Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Grape -- Botrytis Bunch Rot
 
Cause: Many fungal and bacterial organisms, of which Botrytis cinerea is the most important, can infect grapes and result in a bunch rot. B. cinerea has a large host range and grows and sporulates on most of them. The fungus overwinters and oversummers as black sclerotia on old cluster stems, on canes, and on mummified grapes. Spores spread by wind. Young, succulent shoots can be infected in spring, especially if injured by hail. Flower parts frequently are infected and can serve as a source of the fungus within the developing bunch. Wet weather favors infection and disease development, especially near harvest when canopies are dense and berries accumulate sugar. The fungus can quickly spread from berry to berry within ripening bunches and can develop readily on wounded or split berries.

Botrytis cinerea will infect grape berries from 53 to 86°F with as little as 4 hours of berry wetness. The number of berries infected, however, rises as hours of berry wetness increases. A forecasting program assigns a medium bunch rot risk at 60°F with 15 hours of berry wetness but a high risk if berries have been wet 17.5 hours. Fungicides are to be applied after a medium risk occurs during the growing season. This program has not been evaluated in the Pacific Northwest.

'Cabernet sauvignon' is the least susceptible wine grape grown in Oregon. 'Gewurtztraminer' is less susceptible than 'Chardonnay', 'Pinot noir', and 'Riesling'. Bunches that are more tight or compact are at higher risk of the disease.

Under certain environmental conditions, white grapes infected solely by B. cinerea may result in a special rot known as noble rot. These grapes are used to produce a very sweet wine.

Symptoms: Young shoots and leaves may be infected in spring and develop brown, water-soaked areas. These areas generally girdle the shoot, causing it to wilt and die back. The characteristic gray moldy growth may or may not be present.

Generally, rotted berries appear in late summer and autumn as small, brown spots on maturing berries. The berry skin may slip off easily when rubbed. Later, characteristic tufts of gray fungal growth appear on the surface of infected berries. Often, rotted berries are near the center of the bunch. The rot then spreads quickly and may encompass most of the bunch. Other organisms may invade the berries later, producing a large variety of colors, smells, and tastes. Occasionally, immature berries may develop a soft brown rot early in summer.

Note the tattered leaf due to hail injury, and the water soaked margin of necrotic tissue.

Note the brown sporulation typical of this fungus.

Close-up of Botrytis sp. spores.

 
Cultural control:

  1. Remove leaves adjacent to clusters at shatter (Eichhorn-Lorenz stage 27). Removing leaves before E-L 27 may decrease fruit set and yield, and after E-L 27 may be ineffective for Botrytis control. Avoid sunburn by only removing the east (or north) side leaves, which also aids in spray penetration and coverage.
  2. Use appropriate trellising system to increase aeration and sun exposure.
  3. Avoid excessive vegetative growth through rootstock management and judicious use of nitrogen fertilizer.
  4. If adjusting yields by dropping fruit, consider removal of clusters that are in contact with other clusters.
Chemical control: Spray at end of bloom or beginning of fruit set (E-L 25 to 26), just before berry touch (E-L 33), start of veraison (E-L 35), and 3 weeks before harvest before rain is expected. Focus more on applications before rain storms rather than vine growth stage. Fungal strains resistant to chemicals such as iprodione can develop quickly. To delay or prevent building up resistant fungi, tank-mix with other products and try to limit to two sprays, never more than three ('Chardonnay' may need three). Sprays later in the growing season are preferable to earlier ones.

  1. Botran 75 WSB at 2 to 4.7 lb/A. Do not use more than 5.3 lb/A/year. Fruit marking can occur on some cultivars when com-bined with sulfur. Effectiveness is considered low. A group 14 fungicide. 12-hr reentry.
  2. Captan 80 WDG at 2.5 lbs/A. Do not apply more than 24 lb/A/year. May be applied up to the day of harvest. Do not use with oils, lime or alkaline materials. Contact your grape buyer before application, as several wineries and/or countries will not allow its use. Rated from slight to moderate in effectiveness. 3 day reentry.
  3. Copper based products are registered but offer only slight control.
  4. Dithane DF at 1.5 to 2.5 lb/A. Do not use within 66 days of harvest or more than 8 lb/A/season. It is registered but considered ineffective. 24-hr reentry.
  5. Endura at 8 oz/A. Do not use more than 3 times per year or within 14 days of harvest. Better to use this product than Pristine due to rate limitations. A group 7 fungicide. 12-hr reentry.
  6. Elevate 50 WDG at 1 lb/A. Do not use more than 3 lb/A/season. Can be used up to and including the day of harvest. Good control. A group 17 fungicide. 12-hr reentry.
  7. Iprodione-based products. Do not apply more than two (2) applications per season or within 7 days of harvest for wine grapes. Good control when resistance is not a problem. A group 2 fungicide. 48-hr reentry.
    1. Iprodione 4L AG at 1 to 2 pint/A.
    2. Nevado 4F at 1 to 2 pint/A.
    3. Rovral 4 Flowable at 1 to 2 pint/A.
  8. JMS Stylet Oil at 1 to 2 gal/100 gal water. Tank mix with another fungicide. Necrotic foliage may result if applied within 10 days of any sulfur application. Do not tank mix with copper based products when fruit is present. Do not use during freezing temperatures, above 90°F, or when plants are under heat or moisture stress. Do not use when foliage is wet as good coverage is essential. Slightly effective alone. 4-hr reentry.
  9. Pristine can be used at 18.5 to 23 oz/A. Do not use more than two (2) consecutive applications, within 14 days of harvest, or more than three (3) times/year. Do not use on labrusca type grapes such as Concord and Niagra due to possible foliar injury. A group 7 and 11 fungicide. 12-hr or 5-day reentry based on activity.
  10. Scala SC at 18 fl oz/A alone or 9 fl oz/A in a tank mix. Do not apply more than 36 oz/A/season or within 7 days of harvest. A group 9 fungicide. 24-hr reentry.
  11. Topsin M WSB at 1 to 1.5 lb/A plus another fungicide. Do not use within 14 days of harvest or more than 4 lb/A/season. Resistant fungi, which make this product ineffective, are widespread in the Willamette Valley of Oregon. A group 1 fungicide. 7-day reentry.
  12. Vangard 75 WG at 10 oz/A when used alone or at 5 to 10 oz/A when tank-mixing with another fungicide. Buffer to a pH of 5 to 7 when tank-mixing with Rovral. Do not use more than 20 oz/A/season or within 7 days of harvest. Resistance management strategies must be used with this material. More effective at cooler temperatures. Moderate to good effectiveness rating when used alone. A group 9 fungicide. 12-hr reentry.
  13. Ziram 76 DF at 3 to 4 lb/A. Do not use after bloom. Effectiveness is considered low. 48-hr reentry.
Notes: Although Messenger is registered for use it is not recommended due to poor efficacy in the PNW.
Biological control: Serenade MAX (Bacillus subtilis strain QST 713) at 1 to 3 lb/A. Active ingredient is a protein. Variable efficacy in tests in western Oregon. 4-hr reentry.
References:
Johnson, K.B., T.L. Sawyer, and M. L. Powelson. 1994. Frequency of benzimidazole- and dicarboximide-resistant strains of Botrytis cinerea in western Oregon small fruit and snap bean plantings. Plant Disease 78:572-577.

Broome, J.C., English, J. T., Marois, J.J., Latorre, B.A. and Aviles, J, C. 1995. Development of an infection model for Botrytis bunch rot of grapes based on wetness duration and temperature. Phytopathology 85:97-102.
Content edited by: Jay W. Pscheidt on January 1, 2009
 
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