Effects of removing canola shoots and freezing clubs on maturation of resting spores of Plasmodiophora brassicae.

Plasmodiophora brassicae Woronin, causal of clubroot on Brassicas, is an obligate pathogen that survives in the soil as resting spores (RS). RS germinate and infect roots, stimulating the host to produce characteristic clubbing symptoms. RS form and mature in clubs, but the maturation process is poorly understood. To examine some of the factors that affect maturation, the response of a clubroot-susceptible canola (Brassica napus L.) line to four treatments was assessed: (1) immature clubs harvested 6 weeks post-inoculation (wpi), (2) more mature clubs harvested at 8 wpi, (3) plant shoot removed at 6 wpi and clubs collected at 8 wpi and (4) clubs removed at 6 wpi, frozen for 3 days, reburied and collected at 8 wpi. Clubroot severity was assessed and a disease severity index was calculated. RS maturity was assessed using propidium monoazide-assisted qPCR, in which amplification of immature spore DNA is inhibited. Severity and the proportion of mature RS increased in intact plants from 6 wpi (35 ± 10 DSI; maturity = 5 ± 2%) to 8 wpi (68 ± 9 DSI, maturity = 58 ± 24%). For plants with shoots removed, severity at 8 wpi (34 ± 10 DSI) was similar to intact plants at 6 wpi, but maturity (79 ± 19%) was similar to intact plants at 8 wpi. Frozen clubs had more mature RS (39 ± 12%) when assessed at 8 wpi than the control at 6 wpi. In conclusion, RS maturation does not require intact shoots and is not completely stopped by freezing.