Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu
Pythium damping-off of watermelon transplants in the field has been observed this week. In general this disease is less common now that our watermelons are grown from transplants instead of direct seeding to the field. However, it still occurs. Symptoms appear as water soaking or girdled appearance at the soil line. Roots appear stubby and often pinched and discolored. There are different species of Pythium that cause damping-off, and these different species have different temperature optima. Recent cool nights and rain may have contributed to the problem.
The disease can be managed by planting on raised beds, and using transplants that are beyond the first true leaf stage. If possible, reduce irrigation to allow the soil to dry somewhat. Mefenoxam metalaxyl and Uniform, which contains mefenoxam and azoxystrobin, are labeled for Pythium damping-off. Remember that the total amount of mefenoxam that can be applied in a season is limited (see label for details). In addition Previcur Flex also can be applied as a directed spray to the lower stem and soil, or applied through the drip irrigation. For organic growers, Double Nickel and Soilgard are biological fungicides that have OMRI labels.