Occurrence and morphology of small spheroid galls on clubroot-resistant Chinese cabbage

Clubroot, caused by Plasmodiophora brassicae, is a worldwide disease that can lead to significant yield losses in cruciferous crops. It causes root hypertrophy, hampering the uptake of nutrients and water. In the evaluation of resistance, small spheroid galls (SSGs), which are distinct from typical spindle galls, and are generally regarded as a host resistance reaction, were observed. In this study, the occurrence of SSGs was investigated when evaluating the host resistance of seven commercial clubroot-resistant (CR) Chinese cabbage (Brassica rapa) cultivars inoculated with 12 P. brassicae populations collected from different infested fields in Chuanyu region of China. Results showed that while all seven CR Chinese cabbage cultivars exhibited high resistance levels to the major populations tested, SSGs occurred in host-population combinations contained either Chinese cabbage cultivar 'T1-145' or population 'FL'. This indicated that some CR cultivars or P. brassicae populations might be more likely to produce SSGs than others. Thus, recording the occurrence of SSGs separately in the resistance evaluation might be useful in mitigating the potential for selection of SSG producing hosts. External morphology observations revealed that both fresh root weight and gall diameter of spindle galls were significantly higher than SSGs, and histological observations indicated that delayed pathogen development in cortical infection might contribute to the restricted development of SSGs. Additionally, the number of SSGs formed per root was low (2.2 in average). This observation further confirmed that some host-population combinations cannot form large numbers of SSGs on single roots.