Don’t let bunch rots ruin your grapes

If humid conditions persist, Botrytis pressure may be high this year. Remove leaves from around grape clusters and protect clusters with effective fungicides.

Early symptoms of Botrytis bunch rot in grapes.

Early symptoms of Botrytis bunch rot in grapes.

This 2014 growing season, Botrytis bunch rot may be a bigger menace than usual due to prevailing moisture and high humidity. Botrytis bunch rot is caused by the fungus Botrytis cinerea and is the most common cause of pre-harvest losses of wine grapes. Another problem may be sour rot, caused by acetic acid bacteria, yeasts and certain fungi. Sour rot can explode after heavy rainfall during fruit ripening, resulting in swelling and bursting of berries, especially near woods. Fruit flies get in on the melee and help spread the microbes on their bodies. Botrytis and sour rot can be present in the same cluster, in addition to Phomopsis and black rot, so it is important to distinguish the culprits. Bunch rot often begins in one or a few berries, and can consume most of the cluster under the right conditions. Sometimes when rot is prevalent or developing faster than expected, the harvest date has to be moved up to avoid excessive losses.

There are various fungicides available for control of Botrytis bunch rot while very few aid in sour rot control. Sour rot is best controlled by leaf pulling around the clusters between bunch closure and veraison to reduce humidity and increase sun exposure. While biocontrol agents like Serenade (Bacillus subtilis), Actinovate (Streptomyces lydicus) and Blightban (Pseudomonas fluorescens) may help by competing with sour rot microbes, leaf pulling is the main control method. Growers in Ontario apply potassium metabisulfite to “dry out” the clusters and kill yeasts and bacteria.

Botrytis bunch rot
Mid-symptoms of Botrytis bunch rot in grapes.

For Botrytis treatments, Michigan State University Extension advises growers to make sure to alternate fungicides in different chemical classes to avoid fungicide resistance development. The Fungicide Resistance Action Committee (FRAC) code indicates the mode of action; any fungicides with the same FRAC number share the same mode of action. A conventional standard that has been quite effective in our fungicide efficacy trials is a Pristine/Vangard alternation: bloom (Pristine), bunch closure (Vangard), veraison (Pristine) and two weeks before harvest (Vangard). Adding another chemical class (e.g., Elevate) would be even better from a fungicide resistance management perspective. Biological control agents and reduced-risk products also can be used, particularly during lower risk periods (e.g., bunch closure) or when approaching harvest to avoid fungicide residues.

Botrytis bunch rot
Late symptoms of Botrytis bunch rot in grapes.

Below is a list of products with efficacy against Botrytis bunch rot.

  • Elevate: fenhexamid, FRAC group 17, locally systemic; zero-day pre-harvest interval (PHI). Good to excellent preventive and limited post-infection activity.
  • Endura: boscalid, FRAC group 7, systemic, 14-day PHI. Good to excellent preventive and post-infection activity. Use at an 8-ounce rate for Botrytis control.
  • Flint: trifloxystrobin, FRAC group 11, locally systemic, 14-day PHI. Moderately good preventive and limited post-infection activity. Works better at higher rate.
  • Inspire Super: difenoconazole + cyprodinil, FRAC groups 3 and 9, systemic, 14-day PHI. It is mainly the cyprodinil component that provides Botrytis control. Good to excellent preventive and post-infection activity.
  • Luna Experience: fluopyram + tebuconazole, FRAC groups 7 and 3, systemic, 14-day PHI, 10-day REI for leaf pulling, trying and training in wine grapes. Good to excellent preventive and post-infection activity.
  • Pristine: pyraclostrobin + boscalid, FRAC groups 11 and 7, systemic, 14-day PHI. Good preventive and post-infection activity, but only at the high rate of 18.5-23 ounces per acre.
  • Rovral: iprodione, chemical group, FRAC group 2, locally systemic, seven-day PHI. Moderate to good preventive and limited post-infection activity; activity is improved by addition of oil or non-ionic spray adjuvant.
  • Scala: pyrimethanil, FRAC group 9, systemic, seven-day PHI. Good to excellent preventive and post-infection activity.
  • Switch: cyprodinil and fludioxonil, FRAC groups 9 and 12, systemic, 14-day PHI. Provides good Botrytis control. Preventive and post-infection activity.
  • Topsin M: thiophanate methyl, FRAC group 1, systemic, 14-day PHI. Good preventive and post-infection activity.
  • Vangard: cyprodinil, FRAC groups 9, systemic, seven-day PHI. Good to excellent preventive and post-infection activity.

Reduced risk and biocontrol agents:

  • Botector (Aureobasidium pullulans): biocontrol agent, locally systemic, zero-day PHI. Good to excellent preventive and limited post-infection activity.
  • Regalia: giant knotweed extract, FRAC group NC, induced resistance, zero-day PHI. Good to excellent preventive and limited post-infection activity.
  • Serenade (Bacillus subtilis): biocontrol agent, protectant, zero-day PHI. Fair to moderate protectant activity. Organic formulation can be used in organic vineyards.

Dr. Schilder’s work is funded in part by MSU’s AgBioResearch.

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