1. USDA‐ARS, Edward T. Schafer Agricultural Research Center 1616 Albrecht Blvd. N. Fargo ND 58102‐2765 USA
2. Dep. of Plant Pathology North Dakota State Univ. P.O. Box 6050 Fargo ND 58108 USA
3. Univ. of Minnesota Extension Service St. Paul MN 55108 USA
4. Dep. de Agronomia Univ. Federal de Viçosa Viçosa MG 36570–900 Brazil