Morphological and biochemical response of carrots to Pythium violae, causative agent of Cavity Spot

During Cavity Spot disease of carrot (Daucus carota), the surface of the root is penetrated by the fungus Pythium violae causing surface lesions and cell breakdown. Commercial varieties range from the very susceptible Bertan, to the less susceptible Bolero with Narbonne intermediate while the gene bank cultivar Purple Turkey was much less susceptible. Examination of the colonization process in vitro by scanning electron microscopy of Narbonne showed that fungal proliferation occurred in the first 2 days of colonization but this species had disappeared from lesions by day 7. No lesions were evident on Purple Turkey although the fungus had penetrated the root which itself was composed of small regularly arranged cells. Examination of the activity of defence related enzymes during in vitro colonization showed that phenylalanine–ammonia lyase and chitinase activities remained low throughout the first 7 days of infection of commercial cultivars, Bolero and Bertan. Peroxidase and β–glucosidase activity in Bolero increased briefly on day 3 but otherwise were uniformly low. Enzyme activities were generally higher in Purple Turkey. The small cell size within the root and higher constitutive levels of the enzymes may constitute the basis for resistance in Purple Turkey. Potentially this cultivar may provide a source of germplasm for improving the resistance of commercial carrots to Cavity Spot.