Dan Egel, Vegetable Crops Pathologist, SWPAC, Purdue University
It is pretty obvious from figure one that this watermelon plant is wilting, but why is it wilting? If we play plant detective, we could dig down into the soil under the black plastic mulch that is used to warm the soil, keep weeds down and maintain moisture. In this case, we would find that there is plenty of moisture in the soil, thus the plant is not wilting due to lack of water. Let’s look back at figure 1. The entire plant isn’t wilting, only one vine. This is another hint that water is not the problem. Lack of water would cause the entire plant to wilt. Next, let’s dig up the entire plant and cut open the lower stem or crown of the plant. The result can be seen in figure 2. This figure shows a discolored area on the inside of the stem, specifically, the vascular tissue used to transport water from the roots to the rest of the plant. Only one side of the stem shows vascular discoloration, which corresponds to the one-sided wilt observed above ground. As a professor of mine would say, the problem is in the plumbing.
This watermelon plant has Fusarium wilt. The fungus that causes this disease, which is specific for watermelon plants, survives in the soil for years. Should the fungus come into contact with watermelon roots, the fungus invades the plant, resulting in clogged vascular tissue and perhaps death of the plant. The best way to manage Fusarium wilt of watermelon is to avoid planting watermelons in successive years and use varieties that have partial resistance to this disease.
Many other plants may become infected with vascular wilt diseases. Figure 3 shows a red bud tree with Verticillium wilt. The Veriticillium wilt fungus has caused a one sided wilt in the redbud tree in a similar fashion to the one-sided wilt observed with the watermelon plant shown above. The inside of the branch of the redbud tree shown would have similar discolored vascular tissues as shown for the watermelon plant in figure 2. The Purdue University extension bulletin BP-6 describes how to avoid Verticillium wilt in landscape trees by using trees with resistance and how to delay the onset of symptoms with pruning.
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