Fungal Rootlet Colonization and Forage Yields of Alfalfa in Fungicide-Treated Field Plots. J. G. Hancock, Professor, Department of Plant Pathology, University of California, Berkeley 94720. . Plant Dis. 77:601-608. Accepted for publication 24 February 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-0601.

Colonization of rootlets by Fusarium spp. usually did not differ between two nondormant cultivars (Moapa 69 and WL 516) of alfalfa (Medicago sativa) or fungicide treatments in a field trial in the Central Valley of California. Rootlet colonization by Pythium spp. (mainly P. irregulare and P. ultimum) and Rhizoctonia solani AG-4 of Moapa 69 and WL 516 in untreated subplots was negatively correlated over the first two seasons. Cumulative forage yields were higher for WL 516 than for Moapa 69 in each season. Preplant soil treatments with metham-sodium initially reduced rootlet colonization by species of Fusarium and Pythium and increased root-length densities but had no effect on cumulative forage yields during the three harvest seasons. Metalaxyl treatments reduced rootlet colonization by Pythium, and cumulative forage yields were higher with Moapa 69 in treated subplots than in untreated subplots in the first and second harvest years. Forage yields of WL 516 were not affected by metalaxyl treatment. Benomyl treatment had no effect on rootlet infection by the fungi included in this study or cumulative forage yields of either cultivar during the first 2 yr. However, forage yields were significantly higher in benomyl-treated subplots than in untreated subplots of WL 516 in the third year.