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 ca usal agent of cavity spot disease of carrots in Western Australia, to study the relationships between host age, time of infection and develo pment of cavity spot lesions. Pythium coloratum was isolated frequentl y from 3-6-week-old asymptomatic roots of carrots grown in soils infes ted naturally or artificially with the pathogen. Carrots grown in cont ainers 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 t he time of harvest (16 weeks). The disease levels in this treatment we re not different from those transferred to P. coloratum-infested soil. There was no significant (P > 0.05) association between carrot age an d the ability of P. coloratum to infect the roots and to cause cavity spot lesions at harvest.