Effect of seeding date on development of clubroot in short-season Brassica crops

Abstract

Recent studies on the effect of temperature on clubroot of Brassica spp., caused by Plasmodiophora brassicae, have demonstrated that temperature has an important impact on each component of the disease cycle under controlled conditions. The current study was conducted to assess the impact of temperature regime on clubroot incidence and severity on two short-season Asian vegetable crops, Shanghai pak choy (B. rapa subsp. chinensis var. communis) and Chinese flowering cabbage (B. rapa subsp. chinensis var. utilis) under field conditions. Each crop was seeded at about monthly intervals from May to September on a naturally infested site near Bradford, Ontario in 2008 and 2009. A soil-drench treatment with cyazofamid fungicide (trade name Ranman® 400 SC) was included at each seeding date to assess the interaction of fungicide and temperature. Clubroot severity was assessed at weekly intervals from 2–3 wk after planting until about 6 wk after planting using a 0–3 scale. Clubroot incidence and severity were consistently higher on Shanghai pak choy than Chinese flowering cabbage, but both host crops exhibited a similar pattern of symptom development. Clubroot severity was highest in the July seeding (warmest mean temperature), slightly lower in June and August, and lowest in the May and September seeding dates (lowest temperatures). Clubroot levels were also positively correlated with rainfall. Cyazofamid fungicide consistently reduced clubroot levels. We conclude that producers of short-season crops can select seeding dates to avoid losses from clubroot, even in heavily infested fields. Both moderate soil temperatures (17–26 °C) and adequate soil moisture must be present for the development of severe clubroot.