2023 Gummy Stem Blight Control for Watermelon Growers: Early is on Time

From Tony Keinath, Extension Vegetable Pathologist.

The recent freeze in the lower part of South Carolina more than ended the 2022 watermelon season. It’s not too early to plan for gummy stem blight control in 2023 to allow time to buy the right fungicides in case of shortages. As the saying goes, early is on time.

My fall 2022 gummy stem blight fungicide trial was done with a strain of the gummy stem blight fungus highly resistant to FRAC 3 (DMI) fungicides. Miravis Prime rotated with Inspire Super and mancozeb (2 applications of each fungicide, 6 total applications) was clearly the best treatment. Adding 2 applications of Miravis Prime improved control compared to Inspire Super rotated 1:1 with mancozeb.

Mancozeb alone (we used Mancozeb Max for testing purposes) performed better than expected in this trial, probably due to the dry weather this past fall, other than 5.6 inches of rain from Hurricane Ian at the very end of September that started the epidemic. Monsoon (the DMI tebuconazole) and Mettle (another DMI, tetraconazole) rotated with mancozeb performed worse than mancozeb weekly because of DMI resistance. These two fungicides clearly provided little control in the rotation.

Also because of resistance, the DMIs Rhyme (flutriafol), Inspire Super (the DMI difenoconazole premixed with FRAC 9 cyprodinil), and Proline (prothioconazole), all rotated 1:1 weekly with mancozeb, were similar to mancozeb. This means that their past performance, when they typically were better than mancozeb, is greatly reduced. However, Inspire Super and Proline were still better than Monsoon and Mettle. This difference between the 5 DMI products was confirmed in a greenhouse experiment. Similar to the field, mancozeb and Miravis Prime completely suppressed disease in the greenhouse.

The take-home messages are:

1. Don’t use Mettle, Rhyme, and generic tebuconazole products, like Monsoon, Savannah, and Tebuzol, to control gummy stem blight.
2. Add Miravis Prime to your fungicide mix to control gummy stem blight. Note that only 2 applications can be made per crop. This restriction is to prevent resistance to the Miravis active ingredient.
3. Include mancozeb or chlorothalonil (Bravo or generics like Echo) in a watermelon or cantaloupe spray program; see Table 3-52 starting on p. 316 of the Southeast Vegetable Crop Handbook for a list of generic fungicides.
4. The 2023 Watermelon Spray Guide allows one early-season application of tebuconazole so growers can use up existing fungicides. This exception will disappear in future years.