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Alternative strategies for control of Botrytis bunch rot for 2004

Investigations into the causes of Botrytis bunch rot development (Botrytis cinerea) and its control at the Lake Erie Regional Grape Research and Extension Center

Penn State research from 2001 to 2004 has documented the dramatic effect of grape cluster compactness on the development of Botrytis bunch rot. One way to reduce the compactness of clusters is to reduce the percentage of flowers that set fruit. The flow of photosynthate from leaves to clusters around bloom affects fruit set, and competition for photosynthate between clusters, and between clusters and shoots (1, 6), can be manipulated to reduce fruit set and loosen clusters. Research at Michigan State University on Vignoles grape has shown that removal of leaves at nodes 1-4 on fruit bearing shoots at the onset of bloom (trace bloom) starves clusters of carbohydrates and hormones; fruit set and hence bunch rot is reduced (Stanley Howell, personal communication). This technique was applied to Vignoles grape in disease management trials in 2002 and 2003 at the Lake Erie Regional Grape Research and Extension Center in Pennsylvania. The result was a 20%-30 % reduction in fruit set and a reduction in bunch rot of 59 % and 65 % respectively (3, 4). However, leaf removal entails an additional production cost that may be hard to justify in some varieties. Growers want the development of a cluster loosening method that can be applied with conventional spray equipment which would address these economic constraints. Other treatments that inhibit the photosynthetic activity of leaves around the time of bloom may have a similar effect on fruit set, to trace bloom leaf removal.

Research has shown that the application of mineral and plant-based horticultural oils can reduce photosynthetic activity of leaves. The degree of reduction is dependent on oil rate and application volume (2). Unfortunately, when several oil applications are made, particularly around veraison, the reduction in photosynthesis can lead to undesirable reductions in sugar accumulation in fruit clusters at harvest (5). For this reason, it is recommended that growers not apply oils at veraison or later. However, one or two applications around bloom would be expected to have little effect on sugar levels at harvest. Many horticultural oils are already registered as fungicides and their use around bloom can reduce powdery mildew on fruit which may indirectly reduce bunch rot. Horticultural oils are used in organic wine production programs to control powdery mildew and the results of this study may provide a basis for an additional tool for organic bunch rot control. The objectives of this study are to determine the effect of various rates and timings of applications of horticultural oils on fruit set, powdery mildew, Botrytis bunch rot, and juice quality, and to compare their efficacy to more expensive cultural practices like trace bloom leaf pulling and cluster clipping.

1) Coombe, B.G. 1962. The effect of removing leaves, flowers, and shoot tips on fruit set of Vitis vinifera L. J. Hortic. Science 37:1-15.
2) Finger, S.A. 2000. Effects of horticultural oils on photosynthesis, fruit maturity, and yield of wine grapes. M.S. Thesis, Department of Horticulture Virginia Polytechnic Institute.
3) Hed, B. and J.W. Travis. Evaluation of fungicides and cluster loosening for control of Botrytis bunch rot of grapes, 2002. F&N Tests 58:SMF008
4) Hed, B. and J.W. Travis. Evaluation of cultural control and Symspray/Vigor Cal for control of Botrytis bunch rot and total rot of grapes, 2003. Biological and Cultural Tests 19: N004.
5) Northover, J. and C.A. Homeyer. 1998. Efficacy of petroleum oil against powdery mildew and Botrytis bunch rot and its depression of total soluble solids in juice of Canadian grown grapes. (Abstr) Phytopathology 88 S67.
6) Quinlan, J.D., and R.J. Weaver. 1970. Modification of pattern of the photosynthate movement within and between shoots of Vitis vinifera L. Plant Physiology 46:527-530.

Materials and Methods: This trial was conducted with mature Vitis interspecific hybrid 'Vignoles' grapevines trained to a single curtain, high wire cordon system at the Lake Erie Regional Grape Research and Extension Center, North East, Pennsylvania. 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 100 psi and 100 gal/A. Ultra-Fine Oil (petroleum based oil) and SprayTech Oil (soybean oil) were applied with a backpack sprayer at 30 psi and 150 gal/A to ensure good coverage. Treatments received two pre-bloom and two post-bloom applications of conventional fungicides for control of all diseases except Botrytis. Because oils control powdery mildew, we were able to omit the powdery mildew chemical fungicide from the immediate post-bloom application, without sacrificing control. Vangard (10 oz/A) was applied at pre-closure and veraison to most treatments (tables 1 and 2). Oils were applied either 4 days before bloom or at trace bloom, and again at mid bloom. At late bloom (2-3 days after the second oil spray), photosynthetic activity of leaves was measured in all 2 % oil treatments and in the check to document effects of oil applications on photosynthesis. There was little or no phytotoxicity associated with oil treatments. Fruit set, juice quality (acidity, pH, and brix), compactness, average weight of clusters, Botrytis bunch rot, and total rot were determined for each treatment.

