Symptoms

• Alternaria diseases affect the leaves, petals, stems, leafstalks and flower heads. This disease manifests dark-brown, irregularly shaped or circular spots. Stem lesions begin as streaks or black flecks and grow to encompass larger areas of the stems. These symptoms generally do not appear until after the flowering stage.
  
  

Progression

• Alternaria diseases initially appear as lesions on the stems and leaves. These lesions start out brown in color, with a yellow halo. As the disease progresses, the infected area increases and darkens to black. After flowering, the plant's head may also be infected with dark, circular spots on the back. Seedling blight is another possibility on seedlings emerging in rainy weather or under high-moisture conditions in crop residue infected soil.
  
  

Conditions Favorable for Growth

• Wet, warm weather promotes Alternaria disease growth. Regions prone to high humidity and warmer temperatures are susceptible to this disease. Fields that are planted early are susceptible to more severe losses from the affects of the disease than those planted later.
  
  

Control

• As of 2011, there are no known sunflower cultivars resistant to Alternaria disease and no fungicides are available for chemical control. Management occurs through crop rotation and tillage burying infected crops. This encourages more rapid residue decomposition and breakdown. Airborne spores spread alternaria disease. The spores survive and overwinter on infected crop residue. Avoid planting on fields of sunflower stubble and practice regular crop rotation.