Managing Powdery Mildew on Watermelon

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Managing Powdery Mildew on Watermelon

• Anthony (Tony) Keinath, Vegetable Pathologist
• Clemson University, Charleston, SC

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Powdery Mildew on Watermelon

• Strikes mature melon vines.
• Occurs during moderate, dry weather.
• A problem in SC and GA in 1999-2000.
• Also in TX and OK in 2000.
• Spreads rapidly if not treated early.

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First symptom of powdery mildew on watermelon is
a diffuse, yellow spot on the top of leaves.
Directly underneath is the beginning of a powdery
white spot. Fungicide applications should begin
at this stage.
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Typical powdery, white spots of powdery mildew.
These can be found on both top and bottom of
leaves.
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Severe powdery mildew on watermelon. At this
stage, it is too late to apply a fungicide.
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Drying, bronzing, and death of watermelon leaves
affected with powdery mildew. Fruit growth stops
and fruit may sunburn.
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Fungicides Registered to Control Powdery Mildew
on Watermelon

• Old fungicides
• Curative
• Benlate
• Topsin M
• Protectant
• chlorothalonils
• mancozebs and manebs (not effective)

• New fungicides
• Curative and protectant
• Quadris
• Flint
• Nova

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Control of powdery mildew on watermelon with
Quadris, Bravo, and Kocide, 1999,based on amount
of leaf area affected with powdery mildew
Top of leaves
Bottom of leaves
2Q/B
Q/B
B
2Q/K
Q/K
K
Nonsprayed Check
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Control of powdery mildew on watermelon with
Quadris, Bravo, and Kocide, 1999 (continued)

• Since this study was done, the Quadris label has
  been changed. Two back-to-back applications (2Q
  on previous chart) are no longer allowed.
• Quadris (and Flint) must be rotated every other
  application with other fungicides to prevent
  resistance in the powdery mildew fungus.
• Benlate, Topsin M, and Nova should be tank-mixed
  with a protectant to prevent fungicide resistance.

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Susceptibility of Watermelon Cultivars to
Powdery Mildew

• Diploids
• Generally susceptible

• Triploids
• Generally intermediate to resistant

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Jubilee II
Carnival
Baron
Crimson Sweet
Royal Sweet
Royal Flush
Royal Star
Legacy
Jubilation
Fiesta
Pinata
Regency
Juliett
Rebel Queen
Festival
Mardi Gras
Percent powdery mildew on diploid watermelons
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Odem
Vista
GAL
Shadow
AC 5544
Tri-X 313
Crimson Trio
Triple Sweet
Gem Dandy
AC 5032
Genesis
DPS 4502
Millionaire
Disco
0
20
40
60
80
100
Percent powdery mildew on triploid watermelons
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Summary

• Scout watermelon fields for symptoms of powdery
  mildew twice a week beginning in late May. Pay
  close attention to diploid cultivars.
• Apply a curative fungicide as soon as symptoms
  are seen. If weather is dry, continue spraying
  based on label directions.
• Alternate or tank-mix fungicides to prevent
  resistance. Quadris and chlorothalonil will
  control both powdery mildew and gummy stem blight.