White mold (WM) caused by Sclerotinia sclerotiorum (Lib.) de Bary is a severe disease of common bean (Phaseolus vulgaris L.) in North America. Common bean has only partial resistance. However, some accessions of P. coccineus L. and other species of the secondary gene pool (SGP) are highly resistant. The objectives of this study were to (i) introgress WM resistance from the SGP and (ii) compare resistance of interspecific breeding lines (IBL) VCW 54 and VCW 55, developed by congruity backcrossing between ‘ICA Pijao’ and P. coccineus accession G 35172, and VRW 32, derived from recurrent backcrossing of ICA Pijao with P. costaricensis accession S 33720 with known sources of resistance. The three IBL, ICA Pijao, and susceptible (‘Othello’) and resistant (92BG‐7, A 195, G 122, I9365–25, ‘ICA Bunsi’, and VA 19) genotypes were screened in the greenhouse in Colorado and Idaho in 2007 and 2008 and in the field in Idaho in 2007. White mold resistance was successfully introgressed from two of the three Phaseolus species (P. coccineus and P. costaricensis) of the SGP. VCW 54 had the highest WM resistance, and VCW 55 and VRW 32 had similar resistance as previously reported. These sources of WM resistance should be introgressed into common bean cultivars.

The fungus Sclerotinia sclerotiorum, cause of white mold, is known to attack >400 plant species. It is a widespread problem in dry bean (Phaseolus vulgaris) in the United States, causing >30% average yield losses. Low to moderate levels of resistance are found in dry bean. However, some accessions of P. coccineus (commonly known as scarlet runner bean) possess a relatively higher level of resistance. Our objective was to verify the reaction of 13 known white mold-resistant P. coccineus germ plasms and determine inheritance of resistance in accessions PI 433246 and PI 439534. Pinto Othello was crossed with PI 433246, and the resulting interspecific F1 was back-crossed onto Othello and allowed to produce F2 seed. Similarly, pinto UI 320 was crossed with PI 439534. The F1 was backcrossed onto UI 320 and allowed to produce F2 seed. The two parents, F1, F2, and backcross to dry bean of each set were evaluated in the greenhouse using the straw test at Fort Collins, CO in 2004. All 13 P. coccineus accessions and the two F2 also were evaluated using the modified petiole test at Kimberly, ID in 2005. All 13 P. coccineus accessions were variable in a 2002 straw test when rated for white mold reaction on a 1-to-9 scale, because the mean disease score ranged from 1.9 for PI 433246 to 4.4 for PI 189023 and 8.8 for the susceptible check Bill Z. For the petiole test, when rated on a 1-to-9 scale, the accessions exhibited an intermediate white mold score of 4 or 5 in 2005. In 2004, the susceptible check Othello exhibited a mean score of 7.9 compared with 3.4, 3.2, and 2.1 for PI 433246, UI 320, and PI 439534, respectively. The white mold reaction of PI 433246 and PI 439534 was dominant in their respective F1. The F2 segregation further indicated that white mold resistance in PI 433246 and PI 439534 was controlled by a single dominant gene. These two and other white mold-resistant P. coccineus accessions and selected breeding lines from the interspecific crosses should be useful for future improvement of white mold resistance of pinto and other market classes of dry and green or snap bean.

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White mold [caused by Sclerotinia sclerotiorum (Lib.) de Bary] is a devastating disease of common bean (Phaseolus vulgaris L.) in cool‐ to moderate‐temperature and wet‐production regions worldwide. Use of resistant cultivars is crucial for effective and economical white mold control. Partial resistance exists in cultivated and wild common bean and Phaseolus species of the secondary gene pool. The objectives were to (i) develop highly resistant breeding lines (BL) from a recurrent interspecific backcross of common pinto bean ‘UI 320’ with P. coccineus PI 439534, and (ii) compare their response with known sources of resistance. Five pinto BL derived from UI 320 × 2/PI 439534 interspecific backcross population, the two parents, nine known sources of resistance, and susceptible pinto ‘Othello’ were screened in the greenhouse. Sclerotinia sclerotiorum isolates ARS12D and ND710 were used at the University of Idaho, Kimberly, in 2012 and isolates CO467 and NY133 at Colorado State University, Fort Collins, in 2013. All five interspecific pinto BL (VC13‐1, VC13‐3, VC13‐4, VC13‐5, VC13‐6) in Idaho and three (VC13‐4, VC13‐5, VC13‐6) in Colorado exhibited significantly (P ≤ 0.05) higher levels of resistance than PI 439534. Their resistance was either similar to (in Colorado) or higher (in Idaho) than the highest levels available in other interspecific BL (92BG‐7, I9365‐31, VCW 54, VRW 32) derived from the secondary gene pool thus far, as well as medium‐ (USPT‐WM‐1) and small‐ (‘ICA Bunsi’) seeded Middle American and large‐seeded Andean dry (G 122) and green (NY 6020‐4) beans. The effectiveness of the five interspecific pinto BL for controlling white mold with and without fungicides and other disease management strategies should be performed. Genetics of resistance and tagging and mapping of new resistance genes and/or quantitative trait loci should be performed. Also, high levels of resistance from the five interspecific pinto BL should be pyramided across Phaseolus species and/or transferred into cultivars.

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