1. Department of Plant, Soil and Microbial Sciences, Michigan State University, East Lansing, MI 48824, U.S.A.
2. Department of Soil and Crop Sciences, Colorado State University, Fort Collins, CO 80523, U.S.A.
3. Department of Horticulture and Landscape Architecture, Colorado State University, Fort Collins, CO 80523, U.S.A.
4. Department of Crop Science and Horticulture, Sokoine University of Agriculture, Morogoro, Tanzania.
5. Sugarbeet and Bean Research Unit, USDA-ARS, East Lansing, MI 48824, U.S.A.