A few weeks ago I wrote about a new pest arrival to Ohio that attacks our viburnum shrubs.
My own plant came back to life with lots of new growth after being devastated by the larvae of the Viburnum Leaf Beetle, and after researching the insect and speaking to Ohio State University Extension Entomologists and fellow Trumbull County Master Gardeners, I know to be ready when the adult beetle begins to emerge in just a couple weeks to continue its attack.
Just as I was beginning to feel confident in my plan to deal with this formerly unknown garden pest, news of another new garden pest emerged on the scene. This pest, however, doesn't attack viburnums, and it isn't an insect. It's a disease that is slowly making its way up the east coast from Florida and has already begun its spread across the midwest. This disease is after one of our most favored culinary herbs, basil. The disease is called Downy Mildew and was first brought to my attention in an article by Washington Post garden columnist Adrian Higgins, published June 16.
I know what many of you are thinking. Downy Mildew isn't a new disease to midwestern gardeners and you're right. It has been known to attack many vine-like plants such as cucumbers and grapes, but remember, most diseases and even insect pests, are plant specific. The downy mildew that goes after our roses (Peronospora sparsais), is not the same fungus that attacks our cucumbers (Pseudoperonospora cubensis).
But downy mildew is new to basil and growers are not happy about it.
Margaret Tuttle McGrath from the Department of Plant Pathology and Plant-Microbe Biology at Cornell University wrote about the disease last year. In her article, she explained that growers at first didn't realize their plants were afflicted with a new plant disease because the first symptoms are similar to nutritional problems. Those symptoms include yellowing on the upper sides of the leaves. But after further inspection of the undersides of the leaves, gray to purple-looking spots appear, which are the spores of the disease. Basil downy mildew, (Peronospora belbahrii), can devastate a commercial grower who must have pristine leaves to take to market.
According to Higgins' article, leaves that only are slightly infected can still be consumed, but who would want to?
McGrath believes the disease was spread two ways, first by infected seeds and then through wind dispersal of the spores. Spores can travel long distances over a wide area making it easy for the disease to spread far and rapidly.
Since coleus, those lovely multi-colored bedding plants we love to fill our gardens with each spring, are related to basil, we might worry that the disease will attack those plants as well. The good news is, according to McGrath, the pathogens that attack basil do not attack coleus. The bad news is, coleus has its own species of downy mildew disease, which likewise, doesn't attack basil, but that isn't something new. We don't eat coleus.
So now what do we do?
Commercial growers are busy getting updated information on how to combat the disease so they don't lose their entire marketable crop from the experts at the land-grant universities, such as Cornell and OSU.
Those of use who grow a few plants in our backyards each summer for our own use can rest easy that the battle won't be quite as intense. We can pick off affected leaves before the disease spreads over our entire plant collection once we notice the first symptoms.
We also can practice preventive measures, such as thinning out the plants in each row leaving plenty of space between the plants to promote good air circulation. Use soaker hoses for watering directly on top of the soil rather than let water spray onto the leaves. While we can't control the weather, high humidity often contributes to the growth of fungal spores.
We also can try growing disease resistant varieties of basil. Some research has indicated that the more common sweet basil varieties, including purple and large leaf Italian varieties are most susceptible to the disease, while lemon, lime and spice-type basils were less likely to be affected.
Don't let this threat stop you from growing your favorite varieties of basil. Take the time to inspect your plant and be on the lookout for symptoms so you can act immediately to rid your garden of the pest. We've dealt with downy mildew on just about every plant and vegetable we grow, adding one more to the mix might be inconvenient for the backyard gardener, but it shouldn't stop us from growing what we love.