Super Sad Sunflowers: Alternaria leaf and stem spot

Field of sunflowers (Photo: B.B.Shew)

Sunflowers are becoming an increasingly popular alternative crop in North Carolina. A field of sunflowers in bloom is beautiful, but sunflowers are most important as an oil crop. Sunflower oil is lighter in taste and supplies more Vitamin E than other vegetable oils. Sunflower oil is particularly appealing to the food industry because it is trans-fat free and stable without hydrogenation, making it excellent for frying and increasing shelf life. Since most sunflowers are produced in the Great Plains region of the U.S., little is known about which diseases and insect pests may become problematic or limit sunflower production in North Carolina.

As the growing season came to an end, we received several sunflower samples that we diagnosed with Alternaria leaf and stem spot. In the traditional growing areas of the Great Plains, Alternaria is usually of minor importance because conditions are not favorable to severe disease development. Unfortunately for us, North Carolina has hot and humid summers perfectly suited for Alternaria disease development.

Alternaria disease symptoms (Photo: E.C. Lookabaugh)

Alternaria leaf and stem spot is caused by the fungus Alternaria. Two species of Alternaria cause disease on sunflowers, Alternaria helianthi (now known as Alterniaster helianthi) and Alternaria zinniae with A. helianthi being more common. Symptoms include irregular leaf spots, stem lesions, and dark brown spots on the seed heads. Leaf spots are dark brown with grey centers. Yellow halos around spots are seen on younger foliage. The stem lesions begin as dark flecks that enlarge to form large blackened narrow lesions. Severe infections result in defoliation and stem lodging.

Flower head symptoms (Photo: E.C. Lookabaugh)

Stem lesions (Photo: E.C. Lookabaugh)

The pathogen survives on infested plant debris and on the alternate hosts safflower and cocklebur. Cocklebur is native to the U.S and is found throughout NC. Safflower is not present in NC. The fungus is spread by wind and splashing water. Spores splash from infested crop debris onto the lower leaves and stems, where symptoms first appear. Warm, humid conditions favor disease development and spread.

Control of this disease can be achieved through a strict sanitation program, crop rotation, and the use of preventative fungicides. Sanitation is key. Disease plants should be removed and destroyed. Cultural methods that limit splash dispersal and long periods of leaf wetness probably will help to reduce disease. These include avoiding overhead irrigation and using proper plant spacing to promote air circulation. This disease can be seedborne. Use certified, disease-free seed that is produced in dry regions where Alternaria is not a problem. Do not save seed from diseased plants. Do not grow sunflowers two years in a row in the same field because the fungus overwinters on plant debris. Tillage operations promote debris decomposition and reduce the chances of splash dispersal of the fungus. Preventative fungicide applications can be started when symptoms first appear or at flowering, since this is when plants become most susceptible to the disease.

Happy Sunflower (Photo: B.B. Shew)

Post by Emma Lookabaugh and Barbara Shew