Disease Management for Watermelons in North Florida

Managing watermelon diseases is one of the most critical aspects of successfully growing watermelons in North Florida. There are both soil and air-borne pathogens and insect-vectored plant viruses, as well as various types of pathogens causing diseases, including fungal, bacterial, and viral. Below are a few basic considerations and a few pointers on managing the most common watermelon diseases found in North Florida.

Disease Management Considerations:

1. A three-to-five-year non-cucurbit crop rotation is recommended for reducing diseases. However, fields with a Fusarium wilt history should use non-host rotations of more than 5 years. Avoid double cropping cucurbit crops using the same plastic, as this can lead to significant disease issues in the right environments.
2. Inspect transplants. Healthy transplants are critical to managing gummy stem blight, Fusarium wilt, and bacterial diseases as these diseases can be spread by seed. Inspecting transplants and removing diseased seedlings, including symptomless plants surrounding them, will reduce risk.
3. Spray early and consistently to efficiently manage disease. Fungicides are more effective when applied preventatively rather than as a cure.
4. Rotate fungicide chemistries to minimize the risk of selecting for resistant fungal pathogens. The Fungicide Resistance Action Committee (FRAC) developed a number and letter code that can be used to distinguish fungicides’ modes of action. Rotating different modes of action is an important step in prolonging a fungicide’s effectiveness.
5. Fungicides should be selected based on the target pathogen. Many fungicides can control multiple diseases, but in general they are most effective for specific pathogens. Accurate diagnostics are important when selecting proper fungicide products or developing a management plan.

Common Diseases:

Powdery mildew (PM) typically produces white, powder-like signs on the upper and lower surface of watermelon leaves. This disease starts as small, faint yellowish spots on the leaves which continue to develop to neighboring leaf surfaces. PM spread is facilitated by dry conditions; however, moisture is required for infection. Symptoms first appear in the lower canopy on older leaves and can quickly spread throughout a field. PM resistance to

fungicides in FRAC groups 1 (e.g., Topsin M), 3 (e.g., tebuconazole), 7 (e.g. boscalid) and 11 (e.g, azoxystrobin) has been identified. Fungicide options for PM are Vivando, Quintec, Rally, Switch, Luna Experience, and Torino. There are indications of Torino having reduced efficacy against some PM isolates.

Downy mildew (DM) is a continual cucurbit problem in the southern Florida peninsula; however, its incidence varies from year to year in the northern half of the state. Lesions start out as yellow angular leaf spots that will later turn brown to black in color. Leaf curling and water soaking are often associated with DM. A white to grayish fungal growth will appear on the undersides of leaves displaying these lesions when the leaves are wet from heavy dew, rainfall, and high humidity (> 90%). Protectant fungicides (chlorothalonil and mancozeb) provide excellent control early in the season, but their effectiveness is limited once DM develops. DM has been reported to have resistance to Ridomil, Revus, Forum and FRAC group 11 (e.g., Cabrio, Quadris) fungicides. Some strong fungicide options for DM once it is present are Orondis Ultra and Ranman; however, other good early season options are available. These fungicides should be mixed with a protectant fungicide to provide optimal control.

Gummy stem blight (GSB)’s primary symptom is dark circular leaf spots at the leaf surface margin. When severely infected, complete leaf necrosis is common. If a severe outbreak happens before fruit set with heavy leaf drop, yield losses can be substantial as exposed fruits experience sun scalding. The GSB pathogen is known to be resistant to multiple fungicides; hence, a carefully planned fungicide rotation program is necessary to reduce fungicide failure. Fungicides in FRAC groups 11 (e.g., Quadris), 1 (e.g., Topsin-M), and 7 (e.g. Endura) have a high risk of failure. The recommended GSB fungicide rotation programs are FRAC group M5 (e.g., chlorothalonil) with a group 3 (e.g., tebuconazole) or premixes (e.g., Inspire Super, Luna Experience, Aprovia Top, Miravis Prime).

Bacterial Diseases: Bacterial fruit blotch (BFB) symptoms appear as dark greasy spots on the fruit and irregular spots on the leaves. The disease can be managed by foliar application of FRAC group M1 (copper-based compounds) or group P (Actigard) fungicides. Copper compounds should be applied 2 weeks before female flowers open, at bloom, and 2 weeks after bloom. This will help in suppression of BFB and other bacterial diseases including bacterial leaf spot/angular leaf spot. Actigard should also be applied early in the season and before the start of the infection to be effective. These strategies coupled with healthy transplants will ensure that bacterial diseases are effectively managed early in the season, which is the critical infection period.

Fusarium wilt (FW) causes vine wilting and decline. Often symptoms are more pronounced on one side of the plant. Older leaves wilt first during the day and recover at night. Vascular discoloration can be seen when the stem is split. Due to the soil borne nature of FW, few fungicides are effective for control. Prothioconazole (Proline 480 SC) followed by pydiflumetofen (Miravis Prime) can suppress this disease. Please see label information for application instructions specific to fusarium wilt.

Spray Schedules:

A typical spring spray schedule in Florida should start 1 week after transplanting and then use a 7 to 10-day schedule. These schedules should be shortened to 5 to 7 days during prolonged rainfall periods and should only be greater than 10 days if it is very dry and no disease is present. Pay attention to pre-harvest intervals (PHI) for late season sprays as they may be as high as 14 days. Chlorothalonil is a quality early season disease control product but should not be sprayed within 21 days of harvest as it can cause watermelon rind burn.

Consult the Vegetable Production Handbook of Florida for more detailed information: (https://edis.ifas.ufl.edu/collections/vph).

For the complete version of this management guide https://svaec.ifas.ufl.edu/media/svaecifasufledu/docs/pdf/svreports/crops/Watermelon_Spray_Guide_Draft_2025.pdf

By Nicholas Dufault, Mathews Paret, Pamela Roberts, Gary Vallad, Susannah Da Silva, Kiersten Fullem, Tatiana Sanchez-Jones, Jay Capasso, and Robert Hochmuth, University of Florida Extension

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