This week’s sample is cucurbit downy mildew on greenhouse cucumber. Cucurbit downy mildew affects all members of the cucurbit family: watermelons, cantaloupes, squash, pumpkins, and cucumbers. Disease is favored by long periods of high humidity and mild temperatures, which unfortunately, describes most nights in the Southeast during the production season and in greenhouses year round. Downy mildew occurs every year in North Carolina, and in recent years has become increasingly destructive on cucumber.
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| Greenhouse Symptoms (Photo: PDIC Database) |
The most obvious symptoms of downy mildew are small angular spots on the foliage, with older leaves generally being infected first. “Angular leaf spots” describe a type of symptom where the spots are defined by the boundaries of leaf veins, so that the shape of the spots is angular rather than round or blotchy. Downy mildew spots appear pale green to yellow as first, eventually becoming brown and necrotic with age.
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| Angular Leaf Spots (Photo: PDIC Database) |
During humid conditions, brown to purplish fuzzy growth can be observed on the underside of the foliage. This downy growth is actually sporulation of the pathogen.
Dichotomously branched sporangiophores with lemon-shaped sporangia are visible when view through a microscope. As infection progresses, leaves will eventually turn brown and curl upwards. The leaves are the only part of the plant affected. Downy mildew infections result in yield loss and misshapen fruits. Damage from foliar infections also increases sunlight exposure on fruit and leads to sunscald.
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| Fuzzy Sporulation (Photo: Shawn Butler) |
Cucurbit downy mildew is caused by Pseudoperonospora cubensis. P. cubensis belongs to a group of fungus-like organisms called oomycetes (or water molds). This group is also home to other aggressive plant pathogens including Phytophthora and Pythium. P. cubensis is an obligate parasite, meaning it requires living host tissue to survive and reproduce. North Carolina winter temperatures are too cold for the pathogen to overwinter, so it dies out every winter. Unfortunately, Southern Florida has the perfect combination of mild winters and wild cucurbit plants. The pathogen survives all year in Florida and inoculum builds up on wild plants. Spores are windblown and can travel long distances on air currents. Summer weather events, like hurricanes, act as the perfect mode of transportation. Like birds, downy mildew spores fly north for the summer and end up on our cucurbits! In this particular situation, the greenhouse plants probably became infected as spores from nearby field cucumbers blew into the greenhouse over the summer.
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For more information on this disease, click here
For more information on controlling this disease, click here
For more information on forecasting this disease, click here
