Photos and text by Debbie Roos, Agricultural Extension Agent.

Genovese basil showing late stages of basil downy mildew

Basil downy mildew has been confirmed in Chatham and five other North Carolina counties. The disease first appeared in the U.S. in 2007 and has been widespread throughout the country this year with over 200 cases, mostly in the eastern U.S.

Many of our market farmers grow several varieties of both culinary sweet basil and ornamental basil for cut flower bouquets. They often will do multiple plantings to extend the harvest. Basil downy mildew can devastate a basil crop. One Chatham County grower just had to disc in her entire basil crop due to a severe infection of downy mildew (see photos below). She had planted six varieties of basil and all had become infected, from a young crop recently planted in the field to mature plantings. Even the transplants in the greenhouse which were to be planted this week became infected. Basil downy mildew has caused a significant economic loss for this grower and many others.

Basil downy mildew can be overlooked by growers because the initial symptoms tend to be subtle: the plants exhibit slightly yellowing leaves as their most noticeable symptom, which may resemble a nutritional deficiency. The dark fuzzy-looking sporulation occurs on the undersides of the leaves. The disease is serious because it renders the leaves unmarketable. The disease can be spread from infected seed or leaves or from wind-dispersed spores which can travel long distances. It thrives in our humid summers. Really high temperatures knock it back, which is why the disease was not as severe during the summers of 2011-2012 when we had record-breaking heat. The cooler than average temperatures we have experienced lately have likely favored the disease.

Basil downy mildew on young Genovese basil plant; upper surfaces of leaves often turn yellow and eventually become necrotic

Fuzzy sporulation on the underside of a basil leaf

Fuzzy sporulation on the underside of a basil leaf

Close-up of sporulation on underside of leaf

Basil downy mildew symptoms on Genovese basil

Basil downy mildew symptoms on Genovese basil

Basil downy mildew symptoms on Genovese basil

Young basil plants infected with basil downy mildew; they were disced in shortly after this photo was taken

Infected basil leaves

Genovese basil with downy mildew

Genovese basil with downy mildew

Genovese basil with downy mildew

Unfortunately the disease even moved into the greenhouse and infected young seedlings

‘Red Rubin’ basil infected with basil downy mildew

‘Mrs. Burns’ lemon basil infected with downy mildew

‘Cinnamon’ basil infected with downy mildew

‘Oriental Breeze’ basil infected with downy mildew

‘Siam Queen’ basil infected with downy mildew

Genovese basil showing late stages of basil downy mildew

Bed of basil infected with downy mildew

Management Strategies:

North Carolina basil growers are at high risk for seeing this disease on their basil. It will continue to sporulate and spread. Be on the look-out for this disease and try and catch it early. Remove and destroy infected plants to help slow the spread to other rows and/or fields. If you can catch it early enough and cull it you might be able to slow it down. There are no conventional or organic fungicides that have shown to be very effective against this disease.

Anything a grower can do to minimize leaf wetness and reduce humidity within the plant canopy can help slow the disease. This can include using drip irrigation, increasing plant spacing, and improving air flow across beds and in greenhouses.

Dr. Meg McGrath of Cornell University has been working on basil downy mildew for many years. I talked with her today to ask her about seed treatment since basil downy mildew can be seed-borne. She said to not even bother treating seed if you are growing it during its normal season because the spores are ubiquitous outdoors and it won’t make a difference. Folks growing basil in greenhouses during the winter should treat seed, although it’s difficult to do so in hot water or bleach since basil seed produces a gelatinous exudate in water making it very difficult to handle. Some seed companies are now steam-treating basil seeds.

Dr. McGrath has done research on resistant varieties and found that the sweet basil variety ‘Eleonora’ is partially resistant to downy mildew. It still becomes infected but not as severe as other varieties. This variety should be available from Johnny’s Selected Seeds in November. Dr. McGrath is currently conducting research on organic fungicides in combination with this partially resistant variety and will share results of that research later this year on her basil downy mildew web page.

NC State University Plant Pathologist Dr. Lina Quesada is doing DNA and RNA sequencing of the pathogen to build a genome for basil downy mildew. Her lab is collecting samples from the various counties to learn more about this new disease (see photo below of her Lab Manager collecting samples from a local farm).

NCSU researcher Saunia Withers collects samples of diseased basil for DNA sequencing.

If you think you have downy mildew in your basil please contact your county Agriculture Agent and send photos and/or physical samples to the Plant Disease and Insect Clinic to confirm diagnosis of the disease. If  downy mildew is confirmed in your samples by an expert, please consider making a report to warn others that the pathogen is in the area. The report can be anonymous so that only the county information is provided.

For more information:

North Carolina State University Basil Downy Mildew Alert

Expect and Prepare for Downy Mildew in Basil – Cornell University