Development of artificial inoculation techniques for cavity spot caused by Pythium violae to identify new management strategies

The soilborne oomycete Pythium violae is the principal cause of cavity spot of carrots in the UK, a disease which remains a huge challenge for carrot growers as just a few superficial lesions severely reduce crop marketability. Chemical control is limited to application of metalaxyl-M, but efficacy can be variable; hence new approaches for cavity spot disease management are required. However, this is hampered by a lack of consistent inoculation approaches for P. violae that reliably induce cavity spot disease. The aim of this research was to develop robust artificial inoculation systems in both glasshouse and field suitable for identifying alternative control products and sources of disease resistance in the future. In glasshouse experiments, artificial inoculation of growing media using a P. violae oospore inoculum at different concentrations resulted in some damping off of carrot seedlings and stunted carrot roots. Although carrots developed typical cavity spot lesions, there was no clear effect of oospore concentration on the incidence or severity of symptoms. A mycelial inoculum of P. violae resulted in higher cavity spot incidence in carrots. In a field based small plot macrocosm system, the P. violae oospore inoculum successfully induced high levels of cavity spot disease in carrots, although again there was no effect of initial oospore concentration. A subsequent assessment of different crop protection products in the field macrocosms showed that although some reduced cavity spot, this was only statistically significant for SL567A (metalaxyl-M). This research has therefore identified suitable inoculum types and techniques for assessing the efficacy of cavity spot control strategies.