Downy mildew, incited by P. phaseoli Thaxt., became a major threat to lima bean (P. lunatus L.) production in the humid eastern United States in the 1940s. The U.S. Department of Agriculture (USDA) began breeding lima bean for resistance to downy mildew in 1948 in collaboration with the Long Island Vegetable Research Farm at Riverhead, NY (Wester and Cetas, 1959). At that time, no races or variants of the pathogen were known. A physiological race, hereafter referred to as race, is a subdivision of a pathogen species, particularly fungi, distinguished from other members of the species by specialization for pathogenicity to different cultivars of a host (Kirk et al., 2001). The use of differential cultivars has been the principal method to distinguish emerging races of P. phaseoli. Differential cultivars used to distinguish physiological races A, B, C, D, E, and F have been reported by Evans et al. (2007).
The USDA downy mildew resistance breeding program began in 1948 and in 1958, released ‘Thaxter’, its first cultivar with downy mildew resistance (Wester and Cetas, 1959). The resistance in ‘Thaxter’ was conferred by a single dominant gene derived from the P. lunatus landrace PI 164155. A new race of P. phaseoli, which overcame the resistance in ‘Thaxter’, was detected in 1958 and was designated race B (Wester and Cetas, 1959). The landrace PI 189403 was found to be resistant to races A and B (Wester and Cetas, 1959) and was used as the resistant parent in the development of ‘Dover’, which was released by the USDA in 1970 (Stavely, 1991). A new race of downy mildew, designated C, which overcame the resistance in Dover, was identified in 1969 when the cultivar was still in trial (Wester, 1970). The PI 195342 was found to be resistant to the new race C as well as races A and B and were used in the development of the germplasm C-171, which was released in 1973 and was resistant to races A, B, and C (Thomas and Fisher, 1979). In 1975, a fourth race of downy mildew, designated D, was identified. PI 195342, used since 1972 as a source of resistance to races A, B, and C, was susceptible to race D (Thomas and Blount, 1976). A germplasm release designated B2C (PI 549515), made jointly by USDA-ARS and the Delaware Agricultural Experiment Station in 1976, had resistance to races A, B, C, and D. B2C arose from crosses of PI 195342, ‘Bridgeton’ and US 1068, a sibling of ‘Bridgeton’ (Thomas and Fisher, 1980).
The lima bean improvement program conducted by USDA-ARS, Beltsville, between 1949 and 1989, resulted in significant improvement in quality, yield, and most importantly, resistance to the four prevalent races of P. phaseoli (Evans et al., 2007; Kee et al., 1997). From this program, more than 30 cultivars and germplasm releases were made available to plant breeders, seed companies, and producers (Stavely, 1991).
For 20 years, no new races of P. phaseoli were detected in the Mid-Atlantic region (MAR). However, in 1995, a new race of P. phaseoli, designated race E, was detected in Delaware and became more widely distributed than race D (Evans et al., 2002). In 2000, an epidemic incited by race E resulted in an estimated loss of 40% of Delaware lima bean production, equal to a farm gate value loss of $3,000,000 (Scuse and Feurer 2003). Then, race F of P. phaseoli was first detected in 2000 (Evans et al., 2005) and was the most prevalent race by 2004. Because of the emergence of the E and F races, it is important to evaluate the reaction of the cultivars most commonly grown in Delaware to these new races and identify possible sources of resistance. There are currently no cultivars with resistance to both races E and F of P. phaseoli, resistance to race F is needed in horticulturally acceptable germplasm for use in the humid MAR.
In 2004, a lima bean breeding program was re-initiated at the University of Delaware to develop new cultivars for the MAR, including cultivars with resistance to races E and F of P. phaseoli. This study was undertaken to support the disease resistance breeding goals of the program. The first objective of was to evaluate the reactions of commercial cultivars to races E and F of P. phaseoli in the field. The second objective was to identify sources of resistance to those races from diverse germplasm, including cultivars, germplasm releases, and landraces. The third objective was to determine the pattern of inheritance of resistance to these two races to facilitate incorporation of resistance genes into commercially acceptable small, flat-seeded (baby), and large, plump-seeded (Fordhook), lima bean cultivars for production in the MAR.
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