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
- 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.
- 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
- Use resistant varieties.
- Plant your tomatoes in warm, dry, sunny spots.
- 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
Cornell University's Vegetable MD Online
University of California - Davis