Small Fruit Alert: Rainy Conditions and Botrytis (Gray Mold) Management

Botrytis can affect leaves, cause a crown rot, blight blossoms, and infect fruit, so it can be a problem regardless of which part of the state you are in or the growth stage you are at. The biggest concern is that fungal spores that land on the flowers germinate and grow into the receptacle, which will become the berry, causing the fruit to rot during ripening. However, fruit can also be infected later directly from the outside. The typical gray fuzz may not be apparent when the fruit is wet.   

In 2023 and 2024, Pennsylvania botrytis resistance screenings were conducted by Dr. Mengjun Hu at the University of Maryland. Products where botrytis resistance was generally an issue across most farms were two older ones, Topsin M (topsin, FRAC code 1) and Rovral (iprodione, FRAC code 2), and among "newer" products to a slightly less extent, Elevate (fenhexamid, FRAC code 17) and cyprodinil (one of the active ingredients in Switch, FRAC code 9).  FRAC code 11 products (like Cabrio, pyraclostrobin) were not especially strong on botrytis, but that was not the disease they were intended to control in the first place—it was the second ingredient in products like Pristine (pyraclostrobin + boscalid, FRAC code 11 + 7) that provided botrytis control. 

However, this didn't mean that resistance was always widespread everywhere, and this points out why careful rotation of chemical classes (FRAC codes) is so important.  On one farm, only 11% of botrytis isolates were still susceptible to Elevate, whereas on another, 83% of botrytis isolates still were. These two farms had mainly the same cultivars in the same production system from the same nursery source, so the differences seemed to be farm-related.  

Please take a look at this article "Strategies for Effective Management of Botrytis and Anthracnose Fruit Rot in Strawberries," which includes a table at the end that outlines recommended products, taking overall fungicide resistance into account, along with tips for effective management and fungicide use.  If you no longer have a package of thiram labeled for strawberry use on your farm, captan may be substituted.

During rainy spells such as the one we have right now, apply the fungicide(s) before the rain event so residues are in place to minimize sporulation during and after the rain.  Doing so will minimize the number of spores "out there," decreasing the number of chances for resistance development.