Ecology and Management of Stemphylium Leaf Spot of Spinach

Kayla Anne Spawton

doi:10.7273/000006356

Stemphylium leaf spot of spinach has re-emerged as a disease of economic concern for fresh market, processing, and seed production. The two main causal agents, Stemphylium beticola and S. vesicarium, are seedborne and seed transmitted. The overall objective of this study was to elucidate the biology and epidemiology of these pathogens, and to refine management options for the disease by: i) identifying Stemphylium species and development of their teleomorphs associated with Stemphylium leaf spot in spinach seed crops, and species colonizing seed grown in key countries of seed production; ii) screening spinach cultivars for resistance to S. vesicarium; iii) determining the prevalence of resistance to the fungicides azoxystrobin and pyraclostrobin that have been used widely to control this disease; iv) evaluating genetic differences of pathogenic vs. non-pathogenic seedborne isolates of S. vesicarium; and v) sequencing the genome of S. beticola isolates to complement genomes available for S. vesicarium. Isolates of S. beticola, S. vesicarium, and S. drummondii were pathogenic to spinach, but eight other Stemphylium species associated with spinach were not. Cultivars with resistance to S. vesicarium were identified for fresh market and processing spinach production. In vitro and in vivo assays confirmed resistance to azoxystrobin and pyraclostrobin for all spinach leaf and seed isolates of S. vesicarium tested, but not isolates of S. beticola. Resistant isolates all had the G143A mutation in the cytochrome b gene that confers this resistance. The proportion of seedborne S. vesicarium isolates pathogenic to spinach varied among seed lots, and was negatively correlated with genetic diversity of the isolates. Seed populations of S. vesicarium were structured genetically based on pathogenicity to spinach, not by seed lot or country of seed production. Genotypes of pathogenic S. vesicarium isolates from leaf spots were detected on seed lots used to plant the crops, and seed populations were more diverse genetically than foliar populations. The sequenced genomes of two S. beticola isolates from spinach will facilitate genetic comparison of species that cause Stemphylium leaf spot, and development of molecular detection tools. This research provides new insights into Stemphylium leaf spot of spinach with implications for enhanced management of this disease.

Ecology and Management of Stemphylium Leaf Spot of Spinach

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