Crown Rot of Alfalfa in Utah

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Abstract
Alfalfa crowns in Utah are universally affected by a dark, dry necrosis. The isolations and pathogenicity reported here indicated that the cause is a complex of pathogens. Organisms confirmed as pathogens in field plot inoculations were Fusarium solani, F. roseum ''Acuminatum'', and F. tricinctum, Serratia marcescens and Pseudomonas marginalis var. alfalfae. These organisms were all weakly virulent, and no evidence of synergism was detected. The low virulence of the pathogens was reflected in the slow development of disease symptoms. The average extent of necrosis within naturally infected crowns progressed from about 30% in 2-yr old alfalfa to about 50% in an 11-yr old stand. The number of crowns in the 11-yr old field was 7-fold less than in the 2-yr old field. The disease thus was associated with a slow reduction of alfalfa stand density. The remaining crowns compensated for the reduction in crown density, however, by increasing their numbers of stems per crown. Thus, no significant decrease in stems per unit area occurred during the study period.