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Alternative strategies for control of Botrytis bunch rot 2002-03

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

Background: Botrytis bunch rot, caused by Botrytis cinerea, is an economically important disease for wine grape growers in the Eastern United States. Bunch rot typically begins to develop in clusters as fruit begin to ripen. Clusters can be predisposed to disease development by: 1) latent berry infections, some of which are acquired months before ripening, that activate during ripening; 2) retention of floral debris after bloom that provides substrate for the pathogen; 3) wounding of berries, either by insect feeding, weather (wind, rain, sun, etc), or as a result of berry overcrowding in compact clusters. Bunch rot is generally more difficult to control in varieties with compact clusters because floral debris is more likely to be retained, insects and fungal pathogens are more likely to escape the effects of pesticides, and wounds to berries as a result of berry expansion commonly occur. Once clusters become infected, rot spreads more quickly and extensively in compact clusters.

Fungicide trials to determine the best materials and timing for bunch rot control, have been conducted for the past several years at the Penn State/Lake Erie Regional Grape Research and Extension Center (LERGREC) on interspecific hybrid 'Vignoles', a white French hybrid variety with small, compact clusters. Control with fungicides alone has been modest and inconsistent on Vignoles, regardless of material used (Rovral, Vangard, Elevate), number of applications (2 to 4), and timing (bloom, immediately before cluster closure, veraison, and 2 to 3 weeks before harvest). Disease control from Botrytis-specific sprays has generally been additive; control improves as more fungicide applications are made. However, the best timing is still unclear. Reliance on fungicides alone is not the answer to consistent and sustainable Botrytis control, and more integrated approaches are desirable, especially in wine grape varieties with compact clusters. In 2001, studies were initiated to determine the relative importance of some of the factors that predispose clusters to Botrytis bunch rot, such as latent infection, trapped floral debris, and cluster compactness. Subsequently, we began to explore new ways to reduce rot with cultural and alternative chemical treatments.

Latent infections: Latent Botrytis infections of grape clusters are infections that become established in the grape tissue but do not aggressively colonize the tissue. Therefore, the infection site remains symptomless and no rot occurs. If latent infections become active (usually during ripening), they can colonize, rot the berry, and then provide a source of inoculum for more extensive cluster rot. However, the majority of latent infections remain symptomless, as seen in a field sample of ripe, Vignoles clusters where 88 % of symptomless berries sampled at harvest, were infected with Botrytis, but rot severity was only 8 %. What activates latent infections? Three years of greenhouse trials with Chardonnay and Vignoles strongly suggests that the process of ripening alone does not activate latent infections, but that external factors (specifically those causing injury) can influence their activation. In three years of greenhouse trials, inoculation of clusters at fruit set established a high incidence of latent infection. But, all clusters remained healthy through ripening, as long as berries remained physically intact (uninjured). When ripe, symptomless, inoculated berries (with latent infections) were surface-sterilized, subjected to injury by being slightly torn at the berry-pedicel attachment (an injury which commonly occurs as a result of overcrowding in compact clusters), and placed in a high humidity environment. The majority of injured berries quickly rotted, whereas the vast majority of uninjured inoculated berries remained symptomless. The role of injury in the activation of latent infections is being explored further in 2005.

Retained floral debris: Grape clusters often retain dead unfertilized flowers, aborted berries, and caps (calyptrae), following bloom. Wind and rain, during bloom to cluster closure, removes much of this material, but some becomes trapped in clusters as berries expand. Retained floral debris has been shown to harbor Botrytis. Debris collected from symptomless Vignoles clusters at harvest revealed that about two thirds of the pieces of debris were capable of sporulating with Botrytis when placed in a high humidity environment. Retained debris may create a more favorable environment for cluster rot by retaining moisture, increasing humidity, and prolonging wetness inside clusters. In 2001, 02, and 04, hundreds of Vignoles clusters of variable compactness and amounts of retained floral debris were observed for development of Botrytis bunch rot. Debris retention generally increased with increasing compactness, but the correlation was weak. Bunch rot generally increased with debris retention, but the strength of the effect was influenced by compactness. For example, debris removal had no effect on bunch rot severity in loose clusters, but significantly reduced bunch rot in compact clusters.

The compactness of Vignoles clusters generally averages 9-10 berries per cm of the main cluster stem (or rachis). In 2002 and 2004, 20 loose (generally less than 9-10 berries/cm) and 20 tight (generally greater than 9-10 berries/cm) clusters, with and without debris removal, were paired and replicated 5 times (200 clusters/year), and examined to determine how compactness of the cluster (berries per cm rachis) affects the impact of debris retention on bunch rot development. The results appear in the tables below.

