More Evidence that Choosing Grafting or the Right Cultivar is More Important than Using Fungicides to Manage Fusarium Wilt of Watermelon

By Clemson Plant Pathologist Tony Keinath

My 2024 Fusarium wilt experiment at Coastal REC, Charleston, SC, was a streamlined version of the 2023 experiment with 2 fungicides, 2 cultivars, and grafting, except we tested 1 different fungicide and 1 different cultivar. The 2024 results, however, matched the results from 2023 (see title).

Already at 3 weeks after transplanting, Cracker Jack had more diseased plants (5.8%) than Fascination (0.2%) did. Twelve of the 13 diseased plants were in Cracker Jack plots, because this cultivar is susceptible to races 1 and 2 of Fusarium, while Fascination is resistant to race 1.

After the hot spell over Memorial Day weekend, the number of wilted plants jumped noticeably, from 30 the previous week to 106 on May 28. In some plots, over half of the healthy plants developed symptoms, some quite severe, during  a week of hot, stressful weather (stressful to a diseased watermelon plant, that is).

Disease increased by 45% in Cracker Jack but only 21% in Fascination. As a result, there was a clear difference between the cultivars as shown in Figure 2.

Meanwhile, Cracker Jack grafted on Carolina Strongback rootstock had 0.5% wilted plants (1 of 200), and grafted Fascination had 0, which was significantly less than all non-grafted plants, whether they were treated with fungicides or not.

Speaking of fungicides, neither Propulse nor Miravis Prime had any effect on Fusarium wilt, as Figure 3 clearly shows. Fungicides did not work on either cultivar (data not shown).

Meanwhile, Cracker Jack grafted on Carolina Strongback rootstock had 0.5% wilted plants (1 of 200), and grafted Fascination had 0, which was significantly less than all non-grafted plants, whether they were treated with fungicides or not.

As seen in 2023, fungicides labeled for management of Fusarium wilt (Propulse, Proline, and Miravis Prime) have not worked in my heavily infested field. My conclusion, and recommendation, is that they probably also do not work in growers’ fields that may not be as heavily infested (although a few fields in SC are heavily infested).

To manage Fusarium wilt, watermelon growers should choose cultivars advertised with resistance to Fusarium wilt race 1 (Fon1), because that resistance, as seen with Fascination over the past 10 years, also helps reduce Fusarium wilt race 2. Growing susceptible cultivars, like Cracker Jack or Shoreline, will infest fields with a higher level of Fusarium each time they are planted, leading to long-term problems with Fusarium wilt. Grafting is the most effective option.