Virulence Profiling of Bremia lactucae Causing Lettuce Downy Mildew: A Study Across Key Agricultural Areas in Brazil

Abstract

Lettuce Downy Mildew (LDM), caused by Bremia lactucae, is a major disease during the winter, particularly under prolonged wetness periods. Pathogen populations are dynamic and evolve through the emergence of new virulence phenotypes (v-phenotypes). Breeding programs focus on identifying resistance genes that confer broad and durable resistance, particularly against highly virulent populations such as those commonly reported Europe, and USA. Private breeding companies, on the other hand, are engaged to identify resistance genes (R-genes) that offer useful resistance to LDM. Continuous monitoring of B. lactucae population is crucial to aid the development and deployment of LDM-resistant cultivars. This study aimed to monitor the v-phenotypes of B. lactucae in major lettuce production regions in Brazil. A differential set of lettuce genotypes was used to determine the sextet codes corresponding to the v-phenotypes of isolates collected in 2020-2022. The most prevalent v-phenotypes were 31-00-00, 51-00-00, and 31-00-02. The R-genes found in ‘Balesta’, ‘Kibrille’, ‘Bartoli’, and ‘RYZ910457’ provided resistance against most of the isolates collected. The R-gene present in ‘Balesta’ is particularly effective for LDM, breeding programs should aim to improve lettuce similar to this cultivar for the Brazilian market. V-phenotypes corresponding to European (Bl:16EU, Bl:18EU, Bl:20EU, Bl:23EU, Bl:24EU, Bl:25EU, Bl:26EU), and USA (Bl: 8US) Bremia races were identified. Unique v-phenotypes of B. lactucae were found in Brazil, along with the detection of several common to Europe, suggesting that new lettuce cultivars tested for resistance to these European v-phenotypes, are likely to exhibit some degree of resistance in these Brazilian regions.