Powdery Mildew

Powdery mildew is a destructive tomato disease that also affects other plants. The disease doesn't kill the plant but causes decreased plant health and loss of tomato fruit production by up to 50%.

Other susceptible vegetables include melons, pumpkins, squash, cucumbers, beans, and peas (see photos).

How to Identify Powdery Mildew

powdery mildew on pea vine
powdery mildew on pea vine
  • Light green to yellow spots or splotches on the upper surface of the leaves.
  • Initially, spots appear as 1/8-inch to 1/2-inch in diameter.
  • The fruit doesn't show symptoms, only the leaves.
  • As the disease progresses the spots and leaves turn yellow and the leaves wither and fall off.


  • Dry leaf surfaces and cool temperatures encourage development.
  • Infected rosemary has been implicated as a source in tomatoes.
  • It's more common in greenhouses because the leaves stay drier.
  • Epidemics have occurred under dry conditions.
  • Many types of plants can host the disease and carry it from season to season.
  • The disease is spread by windborne fungus spores.

Preventative Measures

  • In greenhouses high relative humidity discourages the disease.
  • Remove infected plants from greenhouses
  • Use fungicides as soon as you observe symptoms
  • Plant tomatoes in sunny locations as cool, shady spots encourage spread.
  • Allow good air circulation by pruning excess foliage.


Natural remedies

  • Use resistant varieties.
  • Plant your tomatoes in warm, dry, sunny spots.

Chemical treatments

  • Spraying with chemical contact fungicides is effective.
  • Begin treatments as soon as you notice the problem and spray all plant surfaces.
  • Contact fungicides to help prevent spread include mineral oils (JMS Stylet Oil) and potassium bicarbonate (Armicarb, MilStop, and Kaligreen). Repeat applications at five day intervals.
  • Systemic fungicides proven effective are Quadris, Flint and the demethylation inhibitor Nova. However, if you don't notice any improvement discontinue use as the disease may have developed resistance.

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Credits for technical content

  1. Cornell University's Vegetable MD Online
  2. University of California - Davis


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