2023 Fusarium Wilt on Seedless Watermelon Experiment – Week 5 Observations

From Clemson Plant Pathologist Tony Keinath

This year’s Fusarium wilt experiment includes 3 seedless cultivars, 3 fungicide treatments applied via drip irrigation, and grafting. (This experiment is supported in part by a USDA SCRI grant.) Transplants were set April 5 in a field infested with Fusarium races 1 and 2.

Right on schedule, symptoms of Fusarium wilt appeared in most plots by 5 weeks after transplanting, May 9.

Typical symptoms of Fusarium wilt on this plant include wilting on one vine (left) with the other vines looking healthy.
Increase of Fusarium wilt between 3 (day 115) and 5 (day 129) weeks after transplanting into infested soil.

It’s a bit early to assess treatments, but, so far, grafted plants have many fewer affected plants (0.2%) compared to non-grafted (15%). At this point, there is no clear effect of fungicides.

After strong winds the weekend of April 29-30, 36 plants on May 2 and 21 more plants on May 9 for a total of 57 plants (12% of the total on April 27, the first rating date) showed symptoms of collapse, dried leaves, twisted or brown stems at the soil line, and complete plant wilting. The symptoms looked like Pythium stem and root rot, but clearly were not Fusarium symptoms, mainly because the entire plant collapsed.

Collapsed plant due to either wind injury or Pythium stem and root rot. Symptoms look similar because both the injury and the disease affect the main stem or roots.

After culturing in the lab, 21% of plants sampled yielded Pythium or Globisporangium (formerly included in Pythium). [Side note: Pythium on cucurbits includes species active in warm soil, while the Globisporangium species typically recovered from diseased cucurbits are active in cool soil. May is a transition month between warm and cool species, particularly under black plastic mulch.]

This year, stand loss from wind damage was a bigger problem than Pythium stem and root rot. In this field, the rows and rye strips are oriented east to west—the wrong orientation along the coast, where the prevailing wind direction is from the west. The rye strips provided little wind buffer. Wind damage is commonly observed at Coastal REC when plants are too short to latch onto each other, but it was worse than normal this year. Grafted plants faired better in the wind than non-grafted plants, as <1% of the grafted plants were lost, but 15% of the non-grafted plants were.

Finally, two counts of the number of SP-7 plants with flowers were done. In one count, more grafted SP-7 had flowers than non-grafted, but in the other count, both grafted and non-grafted plants had similar numbers.

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