Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Most growers should have had at least 2 fungicide sprays for Gray Mold on strawberries already in 2013.
Gray mold is caused by the fungus Botrytis cinerea. The reservoir for this fungus is mycelium in dead strawberry leaves. This mycelium becomes active in the spring and starts to produce spores on this 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: http://extension.udel.edu/ag/files/2012/03/DEvegrecs2013.pdf
Start sprays at 5 to 10 percent bloom, because 90% of fruit infections occur through the flower, and repeat every 7-10 days. Increase spray intervals during persistent dry periods, but decrease intervals to 5-7 days during very wet periods.
Four weekly sprays starting at 5-10% bloom are usually sufficient for season-long control. Tank-mix and rotate fungicides from different FRAC codes to reduce the chances for fungicide resistance development.
Suggested Program:
Application #1, apply one of the following:
Captan–4.0 lb 50WP/A plus Topsin M–1.0 lb 70WP
Switch–11.0 to 14.0 oz. 62.5WG/A
Application #2, apply one of the following:
Elevate–1.1 to 1.5 lb 50WDG/A
Pristine–18.5 to 23.0 oz 38 WG/A
Application #3:
same as Application #1
For subsequent applications, rotate between two or more of the following fungicides:
Captan–4.0 lb 50WP/A,
Captevate–3.5 to 5.25 lb 68WDG/A
Elevate 1.1 to 1.5 lb 50WDG/A
Switch–11.0 to 14.0 oz. 62.5WG/A
Pristine–18.5 to 23.0 oz 38WG/A
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.
The following is information on that testing program from Dolores Fernández-Ortuño and Guido Schnabel, Clemson University
“If you are a strawberry grower and you are interested in getting your farm-specific resistance profile to identify ineffective fungicides, send us around 40 dead strawberry flowers or collect spores from newly infected, decaying fruit with a cotton swab. We need about 10 to 15 of those swabs (each from a different fruit and each fruit from plants far enough apart to represent an acre or so). Make sure that you only collect the fungus spores, do not touch the fruit. Mail the flowers or the swabs to Guido Schnabel, Clemson University, 114 Long Hall, Clemson, SC 29634 and tell us the origin of the sample, your name, and e-mail so that we can send you the report electronically. Upon receipt, we need about 3 (for cotton swabs) to 5 (for flowers) working days to get a report to you outlining farm specific gray mold management recommendations.”