Evaluation and control of Alternaria alternata causing leaf spot in soybean in Northeast China

Aim The aims of this study were to characterize the causal agent of soybean leaf spot and determine the pathogenicity of the pathogen to the main crops in Northeast China and detect the sensitivity of pathogens to the main chemical fungicides and the potted control effect. Methods and results In 2020 and 2021, an uncommon leaf spot was observed in Harbin, Heilongjiang Province (125 degrees 42 '-130 degrees 10 ' E, 44 degrees 04 '-46 degrees 40 ' N), China. The pathogen can infect soybean leaves and cause leaf spot. We collected diseased soybean leaves and isolated four pathogen organisms, all of which were identified as Alternaria alternata through morphological and molecular identification. Koch's postulates were used to confirm pathogenicity. To the best of our knowledge, this is the first report on soybean leaf spot disease caused by A. alternata in northeast China. Moreover, A. alternata had a broad host range and caused leaf spot in most legumes. However, it did not infect medicated lentil (Dolicho Lablab L.) or tobacco (Nicotiana tabacum L.). Assessment of A. alternaria susceptibility to fungicides by spore germination method, isolates of A. alternata were most sensitive to flusilazole, with EC50 values of 0.0040-0.0053 mu g ml(-1). Through two pot experiments, the average control efficacy of 0.1 mg ml(-1) flusilazole on soybean leaf spot caused by A. alternata was 80.7%. Conclusions The work reported that A. alternata is the pathogen organism that causes soybean leaf spot in northern China. The pathogen organism can infect a variety of leguminous plants. Considering the control cost and effect, flusilazole is more suitable for controlling leaf spot disease in the field, and benzoxystrobin can be used as an alternative fungicide.