Written by:  Nick Dufault, UF/IFAS Crop Pathology Specialist

Despite the dry weather, Fusarium Wilt is showing up again in watermelon fields across North Florida, and importantly, we are also observing it in our UF/IFAS variety trials. The variety trial site can serve as an early indicator of the disease since researchers introduced inoculum to better understand disease development with the different commercially available varieties.

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What We’re Seeing in the Field

Symptoms (Figure 1 below) are consistent with what many growers are familiar with:

• Yellowing and wilting of leaves
• Vine decline and necrosis (death of plant tissue)
• Vascular discoloration
• Patchy plant death across the field

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What stands out this season thus far is that after the plants begin to decline, which can be rapid, we have noticed many of the plants start to recover. However, this does not mean the pathogen has left, but more likely it is dormant due to unfavorable environments for disease.

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In-Season Management Remains Limited

One of the consistent challenges with Fusarium Wilt of watermelon is that in-season management options remain limited. We have evaluated tools like prothioconazole (Proline) applied through the drip, and while there are cases where it appears to help slow disease progression, the results have been inconsistent across locations and environments. For a complete list of fungicides available for watermelons and Fusarium Wilt management, please refer to the Florida Vegetable Production Handbook.

Practically speaking, that means:

• This is not a standalone solution
• Expectations should be managed
• Environmental conditions and disease pressure likely play a large role in response

If plants are already collapsing, chemical intervention is unlikely to recover yield in those areas, but may protect from further infections. More information on in season disease management of watermelons can be found here: AgriGator.

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Early fungicide applications may shift disease onset

However, one area where we are seeing some promising movement is earlier intervention.  In our research plots, we are evaluating early foliar fungicide applications, including treatments applied in transplant flats and at transplanting. In these trials, we have consistently observed a two-week delay in disease onset.

That may not sound like much, but two weeks can make a significant difference in reducing overall Fusarium Wilt incidence and protecting yield potential.  This is not a cure, but it may be an important tool to shift the disease pressure later into the season, when the environment is typically less conducive for the disease.

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Environmental conditions remain favorable for disease

Growth of the Fusarium Wilt fungal pathogen is strongly influenced by temperature. Research at the University of Florida, University of Georgia, and Clemson University has shown that Fusarium Wilt tends to be more problematic in early planted watermelons. One reason is that soil temperatures in March and April typically remain below 86°F, a range that favors pathogen activity. With current air temperatures across North Florida reaching highs in the 80s, it is unlikely that soil temperatures are staying elevated long enough to consistently suppress fungal growth. Unfortunately, the current forecast for much of the region does not show temperatures increasing substantially over the next 10 days. As a result, we can expect the disease to remain active in the near term.

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What Should Growers Do Right Now?

Since our options are limited for managing this disease, it is recommended that if you see plants starting to die in-season:

• Document the affected areas
  This is critical for future management decisions.
• Avoid overinvesting in rescue treatments
  At this stage, returns are often limited.
• Maintain overall crop health where possible
  Reducing additional stress (water, nutrition, other diseases) can help preserve remaining yield.

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Bigger Picture: This is still a Preseason Disease

Fusarium Wilt is still fundamentally a preseason management problem, not an in-season one.  Based on what we know, and what we continue to see in trials, the most effective strategies remain:

• Resistant varieties (where available and appropriate)
• Grafting onto resistant rootstocks
• Field selection (history matters)
• Soil fumigation may help in high-risk fields
• Planting timing adjustments where feasible

This pathogen is persistent, adaptable, and capable of causing damage even when we think we understand the system. Documenting the pathogen’s presence can help you plan for next season. Early fungicide applications at or before transplanting may provide an additional tool by delaying disease onset, but they should be integrated into a broader management program.

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