Fungal Infection of Feeder Rootlets of Alfalfa

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Abstract
Isolations of fungi from feeder rootlets of alfalfa (Medicago sativa) grown in commercial fields and experimental plots in the Central Valley of California mainly yielded species of Fusarium (F. oxysporum, F. roseum ''Acuminatum,'' F. roseum ''Culmorum,'' and F. solani) and Pythium (P. dissotocum, P. irregulare, P. paroecandrum, P. vexans, and P. violae), and Rhizoctonia solani (anastomosis group 4). Feeder rootlet-length densities and degrees of rootlet infection by the three genera of fungi were higher in samples taken from the upper 15-cm of soil than those taken at greater depths (15-80 cm). Patterns of rootlet infection and rootlet growth of two cultivars of alfalfa (Moapa 69 and Lahonton) were identical in a plot sampled monthly for 2.5 yr. However, there were marked seasonal differences in the degrees of infection of rootlets by different fungi, especially by species of Pythium. Following fumigation with methane sodium, shoot growth by alfalfa was markedly reduced in soils reinfested with P. paroecandrum and P. ultimum, slightly reduced by P. dissotocum and P. violae, and not affected by F. oxysporum and R. solani.