Getting Downy on the Mildew

Here we are, the middle of April already. Spring is not even a full month old yet, my apologies to anybody reading this where there may still be snow on the ground, but we already have our first confirmed problem in the garden or at least potential problem.
Cucurbit downy mildew has been found on yellow Summer squash and zucchini plants at a Home Depot store here in Charleston, SC. Some plants were likely sold between the dates it was first discovered on Wednesday, April 6th and confirmation of the disease on Sunday, April 10th. The remaining plants were pulled off of the shelves on the day of confirmation. The plants were at bloom stage and being sold in one-gallon pots. The disease was on about 25% of the leaves. Some of the spots were even beginning to produce spores which would lead to further spread. The plants were grown in Miami, Florida which is the only area in which cucurbit downy mildew has been reported so far this season.
Downy mildew is one of the most important leaf diseases of cucurbits and is caused by the fungus Pseudoperonospora cubensis. The Cucurbitaceae or cucurbit family includes such economically important food plants as pumpkin, cucumber, watermelon, muskmelon, Summer squash, Winter squash, and gourd.
Typically, symptoms begin as small yellow areas on the upper leaf surface. As lesions expand, they may become brown with irregular margins.
They will look like this:

A fine white-to-grayish downy growth soon appears on the lower leaf surface.
This growth will look similar to this:

This is the sporulation of the pathogen or how it reproduces.
In most years the disease is an annual, late-season problem on squash and pumpkin in the Eastern and Central US, however, since 2004 it has become one of the most important diseases in cucumber production. The pathogen must overwinter in an area that doesn’t experience a hard frost, such as Southern Florida. That is why discovering it on transplants this early in the season can cause great headaches. It will probably visit soon enough without it getting a toe hold now.
The fungus is easily carried by wind currents, rain or irrigation splash, tools, or the hands and clothes of gardeners. It is favored by cool to moderately warm temperatures, but tolerates hot days, even though long periods of dry hot weather can stifle the spread of the disease. Unlike powdery mildew, it requires humidity to flourish. Therefore, downy mildew is most aggressive when heavy dews, fog, and frequent rains occur. Spores are blown northward each season as favorable seasonal conditions advance. As a result, the disease is most common on late Summer plantings and is infrequently seen on Spring cucurbits. Remember, basically only the leaves are infected. However diseased leaves results in two major effects:
1) Reduced yields and a greater number of mishapen fruit (especially in cucumber)
2) Sunscalded fruit due to increased exposure to direct sunlight (especially in watermelon and Winter squash as the leaves die off).
Controlling downy mildew requires use of resistant cultivars. There have been some developed for cucumber and cantaloupe and to a lesser extent for squash and pumpkin.
Here is a partial list of some of the resistant, or in some cases tolerant of, downy mildew:
Cantaloupe- Mission (tolerant)
Cucumber- Ashley, Burpless, Poinsett 76 and Supersett (resistant)
There may be others out there, it may take a little diligence on your part to find them.
Fungicides are available for the home vegetable garden if the disease becomes severe enough to warrant chemical control, just please make sure that it is labeled for the crop you are wanting to spray and you follow the label directions, it is the law!
There are a few cultural controls that will assist you in preventing or at least delaying the onset of this disease. Choose planting sites with good air movement and without shading. Avoid overhead irrigation in early morning when leaves are wet from dew or late in the day when leaves will not have an opportunity to dry before dew forms.
The disease is sometimes called “wildfire” because of how rapidly it progresses, as if the plants have been burned by fire.

Photo courtesy of NC State University, Department of Plant Pathology
There is a website out there that will assist you when downy mildew is present in your area. It will tell you when to start spraying a preventive fungicide and give you all kinds of other information. It can be found here: Downy Mildew Tracking
It is free to sign up and only takes a couple of seconds.
If nothing else, this whole episode should serve as a reminder to always inspect your plants that you are purchasing. Make sure there are no weird spots or splotches on the plants. Always look for insects, eggs, or insect damage. You should also be aware of diseases that are prone to a particular plant or a particular part of the country.
Luckily, this one was caught early enough and hopefully there were not too many sold.
Here's to a long, happy, healthy gardening season for everybody!
Happy Growing!
Darren