The protist Plasmodiophora brassicae is a biotrophic soil-borne pathogen that causes clubroot on Brassica crops worldwide. Clubroot disease is a serious threat to the 8 M ha of canola (Brassica napus) grown annually in western Canada. While host resistance is the key to clubroot management, sources of resistance are limited.

The agriculturally important genus Colletotrichum is an emerging model pathogen for studying defense in Arabidopsis. During the process of screening for novel pathogenic Colletotrichum isolates on Arabidopsis, we found significant differences in defense responses between detached and attached leaf assays. A near-adapted isolate Colletotrichum linicola A1 could launch a typical infection only on detached, but not attached, Arabidopsis leaves. Remarkably, resistance gene-like locus RCH1-mediated resistance in intact plants also was compromised in detached leaves during the attacks with the virulent reference isolate C. higginsianum. The differences in symptom development between the detached leaf and intact plant assays were further confirmed on defense-defective mutants following inoculation with C. higginsianum, where the greatest inconsistency occurred on ethylene-insensitive mutants. In intact Arabidopsis plants, both the salicylic acid- and ethylene-dependent pathways were required for resistance to C. higginsianum and were associated with induced expression of pathogenesis-related genes PR1 and PDF1.2. In contrast, disease symptom development in detached leaves appeared to be uncoupled from these defense pathways and more closely associated with senescence: an observation substantiated by coordinated gene expression analysis and disease symptom development, and chemically and genetically mimicking senescence.

Clubroot disease caused by Plasmodiophora brassicae Woronin is a concern to the canola (Brassica napus L.) growers in Canada. A crop management strategy that includes deployment of genetic resistance and appropriate cultural practices is needed for long-term management of this disease. Resistance to the P. brassicae pathotypes present in Canada has been found in the primary and secondary gene pools of spring B. napus canola. Some of these sources, such as winter canola ‘Mendel’, rutabaga and Pak Choi (Brassica rapa L.) ‘Flower Nabana’, were used in genetic studies and breeding for the development of clubroot-resistant canola cultivars. A dominant gene in ‘Mendel’ and ‘Flower Nabana’ was found to confer resistance to P. brassicae pathotype 3, while a simple or a complex genetic control of resistance was found in rutabaga. The clubroot resistance (CR) gene in ‘Flower Nabana’ was mapped to chromosome A3, and molecular markers linked to the CR gene were identified for use in marker-assisted breeding (MAB). Using the CR genes from ‘Mendel’ and rutabaga, several clubroot-resistant spring canola lines were developed. Often the CR genes of ‘Mendel’ and rutabaga conferring resistance to pathotype 3 also conferred resistance to other pathotypes of P. brassicae found in Canada, including pathotypes 5, 6 and 8. The CR gene of ‘Flower Nabana’ was introgressed into B. napus and B. rapa canola through MAB. Since single-gene controlled resistance can be eroded, other strategies such as pyramiding different CR genes into B. napus canola should be considered for durable resistance.

This study investigated how the timing of application of the biofungicide Serenade (Bacillus subtilis QST713) or it components (product filtrate and bacterial cell suspension) influenced infection of canola by Plasmodiophora brassicae under controlled conditions. The biofungicide and its components were applied as a soil drench at 5% concentration (vol/vol or equivalent CFU) to a planting mix infested with P. brassicae at seeding or at transplanting 7 or 14 days after seeding (DAS) to target primary and secondary zoospores of P. brassicae. Quantitative polymerase chain reaction (qPCR) was used to assess root colonization by B. subtilis as well as P. brassicae. The biofungicide was consistently more effective than the individual components in reducing infection by P. brassicae. Two applications were more effective than one, with the biofungicide suppressing infection completely and the individual components reducing clubroot severity by 62 to 83%. The biofungicide also reduced genomic DNA of P. brassicae in canola roots by 26 to 99% at 7 and 14 DAS, and the qPCR results were strongly correlated with root hair infection (%) assessed at the same time (r = 0.84 to 0.95). qPCR was also used to quantify the transcript activity of nine host-defense-related genes in inoculated plants treated with Serenade at 14 DAS for potential induced resistance. Genes encoding the jasmonic acid (BnOPR2), ethylene (BnACO), and phenylpropanoid (BnOPCL and BnCCR) pathways were upregulated by 2.2- to 23-fold in plants treated with the biofungicide relative to control plants. This induced defense response was translocated to the foliage (determined based on the inhibition of infection by Leptosphaeria maculans). It is possible that antibiosis and induced resistance are involved in clubroot suppression by Serenade. Activity against the infection from both primary and secondary zoospores of P. brassicae may be required for maximum efficacy against clubroot.