Even a greenhouse cannot protect your cucumbers

by Matthew Lowe, Sustainable Plant Systems major

Photo: Sally Miller, The Ohio State University, Bugwood.org

Photo: Sally Miller, The Ohio State University, Bugwood.org

Pseudoperonospora cubensis, downy mildew, will attack your cucumbers even under the protective glass of a greenhouse. This water mold can cause devastating yield losses in cucumber plants and can even spread to plants such as pumpkins and squash.


The older leaves are the first to show signs of disease pressure. The tops of the leaves will show yellow spots. The undersides of these spots may have mold growth under certain conditions. These yellow spots will eventually turn into brown, necrotic spots on the leaves which will merge together stopping photosynthesis in that leaf causing the reduction in yield.

Disease Cycle

Downy mildew is an obligate parasite requiring a living host at all times therefore it cannot overwinter. To spread its inoculum, downy mildew produces oospores to be spread by wind.

The disease makes its way into greenhouses through air vents. The spores can travel many miles in wind currents so even if downy mildew is killed off due to lack of hosts over winter, it can still return.

The disease is a wet mold so it requires a moist environment to thrive. The best temperatures for infection are 16°C to 22°C. The spores will land on the wet plant and enter through the stomates. Spores are spread even more by human interactions.

Management Strategies

One of the simplest strategies is greenhouse environment management. Keeping humidity down and allowing for good air flow is key.

Make sure watering is necessary as overwatering will increase outbreak potential. Also, keep tools clean and remove infected plants immediately to reduce the chances for spreading.

Spraying fungicides at the first sign of infection can be another good control but it is more expensive than other control methods. Remember to alternate fungicides to prevent the development of resistance.

For more information, go to: http://www.omafra.gov.on.ca/english/crops/facts/09-013w.htm

About Matthew Lowe
I am a Sustainable Plant Systems major with a specialization in Agronomy at The Ohio State University’s College of Food, Agriculture and Environmental Sciences. I earned my Associate of Science Degree in Agronomy from The Ohio State University’s Agricultural Technical Institute (OSU ATI) in the spring of 2015.

This blog post was an assignment for Societal Issues: Pesticides, Alternatives and the Environment (PLNTPTH 4597). The views expressed are those of the author and do not necessarily reflect the views of the class, Department of Plant Pathology or the instructor.



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