| Summary: | Basil (Ocimum spp.) is among the most economically important culinary herbs grown worldwide. Ocimum basilicum includes both the European style sweet basil and the distinct Thai basil. Field, greenhouse, and indoor production practices involve growing dense compact stands that facilitate ideal conditions for the development of significant foliar diseases. One such disease, which has increased in prevalence and severity over the past decade throughout basil production regions in the United States and Europe, is bacterial leaf spot (BLS) caused by the pathogen Pseudomonas cichorii, a common bacterial pathogen with a broad host range including both wild plant species and commercial crops. Under favorable conditions, BLS is capable of causing significant losses before harvest, with growers currently having few economically viable chemical controls available for the disease. At present, no known resistance to BLS exists in commercially available cultivars. This study establishes a rapid pathogenicity screening assay for identification of potential BLS resistant germplasm within the O. basilicum and broader Ocimum genus for use in breeding genetically resistant lines. Three basil accessions, ‘Lettuce Leaf’ (O. basilicum), ‘Mrs. Burns’ Lemon’ (O. africanum), and the coded Rutgers’ ‘RUGBX 88’ (Ocimum spp.), were inoculated with P. cichorii at 108 CFUs and screened for disease severity at the cotyledon, first true leaf, second true leaf, and mature growth stage (four to five nodes). Disease severity scores (0 to 5) were assessed daily from 3 to 6 days post-inoculation (DPI) to determine the best growth stage for rapid screening purposes. Disease severity scores at the cotyledon and first true leaf stage displayed significant variation from mature plants while there were no significant differences (Dunn’s test, α = 0.05) in ratings between the second true leaf stage and maturity for all three accessions; the second true leaf growth stage was then selected to screen a collection of commercial basil cultivars for BLS resistance. Pathogenicity assays at the second true leaf stage revealed substantial variation in disease severity scores with the lowest being 2.28 (‘Queen of Sheba’) and the highest being 4.65 (‘Siam Queen’) across all lines tested. This methodology demonstrates the screening of basils at the second true leaf growth stage can readily be applied to Ocimum spp. Germplasm collections for the identification of potential BLS-resistant lines for incorporation into breeding programs.
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