White mold of dry beans (Phaseolus vulgaris L.) is caused by the fungus pathogen Sclerotinia sclerotiorum (Lib.) de Bary, which can reduce crop yields 30 to 80% in irrigated regions. To reduce disease‐caused losses, germplasm improvement programs are incorporating sources of genetic resistance and desirable plant architectural traits into commercially acceptable market types. This field study was conducted to develop a small plot research technique to measure white mold effects under differential disease pressure, and to test parental and breeding lines for their potential in controlling disease. Sixteen entries were selected for their known resistance to the pathogen and their diverse plant growth habits. Experiments were conducted in 1982, 1984, and 1985 in a Nunn clay loam (fine, montmorillonitic, mesic Aridic Argiustolls) infested with sclerotia of the pathogen. Small research plots (6 m2) treated with benomyl |l‐(butylcarbamoyl)‐2‐benzimidazolecarbamate] and untreated control subplots effectively and reliably differentiated between cultivars and lines susceptible to and less affected by the pathogen on the basis of disease intensity and yield potential. Plant growth habit was not an acceptable predictor of the ability to escape white mold infection, because some Type I (bush) lines were as susceptible as Type III (prostrate vine) lines. Some Type II (upright vine) entries such as ‘A 51’ and ‘83 VEF MXA 222’ had significantly (P = 0.05) less white mold than did other entries in untreated plots. Resistant entries such as P.I. 169787 performed well despite having a Type I habit with compact canopy. Lines having resistance and desirable plant architecture have been identified and incorporated into the germplasm improvement program at Colorado State University.

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