Host Resistance Screening of Baby Kale Against Downy Mildew Isolates Across the Central Coast of California.

Abstract

Hyaloperonospora brassicae, the causal pathogen of downy mildew, presents significant challenges to spring mix greens production in California. Genetic resistance provides a strategy for sustainable management to reduce downy mildew infections and pesticide use in organic and conventional production systems. This study aimed to identify sources of downy mildew resistance to facilitate resistance breeding in baby kale. To achieve this, three host resistance screenings were conducted to assess the resistance of baby kale accessions against eight downy mildew isolates collected from eight distinct locations on the Central Coast of California. Artificial inoculation was performed by spraying a sporangia suspension onto baby kale plants, which were then incubated in a humidity chamber. Disease severity was assessed by examining both surfaces of each leaf for chlorotic and necrotic symptoms and sporulation and then quantified using an established rating scale. Screening of all 212 accessions revealed an average disease severity of 28%, with severities ranging from 0 to 100%. The initial subset screening showed average disease severities ranging from 2.2 to 9.4% depending on the isolate. The final subset screening demonstrated a range of 0.003 to 0.072% average disease severity among the four isolates, with 13 accessions exhibiting 100% estimated resistance probability, 11 accessions between 99.0 and 99.9%, and 1 accession below 99.0%. These results suggested that downy mildew could be effectively managed in baby kale through resistant varieties.