Sunflower Disease Update: Powdery mildew and Alternaria leaf spot developing Back »

Written collaboratively by Emmanuel Byamukama, Febina Mathew, and Ruth Beck.

Alternaria leaf spot and powdery mildew were found at low levels in sunflower scouted in east and central South Dakota, in addition to the sunflower rust and Phomopsis stem canker reported last week.

Alternaria Leaf Spot

Alternaria leaf spot is characterized by irregular dark gray to black lesions on the leaves (Figure 1). The disease develops in lower and middle leaves. Infection is promoted by extended moisture and warm conditions. The Alternaria pathogen overwinters on sunflower residue and crop rotation may help reduce inoculum levels. Fungicides can be used if the middle and lower leaves are found with Alternaria leaf spot and the sunflower has not reached R6. Host resistance to this pathogen is not known because this disease is considered to be minor in the Northern Plains.

Figure 1. Alternaria leaf spot developing on sunflower leaves.
Credit: E. Byamukama

Powdery Mildew

Powdery mildew develops under periods of high humidity. Symptoms start as white spots on the upper side of the leaf on lower leaves (Figure 2). Under severe symptoms, the whole leaf can be covered with white mycelia that reduce photosynthesis. Powdery mildew is caused by a fungal pathogen, Golovinomyces spp. This disease is not commonly found in sunflower in South Dakota and its impact on yield is not known. Powdery mildew inoculum comes from previously infected sunflower stalks and from wild sunflower. Control of volunteer sunflower and crop rotation may reduce inoculum. However, spores of this pathogen can be transported over long distances by wind.

Figure 2. Powdery mildew symptoms on sunflower leaves.
Credit: E. Byamukama

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