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Septoria Leaf Spot

Septoria leaf spot and blight

Septoria leaf spot and blight is a relatively common disease in the U.S. Great Plains, but is not considered economically important in the region. The pathogen (Septoria helianthi) is residue-borne and disease development is favored by moderately warm temperatures, but frequent rains are the dominant factor for disease development. Lesions first develop on the lower leaves as small yellow to dark brown-colored spots (pinhead size). These spots enlarge to circular lesions (up to ¾ inch in diameter) that scatter mostly on the upper side of leaf blade (Figures 1 and 2).

The leaf spots often have a roughly circular appearance and may be delimited by the larger leaf, which gives them an angular or diamond shaped appearance. Over time, the lesions coalesce and the fungus gradually spreads from the lower leaves to upper leaves. The lesions may contain very small fungal reproductive structures (pycnidia), which appear as black specks or bumps when viewed with a hand lens (Figure 3).

This disease most frequently occurs towards the end of the growing season, although visible lesions early in the growing season are not unusual. The disease often goes dormant in periods of warm and dry weather. Active disease management specific to Septoria leaf spot is generally not needed in the US Great Plains, with tillage and crop rotation being sufficient disease-management practices. Yield losses have been documented in China, India and Pakistan, where sunflowers were grown in hot and humid climates with frequent rains. Foliar fungicides are labeled to control Septoria leaf spot, but would not be recommended unless the disease is found to be economically important.

Images

Figure 1. Septoria leaf spot lesions (Charlie Block, Iowa State University) Figure 2. Septoria leaf spot lesions (Charlie Block, Iowa State University) Figure 3. Small black fungal structures (pycnidia) in a Septoria leaf spot lesion (Charlie Block, Iowa State University)

Other NSA Resources

Photo Gallery The Sunflower Magazine's Archive section Approved Chemicals

Disclaimer statements

Information based in part on and reproduced from Kandel, H., Endres, G. and Buetow, R. 2020. Sunflower Production Guide. North Dakota Extension Publication A1995. Informational updates made possible by the Sunflower Pathology Working Group, and is/was supported by the USDA National Institute of Food and Agriculture, Crop Protection and Pest Management Program through the North Central IPM Center (2018-70006-28883).