Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/2007
Publication Date: 6/7/2008
Citation: Davidson, C.R., Evans, T.A., Mulrooney, R.P., Gregory, N.F., Carroll, R.B., Oneill, N.R. 2008. Lima Bean Downy Mildew Epiphytotics Caused by New Physiological Races of Phytophthora phaseoli. Plant Disease. 92:670-674. Interpretive Summary: The mid-Atlantic region is one of the largest producers of lima beans in the United States. For example, Delaware and Maryland plant 18,000 and 3,000 acres, respectively, to lima bean. Downy mildew is a lima bean disease caused by the fungus Phytophthora phaseoli, and it is the most devastating disease associated with lima bean production. Prior to 1995, only four types, or races, of the fungus had been detected. Recently a new race has emerged. Because there is no crop resistance to new races, their appearance is a constant threat and could result in major economic losses to the lima bean growers and the vegetable processing industry. The earlier a change in race is detected, the faster new resistant lines can be developed and available for commercialization. We collected 128 samples from diseased plants in the mid-Atlantic region and determined their race by the symptoms produced on different lines of lima bean. We also used molecular tools in an attempt to identify the races. These tests could lead to the rapid detection of the specific type of fungus present. Three races were detected and identified, and the presence of a new race, race F, was discovered. Molecular tools revealed a very uniform population with few differences. This research will be useful to breeders, seed companies, APHIS, and research scientists concerned with controlling diseases caused by races of this fungus.
Technical Abstract: Prior to 1999 race D was the prevalent phenotype of Phytophthora phaseoli (cause of downy mildew on lima bean) in the mid-Atlantic region of the United States. New races of P. phaseoli were implicated in recent downy mildew outbreaks on previously resistant cultivars. With the development of new races of P. phaseoli and the lack of resistant cultivars to effectively manage them, epiphytotics could lead to the devastation of the mid-Atlantic region's lima bean industry. An investigation of these new races began in the summer of 1999 with the collection of 128 field isolates from Maryland, Delaware, and New Jersey. The use of cultivar differentials, involving inoculation of differential resistant (R) and susceptible (S) cultivars, confirmed the presence of a new race, which was designated as race F. As of the year 2000, race E was the prevalent phenotype in the region. AFLP DNA fingerprinting along with allozyme analysis using both GPI and PEP overlays, were utilized in an attempt to genetically differentiate the races of this pathogen. The results of the molecular characterization illustrated that although races of P. phaseoli were detected though assay of cultivar differentials, allozyme and AFLP analysis did not distinguish genotypic differences between the races.