Ecological and Epidemiological Factors Affecting Carrot Motley Dwarf Development in Carrots Grown in the Salinas Valley of California. MICHAEL T. WATSON, Department of Plant Pathology, University of California, Davis 95616. BRYCE W. FALK, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 78:477-481. Accepted for publication 15 December 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0477.

The geographic and temporal incidence of carrot motley dwarf (CMD) and the partial host ranges of the CMD viruses and their aphid vector, Cavariella aegopodii, were investigated. The CMD viruses—carrot redleaf luteovirus and carrot mottle virus—and C. aegopodii were found to have limited host ranges that overlap in carrot but in no other plant species growing in the Salinas Valley. Field studies assessing the incidence of CMD in spring carrots revealed that CMD development was closely associated with overwintered carrot fields. Little to no CMD developed in spring fields that were distant from overwintered carrot fields or when no overwintered carrot fields were present. Susceptibility of carrot cultivars to CMD ranged from good resistance to extreme susceptibility. These data suggest that time of planting, location in relation to overwintered carrot fields, and carrot cultivar are all important factors in disease development.