Management of clubroot in a dynamic environment

Project Summary

PURPOSE: Clubroot is a very challenging disease to manage. The protist that causes clubroot is complex, with several different life stages that feature characteristics of amoebas, slime molds and fungi. It also has a remarkable ability to spread, with each infected canola plant able produce up to 16 billion resting spores.

This project investigated clubroot management options, including developing economical and effective techniques to eradicate localized infestations using soil fumigants, assessing the impact of cropping rotations which include clubroot-resistant (CR) canola cultivars and optimizing ways to disinfect agricultural and industrial equipment.

RESULTS: These are the key findings from this project:

• Disease severity tended to decline when CR cultivars were rotated, but alternating non-hosts, resistant cultivars or fallow periods with susceptible cultivars did not reduce disease, nor did growing the same resistant cultivar over repeated cycles. All of the commercially available clubroot-resistant cultivars were actually susceptible when grown in soil from fields where resistance loss had been reported, although there was some degree of difference among the cultivars with respect to the extent of susceptibility when inoculum density was ≥ than 100,000 spores per ml of soil and ≤ 1,000,000 spores per ml of soil.
• The use of Vapam, a soil fumigant, was supported as a potential approach in eradicating or containing localized clubroot infestations. Increased efficacy was seen at higher rates, when it was incorporated into the soil and when it was covered with plastic for varying periods of time (although the practicality of this may be more useful to the oil and gas pipeliners than farmers). Although Vapam was effective, it is a non-selective toxin which is both volatile and highly soluble in water.
• Cropping interval has a big impact on resting spore populations in the soil, with large numbers of resting spores dying or disappearing in the first one to two years after a susceptible crop. (Others may survive for many years.) Therefore, even a two-year break between canola crops could reduce yield losses in CR cultivars and prevent selection pressure for new pathotypes.
• It was confirmed that quantitative polymerase chain reaction (qPCR) analysis is an effective tool for evaluating P. brassicae resting spore concentrations in naturally infested soil. Peaks in P. brassicae inoculum were found in the spring following years when susceptible canola was cultivated and inoculum loads declined rapidly after two years without canola cultivation. It was also reported that exposing clubroot spores to 100°C for at least 30 minutes is sufficient to inactivate clubroot spores.
• A special qPCR technique (using propidium monoazide), developed to differentiate between living and dead spores of P. brassicae in field soil, found that resting spore populations declined quickly over the first two years after a susceptible crop, and much more slowly thereafter. Many of the spores in soil in the spring following a susceptible crop were immature, but were likely still infectious (although without a host, these spores rapidly disappeared from the spore bank).
• This project also produced some important methodology developments in quantifying resistance to P. brassicae in Brassica hosts, in examining root cell (cortical) colonization by P. brassicae and in the production of primary and secondary zoospores of P. brassicae. A disease nursery was also developed to help breeders and seed companies screen materials for resistance to clubroot.