The objectives of this study were: (a) to quantify yield losses of plants under natural conditions of infection by Sclerofinia mid-stalk rot, and (b) to determine if there is any relation between the magnitude of such losses and the length of the lesion caused by the disease. Mid-stalk rot infected plants (N:120) showed an average reduction of 357 % in seed field, a lower number of seeds per head (?AVo); a decrease in 1000-seed weight (15%) and lower oil percent (8.1 points) as compared with healthy control plânts (p<0.0001). The reductions in seed yield, seed number, seed weight, and oil content in seed were very significantly correlated with the lesion length of the mid-stalk rot-infected plants. I-,esion length can be utilized as a predictor of such decreases and as a valuable tool in screening genetic materials for tolerance to this disease.