CROP-ROTATION AND N FERTILIZATION EFFECTS ON GROWTH, YIELD, AND DISEASE INCIDENCE IN POTATO

Crop rotation can be an effective mechanism for reducing disease incid ence and contributing nitrogen (N) to succeeding crops. Interactions o f plant pathogen suppression and soil nutrient availability may also e xist, adding to the cropping system complexity. This study examined th e impact of crop rotation, N fertilization, and their interaction on g rowth, yield, and Rhizoctonia solani incidence in potato (Solanum tube rosum L. Norwis). Potato was grown continuously and in two-year rotati ons with annual alfalfa (Medicago sativa L. Nitro), hairy vetch (Vicia villosa Roth), white lupin (Lupinus albus L. Ultra), and oat (Avena s ativa Astro). Fertilizer was banded at potato planting with 0, 45, 90, 135, 180, or 225 kg N ha(-1) as (NH4)(2)SO4. Approximately 58% of con tinuous potato possessed stem lesions caused by R. solani, but only 12 to 22% of potato stems from other rotations possessed lesions. Tuber dry weight was affected by crop rotation in 1989, a dry year, but not in 1990. Apparent N fertilizer replacement values for hairy vetch, Nit ro alfalfa, white lupin, and oat were 65, 43, 26, and 11 kg N ha(-1), respectively. All crop rotations studied appeared to enhance potato pr oduction by reducing stem infection by R solani. Vetch and alfalfa pro vide additional benefits through their N contributions.