Weather thresholds for clubroot development on canola and brassica vegetables

Abstract

Clubroot, caused by Plasmodiaphora brassicae, is an important soil-borne disease of canola (Brassica napus) and other brassica crops worldwide. Temperature and soil moisture are known to affect clubroot development. To understand the influence of weather on development of clubroot, replicated small-plot seeding date trials were conducted from 2011 to 2014 at the Muck Crops Research Station in southern Ontario. Each year, canola cultivar ‘InVigor 5030ʹ was seeded at 2-week intervals, and plants were sampled and assessed weekly for clubroot incidence and severity using a 0–3 scale. A base temperature of 14°C provided the best fit for degree day assessments, based on the assessments in 2011 and 2012. For modelling, data from previous trials on Chinese flowering cabbage (B. rapa subsp. chinensis var. utilis) from the same site were included in the data set, following assessment to ensure that both host species responded similarly across a range of environments. Temperature and rainfall affected clubroot initiation and development, but the results indicated that when conditions exceeded a threshold of temperature and soil moisture, clubroot quickly developed to high levels, irrespective of additional heat units or moisture. This indicated that clubroot initiation and development at sites where resting spores occur at high levels occurred rapidly across a wide range of soil temperature and moisture conditions above a low threshold around 14°C.