Today I observed watermelon transplants still in polystyrene trays that had symptoms of wilt and dieback. When the lower stem (hypocotyl) was sliced longitudinally, a discoloration could be seen in the vascular tissue on either side of the stem. In one case, the stem just below the seed leaves had a large external lesion that did not extend to the soil surface. Watermelon seedlings from an earlier outbreak of Fusarium wilt are shown below.
I have isolated from the seedlings in question. Although I will not know the results of the isolations until Sunday or Monday, I feel sufficiently sure of the diagnosis to let Indiana watermelon growers know of the possibility of Fusarium wilt in watermelon transplants. (Update: I was able to isolate the Fusarium wilt fungus from the transplants.)
Watermelon transplants that are suspected on having Fusarium wilt should not be planted in the field. Seedlings with severe symptoms will quickly die and the Fusarium fungus may survive in the field soil indefinitely. It is possible that apparently healthy seedlings may develop Fusarium wilt in the field after planting.
The fungus that causes Fusarium wilt may survive on used transplant trays and contaminated tools from the field. It is also possible that the Fusarium fungus can survive on seed. Fusarium wilt does not spread from plant to plant.
This year, Proline 480 SC is labeled for Fusarium wilt of watermelon. The label states that the product should be applied through drip irrigation. It is unlikely that any product will help severely affected seedlings.
Watermelon transplant with Fusarium wilt while still in transpalnt tray.
Photo by Dan Egel. (Click image for larger view)