Resistant and susceptible wheats react differently to infection. If a variety is resistant, a reaction develops within the leaf which kills or retards the fungus, and the infection is stopped. Therefore, this disease can increase rapidly and epidemics may occur when susceptible varieties are grown and weather conditions are favorable for rust development. The extent of yield losses caused by leaf rust depends on the severity of infection and the duration of infection. The greatest losses occur if the crop is severely rusted from the seedling stage to maturity.
Wheat leaves manufacture carbohydrates and other nutrients.
The flag leaf is a primary contributor to the nutrients necessary for grain fill. The greater the flag leaf area damaged by rust, the smaller the leaf area available to manufacture nutrients needed for producing plump kernels. Initial infections are found on the lower leaves where high humidity persists for long durations. As the crop develops and matures, leaf rust generally appears on upper leaves of plants and severity increases. In North Dakota, corresponding yield losses have been as high as 30 percent for severely infected susceptible varieties, IF severe infection occured before flowering.