Occurrence of black stem on Helianthus annuus caused by Phoma macdonaldii and resistant evaluation of different sunflower varieties

The first reported case of Phoma macdonaldii Boerema that caused sunflower black stem disease in China was from Xinjiang Uygur Autonomous Region in 2008. Black spindle lesions were observed on sunflower stems in the fields of Inner Mongolia region in China. The causal agents were isolated from the infected stems. Cultures on potato dextrose agar (PDA) plates were whitish with black particles underneath. Dark and pear-like pycnidia with yellowish or dirty whitish exudates were also observed on the surface of the cultures. The cultures produce primarily non-septate conidia. The cultures could infect sunflowers stems by artificial inoculation and developed symptoms consistent with those in the field. Morphological characteristics and molecular identification indicated the isolated causal agents were P. macdonaldii. Phylogenetic analysis based on ITS1, 5.8 s RNA and ITS2 region confirmed the identification. Disease resistance assessments were performed on twenty sunflower varieties and the results suggested that the resistance of seven sunflower varieties to P. macdonaldii was above the average; SC89 and Longkuiza1 were susceptible to P. macdonaldii .