Monitoring Of Bremia Lactucae Races In São Paulo State In 2012.

Lettuce (Lactuca sativa L.) is among the leafy vegetables, economically, the most important to Brazil. In winter, when there is a predominance of low temperatures and high leaf wetness, downy mildew of lettuce, a disease caused by the agent Bremia lactucae, occurs in almost all regions of the world where the lettuce is grown, and is considered one of the most severe foliar diseases for the culture. The use of resistant cultivars is presented as the main alternative for control, being necessary the annual monitoring of races of this pathogen. To obtain information to assist breeding programs for resistance to downy mildew, this study aims to report the emergence of races of B. lactucae in 2012 that occurred in the main producing regions of São Paulo. In August 2012 leaves with sporangia of B. lactucae of main lettuce producing regions of the state were collected, and each sample was considered an isolated, totaling 49 isolates. The sporangia collected were multiplied on susceptible cultivar Solaris, with subsequent inoculation in differential cultivars, performing evaluations on the same day of the first appearance of sporulation in the susceptible cultivar 'Green Tower' (DM 0), according to the code "Sextet". The data allowed to identify two new codes, originating two new races: SPBl: 10 (63/31/02/00) and SPBl: 11 (63/63/18/00). Genes Dm-14, Dm-15, R-17, R-18, R-36, R-37 and R-38 confer resistance to these new races identified. It is recommended, therefore, the use of genes R-17, R-18 and R-38 as sources of resistance in cultivars developed for the State of São Paulo, because they confer resistance to all 11 races already identified. This information is of great importance, since it assists in the development of resistant lines, because it is possible to identify the genes present in the differential cultivars that could be transferred to commercial cultivars with good agronomic traits and, thus, obtain cultivars resistant to all downy mildew races, allowing reduced use of pesticides, and providing hence lower production costs and environmental impact.