Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Strawberry growers in the region should initiate programs to control Botrytis fruit and crown rots in strawberries.
Gray mold of strawberry fruit is caused by the fungus Botrytis cinerea. This pathogen can also cause crown rots that can weaken or kill plants. The crown rot phase of the disease often develops under floating row covers that are used to promote growth or protect against cold events.
The reservoir for this fungus is mycelium in dead strawberry leaves. This mycelium becomes active in the spring and starts to produce spores on the old leaf tissue, which then spread to blooms. Most infections start at the bloom stage but symptoms usually do not develop until close to harvest (the fungus does not become active until the fruit enlarges). Ripening fruits can also be infected. Conditions conducive for infection are temperatures between 70 and 80 °F and wet conditions (rain, dew, fog, irrigation). The most critical period for applying fungicides to control gray mold is during bloom.
Fungicide Recommendations From our Commercial Production Recommendation Guide:
Start spraying at 5-10% bloom, because most fruit infections occur through the flower. Repeat every 7-10 days. Spray less frequently during prolonged dry periods, but spray every 5-7 days during very wet periods. For season-long control it is usually sufficient to spray once a week for 4 weeks. Tank-mix and rotate fungicides from different FRAC codes to reduce the chance of fungicide resistance development.
Application #1, apply ONE of the following: Thiram 480DP 4.4 lb/A, Captan 80WDG 3.7 lb/A, or Switch 62.5WG 11.0 to 14.0 oz/A
Application #2, apply ONE of the following if resistance is NOT SUSPECTED on your farm: Fontelis 1.67SC (except Jewel, L’Amour and Clancy varieties) 16.0 to 24.0 oz/A, or Elevate 50WDG1 1.1 to 1.5 lb/A
If testing, observation, or frequent prior use of the above materials indicates high resistance risk, apply ONE of the following instead: M3 Thiram 480DP 4.4 lb/A, Captan 80WDG 3.7 lb/A, or Captevate 68WDG 3.5 to 5.25 lb/A
Application #3: Same as Application #1
Application #4. For subsequent applications, ROTATE BETWEEN the following fungicides with different modes of action: Fontelis 1.67SC (except Jewel, L’Amour and Clancy varieties) 16.0 to 24.0 oz/A, Switch 62.5WG 11.0 to 14.0 oz/A, or Elevate 50WDG1 1.1 to 1.5 lb /A.
Note: If Pristine is included in the schedule for Anthracnose control, in most cases that may provide satisfactory Gray Mold control and separate application of specific products may not be necessary.
If Botrytis is a problem it can often be traced back to poor sanitation (removing old leaves) from plantings, mistiming of bloom sprays, or a combination of the two. However, we are seeing resistance of Botrytis to several fungicides. If fungicide resistance is suspected, resistance testing may be warranted. There is a laboratory at Clemson University that is doing Botrytis resistance testing.
Strawberries should be cleaned of all dead leaves after row cover removal to limit Botrytis infections. Note freeze damage to flowers (black centers).