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Alternaria Leaf and Stem Spot
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Alternaria leaf and stem spot is an internationally important disease that can be yield-limiting in areas with warm and humid climates. While the disease is common in the U.S. Great Plains, it is seldom severe enough to be considered economically important in this region. However, when sunflowers are produced in the Southeastern U.S., significant defoliation and yield losses can occur. Similarly, the warm, humid areas of south-central Europe, India, China, Brazil and Australia have occasionally reported serious yield losses.
The causal fungus is Alternariaster helianthi with several minor Alternaria species being able to cause similar symptoms. The fungus overwinters on infested crop residue, primarily on old stalks.
In the U.S. Great Plains, the disease is most commonly found towards the end of the season on older (and often senescing) leaves in the lower canopy. Optimal conditions for infection and disease development are temperatures of 77 to 82 F with frequent periods of leaf wetness exceeding 12 hours.
Spores are spread by wind and splashing rain onto the lower leaves and stems. Initial leaf spots are dark and angular in appearance, and may be surrounded by a lighter colored halo. Lesions often concentrate along the leaf margins and leaf tips. Lesions may enlarge and coalesce in large patches, causing the leaf to wither and die. Defoliation occurs from the bottom of the plant upward.
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