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126 WINES&VINES January 2018 GRAPEGROWING PRACTICAL WINERY & VINEYARD SUMMER BUNCH ROT (SOUR ROT) IN CALIFORNIA By Paul Verdegaal Z infandel growers in California are accustomed to being on the lookout for sour rot (summer bunch rot) in any given year, and 2017 was such a growing season. Because of the many factors and pests involved in sour rot, the key to controlling it is still not well understood. Summer bunch rot develops after bunch closure, as berries begin véraison and sugar content exceeds 8° Brix (from personal observation, it seems to be about 75% of clusters with any color or softening, even one berry). It is hard to predict when, where and how much of a problem sour rot can be. Any injury or break in berry skins make clusters susceptible to invasion by a wide variety of fungi. Depending on region, variety and site, the range of fungi and bacteria can be extensive but usu- ally includes yeast species present, Acetobacter spp. and opportunistic molds such as Aspergillus niger, Alternaria carbonarius, Alternaria tenuis, Cladosporium herbarum, Rhizopus arrhizus Peni- cillium sp., Botrytis cinerea at times and others. With véraison, summer bunch rot becomes ap- parent with the odor of vinegar and the signs of "leakers" (clusters leaking juice on the vineyard floor). Ultimately, berries and clusters can dry and shrivel to a mass of rot infested with Drosophila fruit flies. As in nearly all other parts of North America where studies have been conducted, these are the common fruit fly, D. melanogaster, not the spotted wing drosophila (D. sizukii), which seems not to be a wine grape pest in California. Growing conditions are one factor, with actual vine growth and fruit development resulting in thin skins or berry cracking, management of water and nutrients, or damaging agents such as powdery mildew, Botrytis, insects, birds, etc. Tight clusters and varieties with thin skin are most susceptible, with Zinfandel most often affected in Lodi, Calif. Other varieties such as White Riesling and Petite Sirah can have the problem, along with Sauvignon Blanc and Pinot Grigio or old stalwarts like Carignan. In 2017, high rainfall in Lodi led to vigorous and rapid growth of shoots and fruit. Powdery mildew pressure, alternate periods of hot and cool weather and windy days, along with variable soil moisture as soils dried and required irriga- tion during heat waves, may have all combined to weaken skins. Sour rot followed in late July, later than normal. Research indicates some mitigation with preven- tive sprays including Switch, Pristine and maybe copper (see University of California IPM Guide- lines). But these are only slightly helpful and, for the cost, most often not a satisfactory strategy. Careful management of irrigation and nutrients, crop load and shoot growth of the canopy are the best that can be done. The 2017 season was difficult in Lodi in many aspects, and sour rot (especially in Zinfandel) was fairly severe. Paul Verdegaal is a University of California farm advisor for San Joaquin County. OBSERVATIONS FROM THE PAST 20 YEARS Year Sour Rot Level Year Sour Rot Level Year Sour Rot Level Year Sour Rot Level 2016 Very little 2011 Severe (and Botrytis) 2006 Severe 2001 None 2015 Scattered light 2010 Light (with berry shrivel) 2005 None 2000 None 2014 Moderate 2009 Light (mostly Botrytis) 2004 None 1999 None 2013 None 2008 Slight 2003 Light (Botrytis bad) 1998 Light (Botrytis) 2012 Light 2007 None 2002 Moderate 1997 None Periods of alternating hot and cool weather can weaken the skins of wine grapes, promoting sour rot. The 2017 growing season was difficult for Zinfandel growers battling summer bunch rot in Lodi, Calif. Chemical control trials In three spray trials conducted in 2013, 2015 and 2016 in Ge- neva, N.Y., various antimicrobial and insecticide treatments— both alone and in combination— w e r e a p p l i e d t o a v i n e y a r d planted to Vignoles (a tight-clus- ter interspecific hybrid variety) to test their effect on sour rot development. In each trial, alternate vine rows were sprayed with an insec- ticide (Delegate in 2013 or Mus- tang Maxx in 2015 and 2016) or left untreated. One- or two-panel plots were treated with antimi- crobial materials (potassium metabisulfite, Kocide 3000, Oxi- Date 2.0 or Fracture) at various timings and rates (see treatment list and results in the table "Con- trol of Sour Rot with Anti-Micro- bial Sprays +/- Insecticide" on page 124). Note that potassium metabisulfite (KMS), a common disinfectant in the winery, is not registered for use in the field but was included in these tests as a "proof of concept." Insecticides were applied weekly when fruit reached 15.0° Brix, the stage where berries start to become susceptible to sour rot. The start of weekly antimicrobial treatments varied; their timing is listed in the table as: • Pre-symptoms starting at 15° Brix, • Starting when sour rot symp- toms were first visible, • After the first rain following 15.0° Brix, • Following an increase in maxi- mum daily dew point over three days, and • No treatment. Insecticide (plus antimicrobials) provided control In 2013, antimicrobial and insec- ticide treatments applied in com- bination provided significant control of sour rot with a reduc- tion in disease severity (percent of cluster area diseased) of 30% to 54% compared to the un- sprayed treatment. The vines treated solely with antimicrobial sprays did not experience a sig- nificant reduction in sour rot, nor did the treatment in which only insecticide was applied.