Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Strawberry -- Gray Mold (Botrytis Fruit Rot)
 
Cause: Botrytis cinerea, a fungus that overwinters as sclerotia or dormant mycelia in old leaves, petioles, and mummified fruit. Conidia from within the planting are the principal inoculum and are produced readily and for a long time on diseased plant material. Conidia may infect leaves but infections generally remain symptomless until the leaf dies. Conidia readily infect the petals, stamens and pistals but not sepals. Flowers are most susceptible at anthesis. Mycelium then invades the developing fruit. Symptoms are generally delayed until fruit maturity and then progresses rapidly. ‘Northwest’ is very susceptible. ‘Benton’, ‘Hood’, ‘Linn’, ‘Olympus’, and ‘Rainier’ are moderately susceptible. ‘Shuksan’ and ‘Totem’ are least susceptible. No cultivar is immune, and even the least susceptible sustain considerable losses when environmental conditions favor disease development. The disease is more severe in high-density, double-row beds.
Symptoms: Blossom blight is characterized by petals and pedicels turning brown. The entire blossom may die. Fruit rot symptoms may occur on any portion of the fruit. They frequently develop at the calyx end and in tissues contiguous with rotting fruit or diseased flowers. Affected tissue turns light to medium brown. Lesions in green or white fruit develop slowly. The fruit may be misshapen as it enlarges. Fruit rot expands rapidly near harvest. In advanced stages, the fungus produces a gray mold over the fruit surface. Rot may not develop until after fruit is harvested.

Note the gray, fuzzy growth over these berries.

Soft, rotted fruit with a gray mold over the surface.

A basket of moldy berries.

 
Cultural control:

  1. Space plants so foliage dries rapidly after rain and irrigation.
  2. Pick fruit as it ripens and move it quickly to cold storage.
  3. Shorten picking intervals, if possible, when weather favors Botrytis.
  4. Use optimal fertilization.
  5. Time of renovation is important because fall fruit can provide considerable inoculum for the next growing season. In Oregon, it is recommended to renovate 2 to 4 weeks after the last harvest of June bearing types.
  6. Remove blossoms in the planting year.
  7. Under annual production in Florida it was found that removal of senescent, dead leaves reduced the amount of gray mold but did not improve overall yield. Removal of diseased fruit during harvest did not reduce gray mold or increase yield.
Chemical control: Apply during bloom period. Start at first bloom; repeat at regular intervals, especially during wet weather. Applications during fruit ripening are of questionable value. In the absence of floral applications, fruit ripening applications are worthless. In the presence of floral applications, data are incomplete on the value of fruit ripening sprays.
  1. Captan 80 WDG at 1.9 to 3.75 lb/A. May be applied up to the day of harvest. Do not use with oil or alkaline materials. Moderately effective. Hi-Yield Captan Fungicide 50 WP may be used in Oregon home gardens at 5 teaspoons/gal water. 24-hr reentry.
  2. CaptEvate 68 WDG at 3.5 to 5.25 lb/A Do not apply more than 2 consecutive application or more than 21 lb/A/season. Can be used day of harvest. 24-hr reentry.
  3. Elevate 50 WDG at 1.5 lb/A. Do not use more than 2 sequential applications or more than 6 lb/A/season. Can be used up to and including the day of harvest. 4-hr reentry.
  4. JMS Stylet Oil at 3 quarts/100 gal water is registered, but its effectiveness in the Pacific Northwest is unknown. Tank-mix with another fungicide. 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. 4-hr reentry.
  5. Pristine at 18.5 to 23 oz/A. Do not use more than 2 consecutive applications or more than 5 times/year. Can be used day of harvest. 12-hr reentry.
  6. Rovral and other iprodione products (such as Nevado) are registered but can not be used past first flower which makes it useless for controlling fruit rot. It is also registered as a preplant soak for transplants to control crown rot. Use at 2 pt/100 gal water, soak plants for 5 min and plant immediately. 24-hr reentry.
  7. Scala SC at 18 fl oz/A alone or 9 fl oz/A in a tank mix. Do not apply more than 54 oz/A/season or within 1 day of harvest. 24-hr reentry.
  8. Sporan EC (essential spice oils) at 1.5 to 6 pt/A. Unknown efficacy in the PNW. Zero hr reentry.
  9. Switch 62.5 WG at 11 to 14 oz/A. May be used up to and including the day of harvest. Do not apply more than twice sequentially or use more than 56 oz/A/season. Do not replant treated areas to anything other than crops listed on the label for 1 year after last application. 12-hr reentry.
  10. Thiram Granuflo at 3.4 to 4.4 lb/A. Do not use within 3 days of harvest. Moderately effective. 24-hr reentry.
Warning: Tolerant strains of some fungi have become troublesome when Rovral is used exclusively in a spray schedule. Resistant isolates have been found throughout Pacific Northwest production areas. To reduce the possibility of tolerance, alternate or tank-mix them with sprays of fungicides (such as captan or thiram) that have different modes of action. Do not apply more than twice a season.
Notes: Topsin M is registered for fruit rot control but is not recommended because resistant isolates of Botrytis are widespread, even on wild blackberries that have not been sprayed.

Although Messenger is registered for use the commercial efficacy of this product (a bacterial protein) is unknown in the Pacific Northwest and is not recommended.

References:
Johnson, K.B., Sawyer, T.L. and Powelson, M. L. 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.
Content edited by: Jay W. Pscheidt on January 1, 2009
 
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