EVALUATION OF PHYTHOPTHORA SOJAE FROM ARGENTINE SOYBEAN FIELDS D BARRETO1,3, TR ANDERSON2, M GALLY3, and P GRIJALBA3 lINTA, CICA, CC 25 (1712) Castelar; 3Facultad de Agronomia, Universidad de Buenos Aires, Av. San Martin 4453 (1417) Buenos Aires, Argentina; 2Agriculture and Agri-Food Canada, Harrow, Ontario NOR 1 GO, Canada Background and objectives Root rot of soybean caused by Phythopthora sojae M. J. Kaufman and J. W. Gerdemann is one of the most damaging disease in many soybean producing countries of the world. This disease has been found in recent years in Argentine. Field surveys carried out from 1989 to 1992 in the main central soybean producing area of the country indicated that the disease was present in only one county (Pergamino) of the province of Buenos Aires and it occurs in scattered soybean plants. Until that time, all the isolates were race 1. The objective of this study was to evaluate the prevalence and the virulence of P. sojae
  • in the main central soybean area from 1994 to 1997. Materials and methods The pathogen was isolated from symptomatic plants and infested soil. Races were identified with a set of eight soybean differentials using wounded hypocotyls. Plants were rated as resistant or susceptible four days after inoculation [1 ], [2]. Results and conclusions During the 4-year period of 1994-1997, the disease increased in prevalence and incidence mainly under minimum and non-tillage systems and developed into major epidemics in the southern part of the soybean area. The pathogen is now present in several provinces such as C6rdoba, Santa F6, Entre Rios, and Buenos Aires. From 162 isolates obtained from plants and infested soils, 75% was classified as race 1, indicating that this race is still the main component of P. sojae
  • population. In 1996, one isolate was identified as race 4 by Agriculture and Agri-Food Canada in Harrow, Ontario. This race is pathogenic on the Rps 1 -a and Rps 1 -c genes for resistance. A group of 39 isolates did not fit any known race virulence pattern. Our study demonstrated that the disease is increasing and the virulence of the pathogen in Argentina has changed dramatically compared to 1989. References 1. Anderson TR, Buzzell RI, 1992. Plant Disease 76,587-589. 2. Schmitthenner AF, Hobe M, 1994. Plant Disease 78, 269-276.