Influence of crop rotation on the incidence of Pythium- and Rhizoctonia-induced carrot root dieback

The influence of various crop rotations on population densities of Pythium spp. and Rhizoctonia solani, subsequent carrot yields, and incidence of car rot root dieback was investigated. Carrots followed crops of alfalfa, barle y, carrots, cotton, onions, or fallowed soil in two independent trials. Pop ulations of Pythium spp. were greater following alfalfa and barley than oth er crops in one of the 2 years of the study. Populations of R. solani were generally greater following alfalfa and cotton than other crops in each of the 2 years of the study. In 1 year, yields of marketable carrots were redu ced following the alfalfa crop; no other cropping pattern influenced carrot yields. The incidence of root dieback, which resulted in mature carrots wi th misshapen or multiple taproots, was generally greater following alfalfa and barley than other crops in one trial, and greater following alfalfa and cotton in the other trial.