ECOLOGICAL AND EPIDEMIOLOGIC FACTORS AFFECTING CARROT MOTLEY DWARF DEVELOPMENT IN CARROTS GROWN IN THE SALINAS VALLEY OF CALIFORNIA

The geographic and temporal incidence of carrot motley dwarf (CMD) and the partial host ranges of the CMD viruses and their aphid vector, Ca variella 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 spec ies growing in the Salinas Valley. Field studies assessing the inciden ce 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 ca rrot cultivars to CMD ranged from good resistance to extreme susceptib ility. These data suggest that time of planting, location in relation to overwintered carrot fields, and carrot cultivar are all important f actors in disease development.