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Rain and High Relative Humidity = Increased Potential for Disease

Posted: June 15, 2015

Prolonged periods of leaf wetness and high relative humidity create conditions that are perfect for pathogen spread and disease development!
White colonies characteristic of powdery mildew on the upper leaf surface of tomato (Photo credit: Beth K. Gugino)

White colonies characteristic of powdery mildew on the upper leaf surface of tomato (Photo credit: Beth K. Gugino)

Although we have received some much needed rain, the wet and very humid conditions are very favorable for the development and spread of many diseases that affect many vegetable crops. Rain splash can spread the pathogens that cause diseases like early blight, Septoria leaf spot and bacterial spot within and between tomato plants. Prolonged high relative humidity can lead to problems with Botrytis gray mold and leaf mold in high tunnels as well. Taking steps to maximize air circulation in high tunnels and minimizing the handling of plants when they are wet will help reduce potential spread.

In New York, Jud Reid from the CCE Cornell Vegetable Program has reported numerous outbreaks powdery mildew on tomato in high tunnels in Central and Western regions. Given that the disease can be caused by one of several obligate fungal pathogens, it is suspected to have overwintered in high tunnels and greenhouses and the recent prolonged periods of relative humidity are creating favorable conditions for disease development. In contrast to leaf mold, where the olive green-grayish sporulation is concentrated on the underside of the leaf, powdery mildew will develop white fuzzy sporulation on both the upper and lower leaf surface (similar to powdery mildew on cucurbits and other hosts). JMS Stylet oil, Scala or sulfur-based products can help reduce disease spread in a high tunnel or greenhouse.

Contact Information

Beth K. Gugino
  • Associate Professor Vegetable Pathology
Phone: 814-865-7328