Organisms that cause diseases on carrot foliage are present wherever carrots are grown. Research has demonstrated that using fungicides to control these foliar pathogens increases carrot yields by 4-8 tons/acre.

“Alternaria leaf blight (ALB) of carrot and cercospora leaf spot (CLS) contribute to significant and recurrent losses for the production of carrots worldwide. … Foliar pathogens defoliate carrots by infecting and blighting leaflets and petioles, which in turn limits photosynthesis and energy storage in roots. Several field studies have shown 20%-80% yield loss for unsprayed carrots compared with carrots subjected to a standard calendar fungicide program.”

“In addition to yield loss, deterioration of petiole and leaf health may reduce the commercial harvestability of roots since strong, healthy petioles are required to properly lift and remove carrots from the soil. When foliage is weakened by disease, additional crop losses ensue as unharvested roots are left behind in the field. … Repeated fungicide applications are expensive but necessary on susceptible carrot cultivars to maintain crop yield and value.” 

Authors: P.M. Rogers and W.R. Stevenson
Affiliation: Department of Plant Pathology, University of Wisconsin-Madison
Title: Integration of host resistance, disease monitoring, and reduced funigicide practices for the management of two foliar diseases of carrot.
Publication: Canadian Journal of Plant Pathology. 2006. 28:401-410.