Agriculture: Grape Pest Management Guidelines

Summer Bunch Rot (Sour Rot)

  • Summer Bunch Rot (Sour Rot): Alternaria tenuis, Aspergillus carbonarius, Aspergillus niger, Botrytis cinerea, Cladosporium herbarum, Penicillium sp., Rhizopus arrhizus
  • Symptoms and Signs

    As berries ripen and sugar content exceeds 8%, injured fruit become increasingly susceptible to invasion by a wide variety of naturally occurring fungi. Invasion occurs at the point of berry injury caused by insect or bird feeding, mechanical or growth cracks, or lesions resulting from powdery mildew infection or esca (black measles) berry damage that results in cracking. The resulting rot can be severe as it progresses beyond the original injury. Masses of black, brown, or green spores develop on the surface of infected berries. Bunch rots often culminates in sour rot, primarily in the central and southern San Joaquin Valley. Sour rot is caused by a variety of microorganisms, including acetic acid bacteria, which are spread by drosophila flies attracted to the rotting clusters.

    Melting decay or Non Botrytis Slip Skin (NBSS) of Redglobe and Crimson grapes is caused primarily by Hanseniaspora spp. These yeasts colonize the sugary and nutrient rich epidermis of berries after they are covered by the oozing liquid resulting from sour rot infections. Symptoms include hairline cracks in the berry skin, watery discoloration of berries, and general berry breakdown. Decay continues to develop slowly under cold storage conditions.

    Management

    Rotting fruit clusters present during veraison are indicative of summer bunch rot. Management of this disease complex is based on reducing injury or damage to the fruit, thus preventing invasion by bunch rot organisms. Basal leaf removal at or after berry set has given excellent control of summer bunch rot in the San Joaquin Valley. In warmer growing areas, be careful not to remove excessive numbers of leaves, which can lead to sunburned fruit. Remove leaves only from the side of the vine that receives afternoon shade. Also, leafhopper populations and damage caused by omnivorous leafroller have been reduced by this cultural practice. Treat at preclose and veraison if summer bunch rot has been a problem in the past.

    To reduce growth-related damage to the berries, follow proper irrigation, fertilizer, fruit thinning, and canopy management practices. Trellis and prune to achieve vine balance between vegetative growth and cluster number. Also control powdery mildew and damaging populations of omnivorous leafroller and other berry feeders.

    In table grapes, look for symptoms of summer rot on fruit during harvest to assess this year's management program and to prepare for next year. The presence of drosophila flies may indicate summer bunch rot infections. Control of NBSS can be achieved by controlling sour rot in the vineyard and in table grape vineyards, by not harvesting "dripped on" clusters.

    Organically Acceptable Methods

    Copper sprays are acceptable on organically certified grapes; check with your certifier for details.

    Common name Amount per acre** REI‡ PHI‡
    (Example trade name)   (hours) (days)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
    Note: Treatments can be made in conjunction with plant growth regulators and other applications.
    A. CYPRODINIL + FLUDIOXONIL
    (Switch 62.5WG) 11–14 oz 12 7
    MODE-OF-ACTION GROUP NAME (NUMBER1): Anilinopyrimidine (9) and Phenylpyrrole (12)
    COMMENTS: Do not apply in less than 21-day intervals. Do not make more than two sequential applications; rotate to a fungicide with a different mode of action.
    B. PYRACLOSTROBIN + BOSCALID
    (Pristine) 8–12.5 oz 12 14
    MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and Carboxamide (7)
    COMMENTS: Do not use on Concord, Worden, Fredonia, Niagara, or related grape varieties. Do not make more than two sequential applications; rotate to a fungicide with a different mode of action. The R.E.I. is 5 days for treated grapes when conducting cane tying, turning, or girdling.
    C. COPPER#
    (various) Label rates 48 See label
    MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
    COMMENTS: For tank mixes, observe all directions for use on all labels, and employ the most restrictive limits and precautions. Never exceed the maximum a.i. rate on any label when tank mixing products that contain the same a.i.
    ** Apply with enough water to provide complete coverage.
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI The longer of two intervals is the minimum time that must elapse before harvest.
    1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different Group number are suitable to alternate in a resistance management program.
    # Acceptable for use on organically grown produce.

    Important Links

    Text Updated: 12/16
    Treatment Table Updated: 12/16