DEVELOPMENT OF RACES OF PHYTOPHTHORA PHASEOLI, THE CAUSAL AGENT OF DOWNY MILDEW OF LIMA BEAN (PHASEOLUS LUNATUS) AND DEVELOPMENT OF RESISTANCE

摘要

Over the past five years two new races of Phytophthora phaseoli, the causal agent of downy mildew, have been detected in Delmarva lima bean fields. During the 2000 growing season race E of P. phaseoli caused in excess of two million dollars damage tothe lima bean crop. The 2003 growing season was similar climatologically to the 2000 season and a new phenotype of P. phaseoli was dominant, race F, yet losses were minimal. These losses were lessened in part due to the timely application of copper or Ridomil-Gold fungicides and to varietal selection based on a number of years of evaluation by our laboratory. Varietal resistance is the most economical control measure for downy mildew and development of two new races of this pathogen in such a short period of time has caused a problem for breeders providing resistant germplasm for the use of growers.