Tomatoes - Powdery Mildew

The picture below shows what the symptoms on leaves look like. The white spots move from lower leaves to upper leaves.  If not brought under control there can be a significant reduction in fruit sizing and reduction in yields as well.  The fungus in Alberta was identified as Erysiphe sp.  There are some interesting facts about this fungus. It is a disease of low relative humidity although like Botrytis it needs free water to germinate its spores.  One sees this disease starting near the fans and getting worse under dry conditions.

Growers use many reduced risk approaches to reduce its spread such as:

• Spraying water on a daily basis with or without a surfactant.  Sometimes I have seen growing using baking soda, dormant mineral oil.
• Removing infected leaves at an early stage can help reduce the spread of this disease.
• Overhead misting to maintain a relative humidity of around 70% is beneficial.
• If possible keep the temperature below 21C.
• Avoid high nitrogen feed, that is over 250 ppm is considered high.  Increase potassium in relation to nitrogen.
• Intercropping should be avoided when powdery mildew is an issue.
• There are many registered fungicides to control this disease.