Results and Discussion: Rainfall was frequent and abundant throughout the 2004 growing season providing many opportunities for Botrytis to become established in clusters.
Effects on photosynthesis Oils successfully reduced photosynthetic activity of leaves by 36%-50 %, leaf transpiration rate by 20%-42 %, and stomatal conductance by 25%-52 %.
Effects on fruit set and compactness Trace bloom leaf removal and all oil treatments reduced fruit set and cluster compactness. Fruit set reductions ranged from 20 % with leaf removal to 11%-35 % in oil treatments. Compactness was reduced 13 % by leaf removal and 2%-18 % by oils, likely improving fungicide spray penetration into clusters. Ultra-Fine oil (2 %) at trace and full bloom (4 days apart) was the most effective treatment at reducing fruit set and cluster compactness over the check.
Effects on juice quality and cluster weights All Vangard treatments amended with leaf removal or oils had higher brix levels and lower cluster weights than Vangard alone. Amending Vangard with cluster clipping (after shatter), leaf removal, or oils resulted in brix levels that were 0.2 (clipping), 0.6 (leaf removal), and 0.6-2.6 (oils) brix greater than those in Vangard alone. However, oil and cultural amendments also reduced cluster weights, which likely contributed to higher sugar levels. Oil applications and leaf removal reduced cluster weights by 12%-27 % and 17 % respectively, whereas clipping (after shatter) only resulted in a 3 % reduction in cluster weight. Yield loss resulting from lower cluster weights can be offset by increasing bud number, but higher yields may not always be desirable when quality is a top priority. There was little effect of treatments on juice acidity and pH.
Effects on Botrytis bunch rot (BBR) and total rot (TR) control: Tables 1 & 2.
Non chemical cultural programs - Cluster clipping, after shatter significantly reduced the severity of BBR and TR (both by 60 %) over the unsprayed check, was statistically equivalent to two Vangard applications, and was significantly more effective at reducing the severity of BBR and TR than trace bloom leaf removal.
Oil and cultural amendments to a Vangard program - Cultural amendments (leaf removal, cluster clipping) and all oil amendments to Vangard improved BBR control over Vangard alone. However, the improvements were not statistically significant. There were no significant effects of oil rate, timing, or product. SprayTech oil treatments reduced BBR and TR by 22 %, whereas identical Ultra-Fine Oil treatments reduced BBR by 46 % and TR by 42 % over Vangard alone. All oil and cultural amendments to Vangard improved TR control numerically, but only cluster clipping (after shatter) significantly improved TR control over Vangard alone. Amending Vangard with Ultra-Fine Oil (1.5 and 2 % at trace and full bloom), leaf removal, or cluster clipping (after shatter) produced the best results, reducing the level of BBR found in the Vangard program by 42%, 77%, 60%, and 77 %, respectively.

These treatments may serve as alternatives to additional chemical applications in resistance management programs and/or in the production of late harvested Vignoles.
Powdery mildew was almost completely controlled in all treatments. Treatment with oils required one less powdery mildew chemical fungicide application at immediate post-bloom.

Table 1 Botrytis bunch rot

z Timing: 1 = 10 Jun (immediate pre-bloom); 2 = 15 Jun (trace bloom); 3 = 18-19 Jun (full bloom); 4 = 6 Jul (after shatter); 5 = 9 Jul (pre-close); 6 = 24 Aug (veraison).
y Percent Control = control of severity on clusters over that of the unsprayed check.
x Means followed by the same letters within columns are not significantly different according to Tukey-Kramer (P < 0.05).

Table 2 Total rot

z Timing: 1 = 10 Jun (immediate pre-bloom); 2 = 15 Jun (trace bloom); 3 = 18-19 Jun (full bloom); 4 = 6 Jul (after shatter); 5 = 9 Jul (pre-close); 6 = 24 Aug (veraison).
y Percent Control = control of severity on berries over that of the unsprayed check.
x Means followed by the same letters within columns are not significantly different according to Tukey-Kramer (P < 0.05).

The application of oils around bloom reduced cluster rot and enabled us to eliminate a synthetic chemical application for powdery mildew (resistance management) without sacrificing powdery mildew control. At comparable rates and timings, SprayTech Oil was more effective than Ultra-Fine Oil at reducing photosynthesis, transpiration, stomatal conductance, and berry and cluster weight, but less effective than Ultra-Fine Oil at producing intended beneficial effects like reducing fruit set, compactness, Botrytis bunch rot, and total rot. An improved understanding of the impacts of bloom oil treatments may provide more cost effective options for improving the quality of wines made from healthier grapes, while reducing Botrytis and powdery mildew chemical fungicide applications.

The lower portion of Vignoles clusters is typically the most compact and most rot-prone area of the cluster. In 2004, a cluster clipping treatment was added to the trial that involved removal of the rot-prone lower part of the cluster after berry shatter. Earlier trials examined cluster clipping before bloom, and although some reductions in BBR were obtained with this treatment, clipping after shatter may be more effective. Although it is easier to clip clusters before bloom (less canopy, maybe less expensive to implement), pre-bloom clipping can increase fruit set, offsetting some of the benefit. Waiting until after shatter to clip allows one to clearly discern how much to clip and has no effect on fruit set. Clipping after shatter was more effective at reducing rots than leaf removal, oils, or pre-bloom clipping in 2004. This treatment has been added to the protocol for 2005, for a more detailed examination of its effects and benefits.

This work was supported by funding from the Pennsylvania Wine Marketing and Research Board and the Viticulture Consortium/New York Wine and Grape Foundation..