Fusarium wilt of watermelon is one of the most economically important diseases of watermelon in the Midwest. Fusarium wilt typically shows up in watermelon fields just after Memorial Day. This year is no different. After a brief description of the symptoms of the disease, this article will discuss the pluses and minuses of possible fungicide treatments at this time of the season. That is, this article will discuss whether one should apply fungicides for Fusarium wilt of watermelon 3-4 weeks after transplant.
The initial symptom of Fusarium wilt of watermelon that growers are likely to notice is a wilt. The disease will typically affect only one vine, leaving the remainder of the plant unaffected—at least initially. The older leaves on a symptomatic vine will be affected more than younger leaves on the same vine. If the stem is cut close to the ground, the inside stem—the vascular system—will be discolored brown.
There are several management options that growers must consider: long crop rotations, selection of cultivars with partial resistance, etc. But such options are not available at this time of year. Growers with affected crops may wonder what may be done now.
There are several fungicides that are labeled for Fusarium wilt of watermelon. I want to discuss the advantages and disadvantages of using these products 3-4 weeks after transplant. Details about these products may be found in the Midwest Vegetable Production Guide (mwveguide.org) or in the labels. I have not discussed fumigation since this is not a management option that can be considered this time of year.
Fungicides that are labeled for Fusarium wilt of watermelon include:
Proline 480SC®-active ingredient prothioconazole (FRAC group 3); 12-hour REI; 7-day PHI–This product was one of the first fungicides to be labeled for Fusarium wilt of watermelon, based partly on data from Purdue University. Use 5.7 fl. oz. per acre. Only one soil application is allowed per year. Do not use in transplant water. Not labeled for greenhouse use. If using Proline®, it might make sense to use another product for the next application.
Miravis Prime SC®-active ingredients fludioxonil (FRAC group 12) and pydiflumetofen (FRAC group 7); 12-hour REI; 1-day PHI. Apply 11.4 fl. oz. per acre. Apply as foliar spray over top of row; direct nozzles on both sides of row as a drench; or use overhead chemigation. See label for details. Do not use in transplant water or greenhouses. Apply first application shortly after transplanting. Make the 2nd application 14-21 days later.
Velum Prime 4.16 SC®-active ingredient fluopyram (FRAC group 7); 12-hour REI; 0-day PHI. Apply 4-6.84 fl oz. per acre. Apply through drip irrigation. Allow 5 days between applications. Maximum applications 3 per year. Note that Velum Prime® is also labeled for root knot nematode, another soil borne problem.
Applications of any of these products will be most effective if applied shortly after transplant. Such applications should help to protect the root from the Fusarium fungus. After infection has taken place and symptoms have occurred, application of any fungicide will not be as effective as it would have been if applied prior to infection. After symptoms have occurred, a fungicide application will not reverse symptoms. Fungicides applied after Fusarium wilt has become obvious in some vines may help to protect vines that have not become infected yet. Fungicides may help to mitigate symptoms on vines that are in the early stages of infection.
I want to compare fungicide applications for a soil borne disease such as Fusarium wilt of watermelon to fungicide applications for foliar diseases. In both cases, fungicides are more effective if applied before infection takes place. This is true whether the fungicide is a contact or a systemic product.
Fungicide applications to manage foliar disease are applied before the disease appears or becomes too severe. In this way, the fungicide application lessens the disease-causing potential of the spores that will land on the leaf. The benefits of foliar fungicide applications extend to slowing down the progress of the disease as well. That is, the fungicide will help to slow down the number of spores that cause infection as well as the number of spores produced.
Fungicide applications for a soil-borne such as Fusarium wilt will also help to lessen the impact of the disease by slowing or stopping the infection by the pathogenic fungus. In this way, the application is like applications for foliar fungicides. However, Fusarium wilt of watermelon does not spread from plant to plant in the field. Therefore, fungicides applied to manage Fusarium wilt of watermelon will not stop the spread of the disease because the disease does not spread in a field environment. For this reason, a few fungicide applications early in the season are all that is needed for a soilborne disease such as Fusarium wilt of watermelon. For foliar diseases, the fungicide applications should take place every 7-14 days for most of the season to slow the spread and thus the progress of the disease.
Carefully consider the possible advantages of a fungicide application for Fusarium wilt of watermelon after symptoms appear. The benefits of such an application(s) are likely much less than a fungicide application for a foliar disease.