Characterization of Species Associated with Stemphylium Leaf Spot of Spinach ().

Stemphylium leaf spot can result in significant losses to spinach seed, processing, and fresh market crops. isolates ( = 1,775) collected from 2000 to 2022 from spinach seed, leaves, and seed crop stem residues were used to assess the diversity of species associated with spinach. Eleven species were identified based on sequences: (63.6% of samples), (48.9%), (5.1%), (4.5%), (4.0%), (3.4%), and , , , , and (each 0.6 to 1.7%). Only isolates of , , and were pathogenic to spinach. The incidence of spinach seed on which was observed ranged from 2.5 to 73.5% per seed lot, with and predominant. However, only 60.7 and 62.3% of isolates tested for these two species were pathogenic to spinach, respectively. Therefore, the incidence of species on spinach seed may not reflect accurately the risk of a seed lot carrying pathogenic isolates. Fused and genes were detected in isolates of , but only was detected in isolates, which corroborates previous studies that have proposed the two species to be self-fertile. The duration of ascospore dispersal of and from spinach seed crop stem residues in western Washington, the primary region of spinach seed production in the United States, occurred from midwinter to late spring or early fall, potentially serving as inoculum for the next season's spinach seed crops. Growers should incorporate residues into the soil after harvest to reduce inoculum production of these pathogens on spinach seed crop residues.