Transferring White Mold Resistance From the Secondary Gene Pool of Common Bean.

White mold (WM) is a severe disease of common bean (Phaseolus vulgaris L.). Partial resistance is found in Andean and Middle American gene pools. But, higher levels of resistance occur in P. coccineus and other secondary gene pool (SGP) species. The objective of this study was to transfer WM resistance from the SGP to the common bean. One WM resistant interspecific breeding lines (IBL) was developed by recurrent backcrossing (RB) of small tropical black bean ‘ICA Pijao’ with P. costaricensis S 33720 and four IBL from RB of pinto ‘UI 320’ with P. coccineus PI 433246 and two IBL from congruity-backcrossing between ICA Pijao and P. coccineus G 35172. These IBL were compared with other known WM resistant IBL derived from P. coccineus and Andean and Middle American common bean cultivars, breeding lines and germplasm accessions in a randomized complete block design with three replications in the greenhouse in Colorado and Idaho in 2010. The fourth or fifth internode of the main stem was inoculated using a modified cut-stem method. Two mycelial plugs from a 48 hr old culture were used. Also a week later an internode on a branch of each plant was again inoculated using the same technique. White mold was rated 14 and 28 days after the first inoculation using a 1 to 9 scale, where 1 = resistant with no visible disease symptoms and 9 = severely diseased or dead plants. The mean WM scores of IBL derived from P. costaricensis S 33720 and P. coccineus G 35172 and PI 433246 were significantly lower than the susceptible checks and other known sources of WM resistance. These IBL should be used to pyramid WM resistance from diverse sources of germplasm and transfer high levels of resistance into cultivars.