 DF 3 lb	5	33.0 ab	0.77 ab	0.0	0.0		65.74 ab	0.19 a	29.58 a
Untreated Check		95.0 d	2.51 d	41.0	7.54		90.00 c	17.09 b	97.31 c

1 Dates for spray timings: 1 = 12 May; 2 = 26 May; 3 = 9 Jun; 4 = 20 Jun; 5 = 2 Jul; 6 = 13 Jul
2 Means followed by the same letters within columns are not significantly different according to Tukey-Kramer (P < 0.05).
3 No significant differences according to Tukey-Kramer (P < .05).
4 Sixth application on 13 Jul timed by VITIS expert system.

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.