The effect of selected mycorrhizae fungi on clubroot of canola.

In previous studies with commercial biocontrol agents, clubroot (caused by Plasmodiophora brassicae Woronin) severity was reduced under controlled conditions, but only occasionally reduced in the field. The current study investigated suppression of clubroot symptoms on canola (Brassica napus L.) using the mycorrhizae fungi Piriformospora indica Verma, Glomus intraradices Schenk and Smith (AGTIV, Premier Tech), or a mixture of endo and ectomycorrhizal species in a commercial formulation (Root Rescue) in growth room studies that were replicated and repeated. Brassicae species rarely develop mycorrhizal associations, but the research was initiated because it was known that P. indica can colonize the roots of some Brassica crops and promote growth and stress tolerance, and that P. indica can induce disease resistance in other plants. Microscopic observation showed that canola roots were colonized by P. indica and some of the fungi in Root Rescue, but not by G. intraradices. Clubroot severity index (DSI) was slightly reduced in canola plants treated with P. indica (60% DSI as compared to 79% DSI in the untreated check) when plants were inoculated with 5 x 105 resting spores mL-1, but not in plants inoculated with 5 x 104 or 5 x 106 resting spores mL-1. There were no differences among treatments in shoot fresh and dry weights. The reason that symptoms were not reduced at the lowest inoculum concentration for P. brassicae is not known. Colonization by P. indica may protect roots from infection, stimulate root growth, or induce resistance to clubroot by modifying plant metabolism.