Induced Resistance to Blackleg (Leptosphaeria maculans) Disease of Canola (Brassica napus) Caused by a Weakly Virulent Isolate of Leptosphaeria biglobosa.

Blackleg of canola is a disease complex of at least two fungal species: Leptosphaeria maculans and L. biglobosa. Isolates of L. biglobosa typically are weakly virulent or avirulent and are assigned to pathogenicity group 1 (PG-1). Isolates of L. maculans are highly virulent and encompass pathogenicity groups PG-2, PG-3, and PG-4. In greenhouse tests, percent lesion/leaf area (PLLA) on cotyledons of two Brassica napus cultivars, Westar and Invigor 2153, was smaller when L. biglobosa (PG-1) was either pre- or co-inoculated at 0, 12, 24, and 48 h with virulent isolates of L. maculans in PG-2, PG-3, and PG-4. On six-leaf-stage plants of Westar, the PLLA declined significantly compared with the control when the lower leaves were treated with either PG-1 or salicylic acid, then challenged with a PG-2 isolate 24 h later. In addition, the activity of four enzymes (chitinase, β-1,3-glucanase, peroxidase, and phenylalanine ammonia lyase) was greatly enhanced at 48 and 72 h when cotyledons of Westar were inoculated first with PG-1 followed by PG-2 24 h later, compared with a water control treatment. Field experiments conducted in 2003 and 2004 showed decreased blackleg severity in plants inoculated with PG-1 alone or prior to PG-2 compared with plants inoculated with PG-2 alone or prior to PG-1.