Cucurbit downy mildew

Believe it or not another mildew exists that attacks your cucumbers, squashes and melons. It’s called cucurbit downy mildew. Powdery mildew and downy mildew are two different diseases caused by two very different pathogens. Whereas powdery mildew is caused by a fungus, downy mildew is caused by an oomycete, Pseudoperonospora cubensis. For some of the differences between oomycetes and fungi see the post on late blight.

Cucumber plants with downy mildew in a garden plot. You can see the yellowing foliage.

Similar to powdery mildew, downy mildew is caused by an obligate pathogen that requires a living host. It only affects the leaves of the plant; not the roots, stems or fruits. Don’t be fooled into thinking this won’t do much damage however. It turns out that downy mildew is a huge problem, especially on cucumbers. Commercial growers spend hundreds of dollars each year trying to manage this disease, and it can still cause crop losses. This pathogen is dispersed by the wind and can be blown up from the south or down from greenhouse grown plants in the north. The pathogen infects the leaves, begins to take nutrients away from it. Leaves will die, any fruit that are produced will typically be deformed, and the plant will eventually die.

Close up of a single leaf. The yellow patches on the leaf are downy mildew and will eventually turn necrotic as the disease progresses.

Symptoms

The symptoms differ depending on what type of cucurbit it’s on. On cucumbers you typically see very angular lesions, and they will be restricted by the veins. During early stages of infection the leaves show watersoaking which progresses to yellow angular patches. As the infection progresses those angular lesions become necrotic (dead) and if you look on the underside of the leaf will be hundreds of tiny grayish sporangia. They are too small to see well with the naked eye, but it will look like grayish dirt on the underside of the leaf. On melons, the lesions are more rounded, have a cholorotic (yellow) halo and can easily be confused with other leaf diseases (like Alternaria).

This is the underside of that same leaf. You can see the lesions being restricted by the small veins, and if you look closely you can see the grainy appearance of those sporangia inside the lesions.

Management

Unfortunately there aren’t a lot of cultural (or non chemical) controls for downy mildew. There are no varieties currently available with complete resistance to the pathogen, but some varieties are more susceptible than others. (Ask your local extension agent for varieties that are least susceptible in your area). Start cucumbers indoors to maximize their production time. You can slowdown the effect of the disease by scouting for symptoms. Often downy doesn’t show up until the end of July/early August when the weather is warm (not hot), and humid. Some years, if conditions are good, the disease has been seen in Michigan as soon as the first week of July. The Michigan State University Extension service puts out a notice as soon as downy mildew is detected in the surrounding states. They also have some information for home gardeners, I really like the pictures of the sporangia. The pathogen infects and spreads best in warm humid conditions. So, having good plant spacing and limiting your overhead watering can make the microclimate in your canopy less favorable for disease. If you catch the disease early enough you can remove infected tissue by double bagging it, and putting it in the city trash.

Downy mildew on a cantaloupe. The lesions aren't quite as angular and they have a yellow halo around each one.

So does this mean you should give up on cucumbers? Absolutely not! Start those seeds indoors, space your transplants adequately, scout often, and you'll still have a perfectly decent cucumber harvest.

Source contributing information:

http://ohioline.osu.edu/hyg-fact/3000/pdf/3127.pdf

http://msue.anr.msu.edu/news/downy_mildew_spores_detected_in_michigan

http://msue.anr.msu.edu/news/managing_cucurbit_downy_mildew_in_your_garden