 

	Botrytis on clusters
2004	% clusters infected	% Area Clusters infected	debris/cluster	Compactness berries/cm
Loose/debris removal (n=50)	46	2.4	8.5	6.4
Loose/no debris removal (n=50)	32	2.1	25.3	6.5
Tight/debris removal (n=50)	94	21.6	7.1	12.5
Tight/no debris removal (n=50)	96	31.9	29.6	13.1
All clusters/debris removal (n=100)	70	12.0	7.8	9.5
All clusters/no debris removal (n=100)	64	17.0	27.5	9.8

In both years, reducing debris retention did not reduce the percentage of clusters infected with Botrytis (incidence), but did reduce the severity of Botrytis bunch rot and tended to have a greater effect in compact clusters than in loose clusters, where it had little or no effect. Bunch rot generally increased with debris retention, but the effect was dependent on compactness.

Cluster compactness: Vignoles grape clusters generally begin to close 2 weeks after fruit set and fungicide sprays cannot penetrate compact clusters to protect their inside surfaces during ripening, when fruit are most susceptible. To illustrate how cluster compactness affects spray penetration, we sprayed about 50 clusters of varying compactness, individually, by hand with canned spray paint, and then dissected clusters to determine the percent painted surface of the berries. As cluster compactness reaches maximum, spray residue only reaches a little better than half the total berry surface, even when coverage is unimpeded by canopy density, weather, spray equipment limitations, etc. Although it comes as no surprise, it clearly shows how the efficacy of late season sprays can be adversely affected by the compactness of the clusters.

Compact clusters can literally fill up with water during periods of precipitation and may remain wet much longer than loose clusters. This confounds efforts to identify the length of wetting periods useful in disease prediction models. For example, in 2001, 4 short precipitation periods (2-3 hours each) during 2 consecutive days in late August, resulted in only 6 full hours of wetness, registered by leaf wetness sensors. However, from the onset of rain, relative humidity remained above 80 % for the next 48 hours and compact clusters remained full of water for at least 52 hours!

During the growing seasons of 2001, 2002, and 2004, hundreds of clusters in a one acre, mature Vignoles block were examined for determination of effects of berry overcrowding (compactness) on the development of Botrytis bunch rot. Loose and tight clusters were selected as pairs based upon their close proximity to each other (on the shaded north side of the trellis) and their similar cluster length. Vines were sprayed for control of all diseases except Botrytis bunch rot. In each year, compactness had a dramatic effect on both the percentage of clusters infected with Botrytis and the severity of the infection. Loose clusters had significantly fewer berries per cm of the main rachis and significantly less severe bunch rot.

Effect of cluster compactness on Botrytis bunch rot in Vignoles; 2001, 2002, and 2004

Three years of cluster data were categorized according to compactness (3-4 berries/cm, 4-5 berries/cm, etc). Although this removes much of the variation in the data, it clearly illustrates the bunch rot/compactness connection.

Cluster compactness has long been known to be a major factor in bunch rot severity, but few if any practical management options exist for loosening clusters. In 2002, we initiated trials to examine potential methods of reducing compactness in Vignoles grape. Symspray (Agro-K Corporation), a seaweed extract product, is reported to lengthen cluster stems of some varieties, thereby reducing compactness. For 2 years we applied this product in accordance with the company's recommendation; when clusters were about 1" in length (about 10 days pre-bloom). This program was accompanied with 4 to 5 applications of Vigor-cal, a calcium carbonate product, applied at 2 week intervals. In addition, 2 cultural methods were tested: 1) removal of the lower, most compact 33%-50% of the cluster (clipping) where rot most frequently develops and most debris accumulates in Vignoles and 2) trace bloom removal of the oldest 4 leaves on a fruit bearing shoot, a method being studied and developed by Dr. Stanley Howell and his research staff at Michigan State University. Pre-bloom leaf removal starves clusters of carbohydrates during bloom, reducing fruit set and compactness. In 2004 we initiated a study to determine the effects of horticultural oils on fruit set, cluster compactness, and the development of Botrytis bunch rot.

