Phomopsis stem canker in sunflowers

BROOKINGS — The prevalence of Phomopsis stem canker first showed up in South Dakota’s sunflower fields last fall.

After touring around eastern Hughes County the first week in October, it was clear the canker is back.

Phomopsis stem canker is noted to be the most widespread stalk disease in sunflowers in the Northern Great Plains. Prolonged periods of wet weather, high humidity of 90 percent and greater as well as moderate temperatures of 68-75 degrees Fahrenheit encourage the disease’s development.

Yield losses result from smaller heads and lighter seed, and from lodging due to weakened stems, said Ruth Beck, SDSU Extension Agronomy Field Specialist.

She explained that the Phomopsis stem canker fungus overwinters in infected plant residue. “Spores (ascospores) are produced on the previous year’s crop residue and are spread by rain splash or windblown to nearby new crop tissue,” she said.

The infection begins on the margins of lower leaves. A brown necrotic area develops and may be bordered by a pale green or chlorotic margin. The infection spreads down through the veins to the petiole and finally to the stem.

“The distinguishing feature of the disease is the large tan to light brown lesion or canker that typically surrounds the stem at the junction of the leaf petiole and the stem,” Beck said.

Often confused with Phoma black stem and Sclerotinia mid-stem rot, the Phomopsis stem canker lesion is much larger, reaching 6 inches in some cases, and typically has a sunken border. “Phomopsis stem canker can result in pith damage making the plant more prone to lodging. The infected stalk at the lesion can be crushed with moderate thumb pressure,” she said.

Treatment options

There are no fungicide treatments registered in the United States for Phomopsis control at this time.

Beck said crop rotation and planting resistant hybrids are the best ways to manage Phomopsis stem canker. “Crop rotation of two to four years is generally recommended,” she said.

Small grains, sorghum and corn are the least likely crops to host the pathogen.

For information on resistant hybrids, Beck suggested growers visit with seed companies, which often evaluate their hybrids for Phomopsis stem canker resistance.

In 2010, researchers from the public universities and the USDA-ARS along with the National Sunflower Association established an aggressive research strategy that includes; identifying Phomopsis species infecting sunflowers in the U.S., breeding for resistance and determining which fungicides and rates may provide some control. It continues to remain a top research priority of the National Sunflower Association.

For more information, visit iGrow.org.