Studies on the development and management of powdery scab and root gall formation caused by Spongospora subterranea on potato (Solanum tuberosum l.)

The biotroph protozoan Spongospora subterranea causes root gall formation and powdery scab on potato. Symptoms on tubers affect directly the quality and marketability of the harvested product while infection in roots are associated with yield reductions. Moreover, S. subterranea is the vector of the Potato mop-top virus. The management of the disease is difficult due to the limited number of current control options and requires the integration of control measures among which host resistance represents the most economically and long-term approach. This dissertation focuses on the evaluation of management strategies for the control of powdery scab and root gall formation. In the first study, a total of 43 potato cultivars and 80 advanced clones representing a range of skin types were assessed for their response to powdery scab and root gall formation in five field experiments. High levels of resistance was observed among genotypes against the development of disease on tubers and roots which accounted for 32.5% of the evaluated population. Resistance to powdery scab and root gall formation was shown to follow a continuum from very susceptible to very resistant. In the second study, the effect of chloropicrin soil fumigation on the soil populations of S. subterranea and the concomitant development of disease in roots and tubers was investigated in seven field trials. Results indicated a reduction of pathogen soil inoculum, which in most cases was accompanied by an increase in yield and disease on tubers and roots. These results were confirmed in controlled condition experiments in which the amount of pathogen DNA detected in roots increased with the fumigant rate. In the third study, reciprocal grafts between 'Shepody' (susceptible to powdery scab and root gall formation) and 'Dakota Trailblazer' (resistant) were prepared in order to assess the effect of grafting on the translocation of susceptibility factors affecting the infection and development of root galls. The amount of pathogen DNA increased, as did the number of root galls, on graft combinations involving 'Shepody'. The results presented in this dissertation highlight the importance of cultivar selection in the management of root gall formation and powdery scab on tubers.