Botrytis gray mold control in strawberries and raspberries

Rainy weather and high relative humidity promotes Botrytis gray mold in strawberries and raspberries. A range of effective fungicides are available to help manage the disease. Alternate fungicide classes to avoid development of fungicide resistance.

Botrytis gray mold, caused by the fungus Botrytis cinerea, is one of the most important fruit rot diseases affecting strawberries and raspberries. Typical symptoms include a spreading brown rot and fuzzy gray mold on ripening berries. Frequent rains, high relative humidity, and moderate temperatures are conducive to development of this disease. As gray mold develops on infected berries, these become sources of inoculum secondary infections of adjacent berries. The gray mold fungus overwinters on old leaves and plant debris and can sporulate profusely on dead and decaying plant material. The spores are airborne and are usually plentiful in strawberry and raspberry fields. Where possible, remove sporulating berries from the field and destroy them to limit inoculum availability.

There are a number of excellent fungicide choices for gray mold control in strawberries: Switch (cyprodinil and fludioxonil) and Pristine (pyraclostrobin and boscalid) provide excellent control and both have two different active ingredients, one of which is systemic, which broadens their spectrum of activity. Pristine also provides outstanding control of fungal leaf spots and anthracnose fruit rot.

New strawberry fungicides that have excellent activity against Botrytis gray mold are Fontelis (penthiopyrad) and Inspire Super (difenoconazole and cyprodinil). Elevate (fenhexamid) is a locally systemic fungicide with good to excellent activity against gray mold. Captevate is a pre-mix of captan and fenhexamid and has a broader spectrum of activity than Elevate alone as it also protects against anthracnose and leaf spots. Scala (pyrimethanil) is a newer fungicide labeled for Botrytis gray mold control in strawberries and is similar to one of the active ingredients in Switch.

Rovral (iprodione) is an older fungicide with good activity against Botrytis gray mold, but they can only be applied once in strawberries and up to four times per season in raspberries; activity is enhanced by adding a spreader-sticker. With respect to older fungicides for strawberries, a tank-mix of Topsin M (thiophate-methyl) and Captan (captan) provide good efficacy against a broad spectrum of fungi, including gray mold. Thiram (thiram) is a broad-spectrum strawberry fungicide with decent efficacy against gray mold as well but is strictly a protectant.

Just as a reminder, Cabrio (pyraclostrobin) and Abound (azoxystrobin) are not suitable for gray mold control, but are effective against anthracnose and other fruit rot and leaf spot diseases. All fungicides mentioned above have a zero-day pre-harvest interval, except Topsin M (one day), Scala (one day) and Thiram (three days) in strawberries.

Remember to alternate fungicides in different fungicide classes for resistance management purposes. A table showing fungicide classes is available in the “2014 Michigan Fruit Management Guide,” Michigan State University Extension bulletin E-154. Be careful using older fungicides like Captan, Rovral and Thiram when bees are foraging as these fungicides may be toxic to the brood when they are carried back into the hive by the worker bees. Some other fungicides may have toxicity in combination with certain insecticides or adjuvants. It would be best to spray in the evening during dry conditions or to avoid using these materials altogether.

Dr. Schilder’s work is funded in part by MSU’s AgBioResearch.

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