Cucurbit Downy Mildew

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Angular yellow spots on Cucumber leaves

Angular Yellow Spots of Cucurbit Downy Mildew by Shawn Banks

A client brought in a sample of cucumber leaves today with blocky yellow spots on the leaves. She was wondering what was wrong with her cucumbers and what she could do to continue harvesting the loads of prolific cucumbers off her plants.

To diagnose a plant disease the first thing to do is make a note of the symptoms. The angular yellow spots on the cucumber leaves had only appeared a week or so ago, but they were spreading quickly. The angularness of the spots is very telling. It narrows the list of possible diseases down to two.

One is a bacterial disease commonly called ‘Angular Leaf Spot’. We are very original when it comes to naming diseases. This disease is caused by a bacterium (Pseudomonas syringae pv. lachrymans). This disease starts out as a small water soaked or oily looking spot that turns yellow then brown. The bacteria can’t cross over the veins in the leaf so it appears very angular.

A 20X view of the underside of a cucumber leaf with cucurbit downy mildew. Black spore cases are present in the yellow, angular spots.

Black spore cases on underside of cucumber leaf by Shawn Banks

The second disease is called Cucurbit Downy Mildew, which is caused by the fungus Pseudoperonospora cubensis and that’s why we call it Cucurbit Downy Mildew. The name is nearly impossible to wrap your tongue around. The quickest way to tell which one is present is to turn the leaf over and look at the underside of the leaf with a 20X magnifying glass or something stronger. Inside the yellow spot there will be a forest of nearly clear tree trunks topped with little, round, black balls that contain the fungal spores. It’s not easy to see in the picture, but the black spots that look like dirt on the underside of the leaf are those black balls. The spores are released to blow on the wind until they find another suitable host.

According to the Cucurbit Downy Mildew publication from NC State University the only effective products available for controlling this disease contain copper or chlorothalonil. However, the products available for home gardeners aren’t very effective at controlling this disease once it’s present on the plant.