Summer brings cucurbits such as cucumbers, squashes and melons to farmers markets and tables in the Lowcountry. Summer also brings cucurbit downy mildew, a destructive disease that shortens the productive life of these crops.
Cucurbit downy mildew is an unusual character in the plant pathology world in five ways. (Note that downy mildew refers both to the disease and to the organism that causes the disease.)
1. Not all crops are same
Cucumber and muskmelon are the most susceptible crops in the cucurbit family, commonly called the squash family or the vine crops. Cucurbit downy mildew will kill these crops.
Squashes tolerate downy mildew; they will keep producing some fruit even after the plant is diseased. Watermelon does not get downy mildew as often as the other vine crops.
Cucurbit downy mildew turns leaves yellow or brown and takes (absorbs) the nutrients the plant makes.
This is why melons produced on plants with downy mildew aren't very sweet.
2. Mistaken identity
Downy mildews commonly are called water molds.
Everyone used to think they were much like other molds and mildews.
However, when scientists looked at detailed fingerprints of downy mildew DNA, they learned that downy mildews actually are related to the brown algae that form the kelp forests in the Pacific Ocean, where sea otters live.
Although downy mildew is not a true mold, water mold is still a good nickname, because it needs water to complete its life cycle.
3. Winters in Florida
Downy mildew is an example of an "obligate parasite," because it must infect a live plant to grow. It does not survive in soil as some other disease-causing organisms do.
It comes as no surprise, then, that cucurbit downy mildew cannot survive where cucurbit plants freeze in the fall.
Downy mildew is like a "snowbird" that spends winters in southern Florida, south Texas or the Caribbean, growing on winter crops of cucumber or squash or on weeds like Balsam pear.
4. Prefers air travel
Rain storms are the perfect way for downy mildew to spread from place to place. The wind, rain and clouds provide ideal conditions for spores to travel by air.
Wind moves the spores up to 1,000 miles away from their source in 24 hours. This is how downy mildew is spread northward each spring from the Southern regions where it over-winters.
Rain washes spores out of the air and deposits them on leaves. At the same time, rain wets the leaves, providing the moisture necessary for downy mildew spores to germinate like a seed and infect leaves.
Cloudy weather protects spores from UV rays. Spores remain viable longer, and can travel farther during cloudy weather than during sunny periods. Three of four airborne spores will still be alive after 12 hours on a cloudy day, but on a sunny day, they would all be dead.
I used to think downy mildew flourished in warm, wet weather. Since the first sightings of downy mildew in the Lowcountry are usually between mid-May and early June, its weather preferences made sense.
I recently learned, however, that downy mildew grows quite well at only 70 degrees; it doesn't need warm weather at all. Once downy mildew is inside a leaf, it also doesn't need dew, because it uses the water inside the plants. It doesn't matter if the outside of the leaf is dry.
Some varieties of cucumber were completely resistant to downy mildew before 2004, when a new race of downy mildew that could attack resistant varieties came into the United States. Resistant varieties such as Marketmore 76, General Lee and SV4719CS, get downy mildew, but symptoms will show up about a week later, and plants usually produce fruit longer than susceptible varieties.
Even though algae are very sensitive to copper fungicides, they do not work well against downy mildew. Once downy mildew gets inside a plant, copper can't reach it. Broad-spectrum fungicides that work against most plant diseases, like chlorothalonil and mancozeb, control downy mildew.
There are no organic controls for cucurbit downy mildew. The best approach is to plant crops early. Vining cultivars of cucumber should be tied to trellises, so that dew on the leaves dries quickly.
Anthony Keinath is professor of plant pathology at the Clemson Coastal Research & Education Center in Charleston. His expertise is in diseases of vegetables. He also is an avid gardener. Contact him at tknth@ clemson.edu.