Pythium Damping-Off of Seedless and Pollenizer Watermelons

From Clemson Plant Pathologist Tony Keinath.

Stand loss in transplanted spring seedless watermelons is usually not a serious issue in South Carolina, due to relatively sandy soils in the major production areas. Two pathogens that can cause post-transplanting damping-off and plant death are the fungus Fusarium and the water mold Pythium. Fusarium can kill seedlings of very susceptible cultivars, such as those with no resistance to race 1, and cause slow plant death of cultivars resistant to race 1 but susceptible to race 2 for 3 to 6 weeks after transplanting. Young watermelons with Fusarium wilt will, well, wilt, but stay green for a bit, and the stem doesn’t collapse.

A young watermelon affected with Fusarium wilt 4 weeks after transplanting. Note that the wilted leaves and vine are still green.

Symptoms of Pythium stem and root rot on seedless and pollenizer watermelon included collapse and browning of the entire young plant. Girdling of the lower stem at the soil line may be visible. The “wiggle test” can be used to check root growth of diseased transplants without pulling up the plant. Grasp the seedling at the base of the stem and wiggle it back and forth. If there is resistance to movement, the root ball is intact, and the problem is probably Fusarium. If the plant moves easily, the problem is likely Pythium.

A young pollenizer watermelon affected with Pythium stem and root rot. Note the brown leaves and girdled lower stem above the roots.

A few of the many seedless watermelon cultivars were screened to see how susceptible they are to Pythium. Estrella is very susceptible, while Joyride is pretty resistant. Growers should be cautious about using Estrella as a pollenizer because of its extreme susceptibility to Pythium.

Some pollenizers, like SP-6 and new SP-7, are resistant to both race 1 and race 2 of Fusarium wilt. If these two cultivars are dying 3 to 4 weeks after transplanting, the most likely cause is Pythium. Pythium stem and root rot was found on 12-15% of SP-6 and SP-7 pollenizers in a watermelon cultivar trial at Coastal REC in April 2022. Five species of Pythium, three cool-season species and two warm-season species, were identified by the Clemson Plant and Pest Problem Clinic.

In a growth chamber test, Estrella and SP-6 had more damping-off than SP-7. Cool-season Pythium caused more damping-off than warm-season species. This difference also was seen in a field study, where Pythium stem and root rot were more severe in the spring, when plants were transplanted into the soil around 70°, than in the fall, when transplants went into warm soil. In general, growers may want to choose SP-7 instead of SP-6 as a pollenizer because it is less susceptible to Pythium.

Damping-off of Estrella seedings in a growth chamber test caused by a warm-season species of Pythium.

Other pollenizer watermelons, such as Wild Card Plus, Ace Plus, and Wingman, were not included in this test. All three have resistance to Fusarium wilt race 1 but not to race 2. Thus, death of these transplants could be caused by Fusarium race 2 or Pythium. Because race 2 Fusarium is widespread in South Carolina soils, pollenizers with resistance to Fusarium races 1 and 2 should be chosen to ensure a good stand of pollenizers to provide enough pollen for a good fruit set.

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