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Evaluation of fungicides for control of Botrytis bunch rot of 'Vignoles' grape, 2000.

This trial was designed to 1) compare the efficacy of 2 to 4 applications of Vangard at predetermined stages of cluster development, 2) evaluate Elevate, Switch and 3 strobilurin fungicides for efficacy on Botrytis, and 3) evaluate the use of 2 Botrytis models for timing fungicide applications1, 2. During flowering, the flower infection model by Nair and Allen, 1993, was used to advise the need for fungicide applications. After flowering, the model by Broom et al., 1995, was used.

The trial was conducted with ten year old vines trained to a single curtain, high wire cordon system at the Lake Erie Regional Grape Research and Extension Center, North East, PA. Treatments were applied to 3-vine plots in a randomized complete block design with 4 replications. Fungicides were applied with a Friend covered-boom plot sprayer at 200 psi and 100 gal/A. Other diseases were controlled with applications of Penncozeb 75DF (4 lb/A), Nova 40W (5 oz/A), Rubigan 1EC (4 oz/A), and Elite 45DF (4 oz/A), using a Kinkelder air blast sprayer. Rainfall for May, Jun, Jul, Aug, and Sep was 5.26, 5.04, 5.63, 2.12, and 5.69 in respectively. Botrytis incidence and severity were determined on 29 Aug and 26 Sep from 50 randomly selected clusters in the center of each plot. Severity was rated using the Barratt-Horsfall scale and converted to percent using Elanco conversion tables. Weather information for the infection models was obtained from an onsite Campbell Scientific weather station and Skybit weather service that provided 2 days of predictive weather forecast data. Incidence and severity of bunch rot were recorded at the first appearance of disease and one week before harvest.

Results

Infection pressure from Botrytis was considered high during bloom, due to sufficient wetness and temperature for close to maximum incidence of flower infection almost every day of the bloom period1. The first application of fungicide made to the model treatment coincided with the scheduled full bloom spray on 19 June. After bloom, there were 13 high risk and 8 medium risk infection indices generated from fruit set to harvest2.

For the model treatment, spray alerts that occurred between cluster closure (early July) and veraison (mid August) were ignored. It was assumed that berries were relatively resistant during this time and that spray penetration after closure of tight clusters would be poor. After veraison, five high and five moderate risk indices occurred and three more applications of fungicide were made to the model treatment on 17 August (3 days after veraison), and 6 and 19 September.

Only full season programs of Rovral, Vangard, and the strobilurin Flint (applied at a rather high 4 oz/A rate) provided significant reductions in bunch rot incidence and severity. Among the two full season Vangard programs, no differences were observed when the first spray was applied pre-bloom instead of at full bloom. Switch provided a significant reduction of incidence but not severity of bunch rot. Full season Elevate, Sovran, Switch, and BAS500 and all partial Vangard programs failed to significantly reduce bunch rot severity over that of the untreated check. Control of bunch rot in the model advised program was not significant.

In retrospect, reacting in a timely manner to every spray alert from bloom to harvest would have resulted in about 6 applications of Botryticide, assuming a 10-14 day period of protection. However, the addition of a second early spray before cluster closure and shortening the intervals between post-veraison sprays may have improved control in this treatment. The models may be useful to gauge infection pressure during times when sprays, based upon phenology, are normally scheduled (bloom, pre- bunch closure, veraison, and pre-harvest). This could help growers make more informed decisions on whether or not to spray, particularly during bloom and early fruit set. Year 2000 was a relatively wet year. The models may be more useful in reducing the number of sprays necessary in a relatively dry year.

1 Nair, N.G. and R.N. Allen. Infection of grape flowers and berries by Botrytis cinerea as a function of time and temperature. Mycol. Res. 97:1012-1014. 1993.

2 Broome, J.C., English, J.T., Marois, J.J., Latorre, B.A., Aviles, J.C. Development of an infection model for Botrytis bunch rot of grapes based on wetness duration and temperature. Phytopathol. 85:97-102. 1995.