Wheat streak mosaic virus infection is often found to be highly severe at the point of entrance in a wheat field and then decrease in severity as it moves across the field. This movement creates a severity gradient from highly diseases to less severe diseases in specific areas of the field. A study was conducted to determine if model of the relationship between disease severity and yield could be created. Disease severity was determined using reflectance readings beginning at the highly diseased edge of a field along its diseases gradient to the less diseased area within the field. Disease readings were correlated with yield and water use efficiency data. The study determined that as disease severity decreased from the edge of the field, yield and water use efficiency increased and a time series model was described (Workneh, 2009).