Effects of host age on development of cavity spot disease of carrots caused by Pythium coloratum in Western Australia

Three experiments were conducted with Pythium coloratum Vaartaja, a causal agent of cavity spot disease of carrots in Western Australia, to study the relationships between host age, time of infection and development of cavity spot lesions. Pythium coloratum was isolated frequently from 3-6-week-old asymptomatic roots of carrots grown in soils infested naturally or artificially with the pathogen. Carrots grown in containers of soil artificially infested with P. coloratum, but not those in naturally infested field soil, developed cavity spot lesions after 6 weeks. Early infection of carrot seedlings at or before 3 weeks by P. coloratum in artificially infested soils followed by their transfer to pathogen-free soil was sufficient to cause cavity spot disease at the time of harvest (16 weeks). The disease levels in this treatment were not different from those transferred to P. coloratum-infested soil. There was no significant (P > 0.05) association between carrot age and the ability of P. coloratum to infect the roots and to cause cavity spot lesions at harvest.