From Clemson Plant Pathologist Tony Keinath.
Cucurbit growers should reduce the use of tebuconazole to manage gummy stem blight as the pathogenic fungus has become moderately resistant to this very commonly used fungicide.
Seven watermelon fields were sampled in 2019 (six fields) and 2021 (one field) in Beaufort, Bamberg, Barnwell, and Hampton counties. Of 229 samples of the gummy stem blight fungus, 93% were moderately resistant to tebuconazole when grown in the lab on culture medium plus tebuconazole. Only 3% of the samples were sensitive to tebuconazole, and 4% were highly resistant. Moderately resistant isolates were found in all seven fields at a rate of 86 to 100% of the samples per field.
Isolates collected in 2015 were sensitive to tebuconazole, so resistance seems to have developed in the period between 2016 and 2019. In 2015-2017, 42% of watermelon fields were sprayed with tebuconazole within a 2-week period. Put another way, 26% (one quarter) of all fungicide applications to watermelon during this period were tebuconazole. Widespread frequent use of generic tebuconazole likely contributed to resistance.
These results lead to the question: Does tebuconazole (Monsoon and other generics) still provide any control of gummy stem blight? In the greenhouse, tebuconazole was 99% effective against sensitive samples and still provided some control (74% of normal) against moderately resistant samples, but provided only 45% control of highly resistant samples. Most watermelon growers likely will see less efficacy of tebuconazole sprayed in the field to manage gummy stem blight.
Expected control of GSB with tebuconazole*
Alternatives to tebuconazole include mancozeb and chlorothalonil early in the season and Inspire Super and Miravis Prime later in the season. In the updated 2022 Watermelon Fungicide Guide I recommend only ONE spray of tebuconazole per season to reduce the risk that resistance will shift to highly resistant, as it has in other pathogens resistant to the DMI class of fungicides (FRAC Code 3).