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 carrot root dieback was investigated. Carrots followed crops of alfalfa, barley, carrots, cotton, onions, or fallowed soil in two independent trials. Populations of Pythium spp. were greater following alfalfa and barley than other 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 reduced following the alfalfa crop; no other cropping pattern influenced carrot yields. The incidence of root dieback, which resulted in mature carrots with 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.