The Diversity of Isolates Collected from Tomato Plants in U.S. High Tunnels.

High tunnels extend the growing season of high value crops, including tomatoes, but the environmental conditions within high tunnels favor the spread of the tomato leaf mold pathogen, (syn. ). Tomato leaf mold results in defoliation, and if severe, losses in yield. Despite substantial research, little is known regarding the genetic structure and diversity of populations of . associated with high tunnel tomato production in the United States. From 2016 to 2019, a total of 50 . isolates were collected from tomato leaf samples in high tunnels in the Northeast and Minnesota. Other species were also isolated from the leaf surfaces. Koch's postulates were conducted to confirm that . was the cause of the disease symptoms observed. Race determination experiments revealed that the isolates belonged to either race 0 (six isolates) or race 2 (44 isolates). Polymorphisms were identified within four previously characterized effector genes: , , , and . The largest number of polymorphisms were observed for . Both mating type genes, -- and --, were present in the isolate collection. For further insights into the pathogen diversity, the 50 isolates were genotyped at 7,514 single-nucleotide polymorphism loci using genotyping-by-sequencing. Differentiation by region but not by year was observed. Within the collection of 50 isolates, there were 18 distinct genotypes. Information regarding . population diversity will enable better management recommendations for growers, as high tunnel production of tomatoes expands.