Phomopsis stem canker in sunflower is back, says Ruth Beck, SDSU Extension agronomy field specialist.
It's a sunflower disease that reduces head and weakens size and weakens stems. Lodging can be significant.
Phomopsis stem canker fungus overwinters in infected plant residue. Spores are produced on the previous year's crop residue and are spread by rain splash or windblown to nearby new crop tissue. 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. The infected stalk at the lesion can be crushed with moderate thumb pressure.
There are no fungicide treatments registered in the United States for Phomopsis control at this time.
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 says.
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.
Source: SDSU Extension