Sweet basil cultivars resistant to downy mildew evaluated under open field conditions

Sweet basil is the most widely grown herb in the U.S. Downy mildew is the most common disease. Management is necessary because there is no tolerance for disease symptoms on leaves marketed for fresh consumption. Host resistance is a preferred management practice. Research was initiated to evaluate cultivars bred to be resistant to downy mildew when they started to become commercialized. Five replicated experiments were conducted between 2018 and 2022 with field-grown basil exposed to naturally-occurring wind-dispersed sporangiospores of Peronospora belbahrii. Eleonora, which is one of the first resistant cultivars commercialized, exhibited limited suppression in this study (26 and 34% control based on AUDPC for incidence of leaves with symptoms compared to a susceptible cultivar). Amazel, the related cultivar Pesto Besto, and Prospera plus related cultivars from Genesis Seeds provided excellent to complete suppression (usually 100%). They have resistance gene Pb1 . The four Rutgers DMR cultivars (Devotion, Obsession, Passion, and Thunderstruck) provided good to excellent control with some yearly variation (46 -99% control). The recommended management program for downy mildew in field-grown sweet basil in an integrated management program with fungicides applied to resistant cultivar(s), plus weekly checking the crop for symptoms as well as staying current on knowledge about changes in the pathogen, commercialization of new resistant cultivars, and registration of new fungicides.