Recurrent selection for physiological resistance to white mould in dry bean

P>White mould (WM) [caused by Sclerotinia sclerotiorum (Lib.) de Bary] is a widespread disease of dry and green bean (Phaseolus vulgaris L.) in North America and elsewhere. Our objective was to determine the effectiveness of recurrent selection (RS) for pyramiding WM resistance from Andean, Middle American and inter-specific breeding lines. Two double-cross populations, Pop I = USPT-WM-1/CORNELL 601//USPT-CBB-1/92BG-7 and Pop II = 'Chase'/I 9365-25//ABL 15/A 195 were developed. Each double-cross comprised 848 F-1 plants for RS cycle one (C-1). Twenty six WM-resistant plants from each double-cross were used to produce RS cycle two (C-2). To measure the gain for WM resistance, 13 selected S-2 families from each of C-1 and C-2 of each population and their four parents (C-0) were tested in a randomized complete block design with three replicates in two greenhouse environments in 2008. Two separate inoculations on each plant one week apart using a cut-stem method were made. The WM reaction was scored at 16, 23 and 33 days post inoculation (DPI) using a 1 (no disease symptoms) to 9 (severely diseased or dead plants) scale. The effects of environment were significant (P = 0.05), but replicate and population effects were not significant (P > 0.05). The RS was effective in changing the mean WM score of selected families from C-0 to C-1 and C-2, but the gain between C-1 and C-2 was not significant in Pop I. The gain in WM resistance for Pop I was 12%. Selection gain in Pop II was 5%, but no significant differences in mean WM scores of C-0, C-1 and C-2 were found. However, selected S-2 families had WM scores similar to those of the best parent and selected families from Pop I. Thus, one cycle of RS applied to double-cross populations was adequate for improving WM resistance in common bean.