Watermelon Fungicide Guide for 2021

Four foliar diseases reduce watermelon yields in South Carolina. Gummy stem blight and powdery mildew are the most common diseases in spring crops.

Gummy stem blight is large, round spots on the edges of leaves and dry cankers on the main stem (figure 1). The best fungicides are Miravis Prime, Switch, Inspire Super, or Luna Experience. Powdery mildew is yellow spots on the top of leaves and white powdery mildew on the bottom (figure 2). It shows up during dry periods. The best fungicides are Gatten, Vivando, or Quintec.

leaf with gummy stem blight present

Figure 1. Gummy stem blight. Image Credit: Anthony P. Keinath, Clemson University.

leaf with powdery mildew present

Figure 2. Powdery mildew. Image Credit: Anthony P. Keinath, Clemson University.

Downy mildew starts as brown or yellow, round or irregular spots that spread quickly (figure 3). If seen, spray immediately with Ranman or Orondis Ultra, followed by the other fungicide the next week. Anthracnose starts as dark, irregular spots with points like a star (figure 4). It also affects vines and young fruit. The best fungicides are Flint Extra, Cabrio, mancozeb, or chlorothalonil.

downy mildew on a leaf

Figure 3. Downy mildew. Image Credit: Anthony P. Keinath, Clemson University.

Anthracnose present on a leaf

Figure 4. Anthracnose. Image Credit: Anthony P. Keinath, Clemson University.

Scouting for Diseases

  • Walk fields weekly and the day before spraying.
  • Yellow spots on leaves are symptoms of powdery mildew or downy mildew. Look for spores on the underside of leaves in the early morning. Call your local Extension office if you can’t identify the disease.
  • Brown spots are usually gummy stem blight or anthracnose. Anthracnose also shows up as tan, narrow, sunken spots on vines.

How to Get the Most Out of Spraying

Table 1 provides details for fungicide spray programs for spring and fall crops. Follow these steps to get the most out of your fungicide program.

  1. Rotate fields to crops other than cucurbits for two years in between watermelon and cantaloupe to help control gummy stem blight and anthracnose.
  2. Start spraying when vines start to run, no later than when the first (male) flowers open.
  3. From vine run until mid-May, spray every ten days.
  4. After mid-May, spray every week through harvest regardless of the weather. Downy or powdery mildew can attack any time a crop goes more than a week without a fungicide.
  5. Apply fungicides, allowing time to dry, before a predicted rain rather than after.
  6. For best control after disease starts, use systemic fungicides (see option “b” in the first column of table 1).
  7. Do not stop spraying until one week before the final picking.

Fungicide Programs

  • Chlorothalonil and mancozeb protect against gummy stem blight, downy mildew, and anthracnose; chlorothalonil also protects against powdery mildew.
  • The spring program is designed to manage bacterial fruit blotch, bacterial leaf spot, gummy stem blight, powdery mildew, anthracnose, and downy mildew on watermelon.
  • The fall program is designed to manage gummy stem blight, downy mildew, anthracnose, and powdery mildew.
  • Week-by-week rotation is built into the spray programs for systemic fungicides, like tebuconazole, to reduce the risk of fungicide resistance.
  • Tebuconazole, Inspire Super, and Luna Experience have a 7-day pre-harvest interval (PHI) on watermelon. These fungicides should not be sprayed during the harvest period (weeks 6-8 in the spray programs). When harvest begins early, skip ahead to spray number 6a or 6b listed in table 1. Mancozeb and Gavel have a 5-day PHI.
  • See the current edition of the Southeastern U.S. Vegetable Crop Handbook for more information.1

Table 1. Fungicide guide for watermelon

Spray Number Fungicide Program for Spring Watermelon* Comments on Spring Program Fungicide Program for Fall Watermelon*
1 (vine run) mancozeb + fixed copper To prevent bacterial leaf spot and fruit blotch. chlorothalonil
2 chlorothalonil If fruit blotch or bacterial leaf spot is a concern, use mancozeb + fixed copper instead. Do not tank mix copper with chlorothalonil. tebuconazole + Gatten
3a** tebuconazole or Viathon Tebuconazole controls only gummy stem blight. chlorothalonil + Ranman
3b** or

tebuconazole (+ Flint Extra)

Add Flint if anthracnose fruit rot was found on your farm the previous year.
4 chlorothalonil or mancozeb If fruit blotch or bacterial leaf spots are a concern, substitute mancozeb + fixed copper. Quadris Top
5a** mancozeb + Vivando Starting week 5, use mancozeb to avoid injury to fruit on hot, sunny days. Note 5-day PHI on mancozeb. Use Inspire Super if gummy stem blight is present but note 7-day PHI. mancozeb plus Orondis Ultra
5b** or

Inspire Super

6a Gavel Note 5-day PHI on Gavel. Switch
6b or mancozeb plus Orondis Ultra Tank mix Orondis products with mancozeb to protect against gummy stem blight and anthracnose. Apply Orondis if downy mildew has been reported on watermelon in GA or SC.
7a** mancozeb + Gatten Use Switch if gummy stem blight is present. Gavel
7b** or Switch + Gatten
8 mancozeb + Ranman chlorothalonil
9-12 If more sprays are needed after the eighth spray, repeat sprays 5-8 again until one week before the last harvest.

*Fungicides for downy mildew are in bold and should be used if downy mildew has been reported on watermelon in the current season (visit the downy mildew forecasting site). Fungicides for powdery mildew are italicized and should be applied all spring and during dry periods in the Fall. Cantaloupe growers who are using this fungicide program can omit underlined fungicides for powdery mildew on hybrid varieties.

**Option “a” is a lower-cost treatment that may be less effective. Option “b” is a more expensive systemic fungicide that is more effective when disease is already in the field or when weather conditions favor disease getting worse.

References Cited

  1. Kemble JM, senior editor. Meadows IM, Jennings KM, Walgenbach JF, editors. Vegetable crop handbook for the southeastern US. Willoughby (OH): MeisterMedia Worldwide. 2019. Growing Produce.

References Consulted

Rennberger G, Gerard P, Keinath AP. Occurrence of foliar pathogens of watermelon on commercial farms in South Carolina estimated with stratified cluster sampling. Plant Dis. 2018. 102: 2285-2295. doi:10.1094/PDIS-03-18-0468-RE.

Rennberger G, Gerard P, Keinath AP. Factors influencing the occurrence of foliar pathogens in commercial watermelon fields in South Carolina based on stratified cluster sampling. Plant Dis. 2019. 103: 484-494. doi:10.1094/PDIS-07-18-1188-RE.

Additional Resources

Keinath AP. Cucurbit downy mildew management for 2019. Land-Grant Press by Clemson Extension. 2019; LGP 1002.

Keinath AP, Rennberger G. Powdery mildew on watermelon. Land-Grant Press by Clemson Extension. 2019; LGP 1019.

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