Virulence of Phytophthora sojae in the Pampeana Subregion of Argentina from 1998 to 2004

Abstract Phytophthora root rot is one of the most serious diseases of soybeans in Argentina. Surveys of commercial fields and trial plots of soybean were conducted throughout the northern Pampeana subregion (Argentina) between 1998 and 2004. A total of 193 isolates of Phytophthora sojae were collected and classified into races or virulence formulae. Among the 173 isolates tested on 8 differentials, 42 different pathotypes were detected, including 18 described races. Races 1, 4, 5, 7, 9, 13, 23 and 24 were found in both plants and soils, whereas races 2, 3, 6, 8, 11, 14, 15, 17, 43 and 44 were only isolated from plants. An additional 19 pathotypes were described from 20 isolates tested in Canada on the expanded set of 14 differential cultivars. Currently, all Rps genes/alleles associated with resistance have been defeated, indicating an increased complexity of virulence within the P. sojae populations in the region. The great increase in virulence complexity found in this study is most likely a result of a long period of continuous production of soybean cultivars with Rps genes and the extensive adoption of the no‐tillage system.