Strategies for Effective Management of Botrytis and Anthracnose Fruit Rot in Strawberries

This article describes disease management strategies designed to slow further resistance development while also providing specifics for managing these two common fruit rots.

Botrytis is a major fungal pathogen causing pre- and post-harvest fruit rot. Use of fungicides remains a main tool for Botrytis management, but resistance in this pathogen has been detected to multiple fungicide groups. The resistant strains can survive and cause fruit rot in fungicide-treated fields. Resistance to iprodione (FRAC code 2, Rovral), fenhexamid (FRAC code 17, Elevate), boscalid (FRAC code 7, one of the active ingredients in Pristine), and cyprodinil (FRAC code 9, one of the active ingredients in Switch) is common. Boscalid has less inherent activity on botrytis species in general compared to other group 7 (SDHI) fungicides. Resistance to fludioxinil (FRAC code 12) and newer SDHI fungicides such as isofetamid (Kenja), penthiopyrad (Fontelis), and fluopyram (Luna series) has also been detected but at lower frequencies. Strobilurins (FRAC code 11) and DMIs (FRAC code 3) are ineffective against Botrytis.

Anthracnose fruit rot, caused by Colletotrichum species, is another major strawberry disease. The fungal pathogen has also developed resistance to single-site fungicides.  In particular, resistance to commonly used strobilurins has been widely detected in isolates from strawberries and other small fruit crops in the region.

The active ingredients listed above for these diseases are single-site fungicides, indicated by a single number in their FRAC code, aka activity group.  They target a single step in the fungus’s processes, so the fungi find it rather easy to overcome the fungicides’ effectiveness. Thiram, which works for Botrytis, and Captan are multi-site fungicides, as indicated by the "M" in their FRAC codes, so fungi are less able to overcome the multiple ways that the fungicides affect them.  Both play an important role in resistance management. Thiram and Captan are both protectants, meaning that they form a protective layer on the surface of plant tissues, thus limiting disease development. However, new growth that does not have this protective layer is vulnerable, and while the materials may simply be redistributed with light rains, heavy rains may wash the fungicides off.

More information related to fungicide resistance and the categorization of products can be found on the Fungicide Resistance Action Committee website.

What Can You Do to Manage Botrytis and Anthracnose?

Use all cultural methods that you can to maximize foliage drying.  Use sufficient plant spacing and keep weeds under control. Weedy fields stay wet longer, and Botrytis thrives under wet conditions. The optimum temperature for growth of Botrytis is 65 to 72°F, which is also good for strawberry growth, resulting in production of tender, easy-to-infect tissue.  If foliage stays wet for 14 hours or longer, Botrytis infections are favored, and spores are produced that easily drift onto blossoms, fruit, and leaves. Anthracnose is favored by wet conditions of 7 hours or longer, and its optimum temperatures for development are warmer (75 to 82°F). It is primarily a rain-splashed disease that does not spread over long distances.

Target Botrytis early in the season (i.e., during bloom) when applying fungicides, and both Botrytis and anthracnose later when temperatures are warmer using these strategies:

1. Use multi-site fungicides as the backbone of your spray program to minimize resistance development to the single-site fungicides mentioned above. Maintain continuous use of thiram (more effective for gray mold) and captan (more effective for anthracnose) during the critical disease control periods of bloom and fruit ripening. If you cannot find a source of Thiram, substitute Captan.  Use these products alone during dry spells when disease pressure is low. Apply before rain events so the material is affixed onto the foliage before the rain occurs.
2. Add single-site fungicides only when disease pressure is high (extended periods of moisture). Recommended single-site fungicides during bloom for Botrytis are category 7 (SDHI) fungicides (Merivon, Kenja, Fontelis, and certain products in the Luna series). They can also be applied during ripening if needed. If conditions conducive to anthracnose development occur earlier than usual, products effective against anthracnose may be applied.
3. Save Switch or Miravis Prime for late-season Botrytis and anthracnose management. Other fungicides effective on anthracnose appear in the table below. Ph-D or OSO is another good choice for B. cinerea control during harvest.

Overall, how you deploy these strategies will vary with your production system. Growers using plasticulture may need to focus more on anthracnose management, while growers using matted row production may need to focus on controlling Botrytis. Growers with day-neutral cultivars, which continually bloom and fruit over a long period, should try to focus on using the multi-site protectants thiram and captan before major or consecutive rain events.

All growers should use single-site fungicides only when necessary, such as when protectant sprays were missed prior to rain events. Rotate chemistries carefully, and minimize fungicide use during warm dry spells when fungicides are less likely to be needed.

The table below summarizes the information presented in this article with details on specific recommended fungicides, considerations for making the best use of them while minimizing resistance development, and their activity groups. Check individual product labels for rates, pre-harvest intervals, re-entry intervals, and any other restrictions before applying and make sure the products are registered for use in your state. Specimen labels can be found at CDMS, TELUS, and individual company websites.

Table 1. Management strategies with specific recommended fungicides and extra recommendations for early-season gray mold control and harvest time issues.

Written by Dr. Mengjun Hu, University of Maryland, and Kathy Demchak, Pennsylvania State University