Peanut stem segments 8.5-cm long were inoculated at leaf nodes with mycelial plugs of Sclerotinia minor and incubated in moist chambers at 20 C. Water-soaked lesions were often visible after 24 hr. The rate of lesion elongation was used to quantitatively assess the physiological resistance of peanut genotypes to Sclerotinia blight, screen chemicals for fungicidal activity, and evaluate the pathogenicity of S. minor isolates. The age and/or developmental state of lateral limbs of plants had a marked effect on lesion development; 3 days after inoculation, mean lesion lengths were 58.8, 46.8, and 38.9 mm for terminal, median and basal parts of stems, respectively. The speed, simplicity and adaptability of this method make it a valuable tool for research on Sclerotinia blight of peanut.