Evaluation of fungicides and cluster loosening strategies for control of Botrytis bunch rot (Botrytis cinerea) of grapes, 2002, 03.
B. Hed* and J. W. Travis**
*Lake Erie Regional Grape Research and Extension Center, North East, PA 16428
**Penn State Fruit Research and Extension Center 290 University Drive, Biglerville PA 17307-1330

Trials were conducted with mature Vitis interspecific hybrid 'Vignoles' trained to a single curtain (no tie) 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. Botrytis-specific fungicides and Symspray/Vigor-Cal were applied with a Friend covered-boom plot sprayer at 200 (2002) and 100 (2003) psi and 100 gal/A. Using a Kinkelder air-blast sprayer, all plots received regular applications of a standard fungicide program for control of all other diseases. Rainfall in 2002, for May, Jun, Jul, Aug, and Sep was 8.13, 2.33, 2.61, 3.16, and 6.58 in., respectively. Rainfall in 2003 for May, Jun, Jul, Aug, and Sep was 9.83, 3.51, 6.53, 6.25, and 10.41 in., respectively. In 2002, Bunch rot incidence and severity were determined on 5 Sep (21 Brix) and 19 Sep (24 Brix) from 50 clusters in the center of each plot. In 2003, Botrytis bunch rot was determined on 13 Sep from 25 clusters per plot, and Botrytis bunch rot and total rot were determined on 30 Sep (21 Brix) from 30 clusters per plot.

2002 Results
Only trace amounts of rainfall occurred from pre-bloom to bunch closure, and relatively dry conditions characterized most of the midsummer period until shortly after veraison. Berry damage from overcrowding in compact clusters played an increasingly more important role in the development of bunch rot as ripening progressed. At 24 Brix, berries were extremely fragile and skins were easily compromised. Differences among treatments, particularly the amended treatments, reflect their effectiveness at reducing cluster compactness. All treatments significantly reduced bunch rot incidence through 5 Sep (21o Brix), and severity through 19 Sep (24o Brix) over that of the untreated check. Leaf removal controlled incidence through 19 Sep, over the check. All cluster loosening amendments further reduced bunch rot numerically, but only leaf removal resulted in a significant additional reduction in severity over Botrytis-specific fungicides alone.

z Application dates: 1 = 26 Jun (full bloom); 2 = 8 Jul (pre-close); 3 = 13 Aug (9 o Brix); 4 = 6 Sep (about 21o Brix).
y Severity was rated using the Barratt-Horsfall scale and was converted to % area infected using Elanco conversion tables.
x Actual data are shown. Data were transformed by arcsinsqrt transformation before statistical analysis.
w Means followed by the same letters within columns are not significantly different according to Tukey-Kramer (P < 0.05).
v Removal of four most basal leaves on 22 Jun at trace bloom (Dr. G.S. Howell, personal communication)
u Symspray applied on 10 Jun when clusters were at 1-2" in length
t Removal of lower third of cluster on 22 Jun at trace bloom.

2003 Results
Rainfall was frequent and abundant throughout the growing season. Overcrowding in clusters played an important role in the development of bunch rot, and differences among treatments reflect their effectiveness at reducing cluster tightness. As in 2002, most of the cluster rot was caused by Botrytis. All treatments significantly reduced the incidence and severity of Botrytis bunch rot (BBR) and total rot (TR) over that of the untreated check on 30 Sep. Treatments amended with leaf removal (at trace bloom) and cluster clipping (at 1 inch clusters) + Symspray/Vigor-Cal, were the most effective treatments and reduced BBR and TR over the sprayed check (Vangard alone), by 65%, 60%, 47%, and 44 %, respectively. However, none of these improvements in control were statistically superior to Vangard alone.

z Treatment Timing: 1 = 16 Jun (1 inch clusters); 2 = 24 Jun; 3 = 1 Jul (trace bloom); 4 = 13 Jul (pre-bunch closure); 5 = 17 Jul; 6 = 31 Jul;
7 = 13 Aug; 8 = 26 Aug (veraison).
y Severity was rated using the Barratt-Horsfall scale and was converted to % area infected using Elanco conversion tables.
x Actual data are shown. Data were transformed by arcsinsqrt transformation before statistical analysis.
w Means followed by the same letters within columns are not significantly different according to Tukey-Kramer (P < 0.05).
v Leaf Removal = removal of four most basal leaves at trace bloom (Dr. G.S. Howell, personal communication).
u Cluster clipping = removal of lower third of cluster.
t Percent Control = control of severity on berries over that of Vangard 75 WG 10 oz.

Conclusions: Over the last 2 years, leaf removal, integrated with fungicides, has been the most consistent and effective treatment with an average additional Botrytis bunch rot reduction of 62 % over fungicides alone at 21 Brix. In comparison, additional reductions from pre-bloom clipping have averaged about 29 % at 21 Brix. The Symspray/Vigor-cal program has been the least consistent and effective, reducing bunch rot by an additional 26 % on Vignoles in 2001, but providing no further reduction in 2002 on Vignoles, Pinot gris, or Riesling. There was never a significant cluster lengthening effect of Symspray/Vigor cal on these varieties (data not shown).

This work was partially supported by funding from the Viticulture Consortium/New York Wine and Grape Foundation and by materials supplied by Agro-K Corporation.