Bloom is a critical time for control of Botrytis gray mold in strawberries

Applying fungicides at bloom will reduce problems with Botrytis gray mold in strawberries at harvest.

Botrytis gray mold, caused by the fungus Botrytis cinerea, is one of the most important fruit rot diseases affecting strawberries. Typical symptoms include a spreading brown rot and fuzzy gray mold on ripening berries. Wet weather and moderate temperatures are conducive to development of this disease. The bloom period is the most important time for control of gray mold, since primary infections take place almost exclusively through the blossoms. The infections then remain dormant until the berries start to ripen. As gray mold develops on infected berries, these become sources of inoculum secondary infections of adjacent berries. Ripe and overripe berries in particular are very susceptible and gray mold can spread rapidly at that time.

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 fields. If the bloom period is dry or good fungicide coverage is maintained, incidence of gray mold at harvest will be low. However, if primary infections get established, it will be harder to control the disease both before and after harvest. 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; both have two different active ingredients – one of which is systemic –that broaden their spectrum of activity. Pristine also provides outstanding control of fungal leaf spots and anthracnose fruit rot. 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 and Iprodione (both iprodione) are older fungicides with good activity against Botrytis gray mold, but they can only be applied once and not after first fruiting flower. Also, their activity is enhanced by adding a spreader-sticker.

With respect to older fungicides, a tank-mix of Topsin M (thiophate-methyl) and Captan (captan) has good activity against a broad spectrum of fungi, including gray mold. Adding Kocide (copper hydroxide) or Cuprofix (basic copper sulfate) can help tackle angular leaf spot as well. Thiram (thiram) is a broad-spectrum fungicide with fairly good 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). Copper products have a 24-hour re-entry interval. Remember to alternate fungicides in different fungicide classes for resistance management purposes. A table showing fungicide classes is available in the 2012 Michigan Fruit Management Guide (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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