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Studies on chemical control of white mold, conducted between 1994 and 2001 at several locations in North Dakota, resulted in diverse levels of white mold incidence and severity. Navy bean cultivars were evaluated in on-farm trials between 1994 and 1996, while pinto bean cultivars were used between 1997 and 2001. The relationship between yield and white mold incidence in these trials was examined using correlation and regression analysis. White mold incidence was correlated to severity using a second-degree polynomial equation (R 2 = 0.90, P = 0.0001) in pinto bean experiments. For every percent unit increase in white mold incidence, yield was reduced by 12 kg/ha (range 7 to 19 kg/ha) in pinto bean and by 23 kg/ha (range 19 to 26 kg/ha) in navy bean. In both instances, the coefficients of determination were significant (P < 0.04) for most locations or years, and ranged from 0.42 to 0.87 for pinto bean and from 0.98 to 0.99 for navy bean. Fungicide-protected plots had an average white mold incidence of 34 and 50% compared with 76 and 73% in nonprotected plots for pinto and navy bean, respectively. Fungicide applications increased yields by 33 and 26% (P ≤ 0.05) for pinto and navy bean, respectively.

Bean rust (Uromyces appendiculatus) reached epidemic proportions in North Dakota and Minnesota from 1993 to 1996. Although U. appendiculatus is a macrocyclic autoecious rust, neither pycnia nor aecia have been observed in commercial dry bean (Phaseolus vulgaris) in North Dakota fields. The source of initial inoculum is not clearly understood. This study determined the potential for urediniospore survival overwinter. Uredia-bearing bean leaves from artificially inoculated greenhouse-grown plants were kept outside near a field from November to May from 1990 to 1996. Based on bioassays urediniospores survived overwinter, but viability declined over time. Overwinter survival indicates urediniospores may function as initial inoculum.

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