Development of inoculation methods for Pythium violae to evaluate resistance of carrot cultivars and efficacy of crop protection products for cavity spot control

Carrot is a major root vegetable crop grown in many parts of the world. In Europe, cavity spot disease continues to have a major impact on marketable yield. The disease is caused by several Pythium spp., with P. violae the main pathogen in the UK and results in small black lesions on mature carrot roots. The lack of reliable inoculation methods for these Pythium pathogens has, for many years, hampered the identification of new effective crop protection products or carrot varieties that are resistant to the disease. In this research, inoculation methods were developed for P. violae using mycelium or oospores, each of which successfully induced typical cavity spot symptoms in both pot- and field-grown carrots as well as consistent root stunting in the former. These methods were also used to successfully identify carrot cultivars with resistance to cavity spot and confirmed the efficacy of the fungicide metalaxyl-M against the disease. Results therefore demonstrated that the inoculation methods should be reliable for identifying the efficacy of crop protection products, assessing cavity spot resistance and for further studies investigating the biology and epidemiology of the pathogen.