Inheritance of Resistance in Lima Bean to Phytophthora phaseoli, the Causal Agent of Downy Mildew of Lima Bean

The oomycete Phytophthora phaseoli is one of the most threatening pathogens of lima bean (Phaseolus lunatus) in the humid Mid-Atlantic United States. In the last 60 years, P. phaseoli has evolved to overcome genetic resistance in the host and several physiological races have been identified during the last 6 decades. Six physiological races A, B, C, D, E, and F have been identified over the years. Only race F has been detected in the field over the past decade. Identifying and characterizing sources of resistance to this pathogen and determining the nature of resistance were the main objectives. Eight commercial cultivars and 35 germplasm accessions of P. lunatus were evaluated for their reaction to races E and F. Four commercial cultivars and four accessions with resistance to race E, and two cultivars and four accessions with resistance to race F were identified. None of the germplasm evaluated were resistant to both races. Five populations of F2 plants and a recombinant inbred line (RIL) population were produced and inoculated to investigate the inheritance of resistance to races E and F. Resistance to races E and F was determined to be conferred by single, independent, dominant genes.