Flax black spot caused by Alternaria alternata , a new disease afflicting leaves and the exocarps of flax in China

Abstract Flax black spot was found in Ordos City in 2022, mainly infecting leaves and the exocarps, causing leaves turn yellow and brownish with black leaf tips before drying and damage, and exocarps became brown or black with a layer of black mould which significantly affected the yield and quality. The Y3 and K1 strains from infected leaves and the exocarps were the same pathogen of flax black spot which was identified as Alternaria alternata by morphological characteristics and molecular identification. In the pathogenicity test, A. alternata caused severe black spot on leaves and brown or black exocarps with mould of flax consistent with those observed in the field. The study for pathogenic identification ascertained a new disease of flax in China which lays the foundation for subsequent studies on disease control.