A destructive plant disease known as basil downy mildew has been making its way around the U.S. since 2007 and will likely appear in Kansas again this year as temperatures warm.

The disease is fatal to basil if left unmanaged, but early detection and diagnosis may prevent or reduce crop loss.

The key to diagnosis of basil downy mildew is the observation of purplish gray spores on the undersides of leaves with yellowing between the veins on the leaf surface. The growth on the underside of the leaves, which looks like an airy bread mold, is the reproductive part of the mildew. The yellowing on the leaf surface is the result of the mildew feeding on the leaf.

Basil downy mildew is most prevalent in midsummer and is worse in years with ample moisture.

If basil leaves are yellowing without noticeable spores on the undersides, place a few leaves in a plastic bag with a wet paper towel. Then place the bag in the dark for a day or two to encourage sporulation if the pathogen is present. If the undersides of the leaves still lack mildew after this test, the yellowing is probably related to a nutritional deficiency, too much or too little water, or some other sort of environmental stress.

If basil downy mildew is observed on plants, work to minimize leaf wetness and humidity. Space plants out or move them to allow for better air circulation, use drip irrigation rather than sprinklers or overhead irrigation and ensure plants are in full sun.

For large crops, growers may be also able to use heat and light treatments to minimize crop loss.

Plants that succumb to basil downy mildew should be removed and destroyed to reduce buildup of the disease.

Plant breeders are working to produce hybrid basil with downy mildew resistance, but until those are released growers have to rely on trying to find pathogen-free seed. One variety of sweet basil, Eleonora, has moderate resistance. All other sweet basils, including the ever-popular Genovese and Italian large leaf, are highly susceptible. Lemon, spice and ornamental basils show moderate resistance but have a different flavor.

Basil downy mildew was reported in Uganda in 1933. It appeared in Florida in the fall of 2007. The following year, the disease was confirmed in Kansas and several other states in field and greenhouse basil crops. Scientists are unsure of what caused the resurrection of the disease, but suspect the pathogen was able to move long distances by traveling on seed and asymptomatic plants. It was also able to build up quickly because of misidentification by growers in the first few years it appeared in the U.S.

Downy mildew and infected plants do not pose a human health risk.

— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation. She is the host of “The Garden Show.” Send your gardening questions and feedback to features@ljworld.com.