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

Canola (Brassica napus L.) is the second largest cash crop in Canada (4). Blackleg disease, caused by Leptosphaeria maculans (Desmaz.) Ces. & De Not. (anamorph = Phoma lingam (Tode:Fr.) Desmaz.), is one of most economically important diseases of canola (21). In the past decade, canola production has expanded significantly worldwide and interest in the blackleg fungus, particularly its biology, pathogenicity, and control, has increased. Blackleg of canola is a disease complex and consists of at least two types of fungal isolates: L. biglobosa (32), which is weakly virulent and classified as B group or type isolates; and L. maculans, which is aggressive, highly virulent, and classified as A group or type isolates (22). L. biglobosa typically causes Phoma leaf spots or superficial stem lesions, but L. maculans causes the more serious stem cankers. Based on the pathogenicity of isolates to three B. napus cultivars, Westar (susceptible), Glacier (Rlm2 and Rlm3 resistance genes), and Quinta (Rlm1 and Rlm4 resistance genes), the pathogen complex can be categorized into several pathogenicity groups (PGs) (26). L. biglobosa belongs to PG-1 and L. maculans comprises PG-2, PG-3, and PG-4. In addition to isolates of L. biglobosa, isolates in A group causing an incompatible reaction on resistant cultivars also are termed weakly virulent isolates of L. maculans (30). Induced resistance to disease in plants is a state of enhanced plant defense capacity dependent on the host plant’s physical and chemical barriers, activated by biotic or abiotic agents (3). Weakly virulent isolates have been shown to induce resistance in the host plant against virulent isolates in many pathosystems. Two avirulent races of Fusarium oxysporum f. sp. niveum induced resistance to Fusarium wilt of watermelon when challenged with a virulent race (24). Weakly virulent strains of Phytophthora infestans induced systemic resistance in potato to late blight (33) and avirulent isolates activated systemic resistance in broccoli against downy mildew (27). Nonpathogenic races of F. oxysporum f. sp. ciceris and F. oxysporum f. sp. lycopersici induced resistance to Fusarium wilt in chickpea and tomato (16,20). Similarly, resistance in a susceptible mustard cultivar to the highly virulent Alternaria brassicae isolate A and moderately virulent isolate C was induced using the avirulent isolate D (35). In the Brassica spp.–blackleg system, a rapid necrosis of guard cells in the leaves of Indian mustard occurred following inoculation of a resistant cultivar with an avirulent isolate of L. maculans (9). A hypersensitive reaction (HR) and an accumulation of phytoalexins were detected in Brassica spp. after inoculation with a weakly virulent isolate of L. maculans (28). An elicitin, cryptogein, and a weakly virulent isolate of blackleg pathogen both were capable of triggering HR on cotyledons of B. napus (30). Accumulation of pectin in the lumen of xylem vessels and the production of callose and lignin were confirmed to play a role in inhibiting systemic infection caused by L. maculans (19). In addition, induction of pathogenesis-related (PR) proteins, β-1,3-glucanase and chitinase, also has been documented (10). The above studies were focused on the induction of HR in Brassica plants by preinoculation with a weakly virulent isolate. It might be beneficial, however, if weakly virulent isolates of L. biglobosa (PG-1) could trigger systemic acquired resistance (SAR) instead of the HR, because SAR can provide the plant with longer-lasting protection against a broad spectrum of pathogens. SAR has been described in over 30 plant species (3) and elicitors of SAR comprise both biotic and abiotic agents. Mahuku et al. (23) reported SAR in canola to blackleg when a weakly virulent isolate was introduced 64 h prior to infection by a highly virulent isolate. In addition, the size of the lesions on the inoculated leaf and on neighboring leaves (above and below the inoculated leaf) and stem was reduced. However, the mechanism involved in this SAR still remains unknown and the performance of such SAR under field conditions has not been investigated. In this study, we evaluated the ability of a weakly virulent isolate of L. biglobosa (PG-1) and the elicitor salicylic acid to induce resistance in B. napus against blackleg disease caused by virulent isolates of L. maculans (PG-2, PG-3, and PG4) in both the greenhouse and the field. The activity of four PR proteins (chitinase, β-1,3-glucanase, peroxidase [PO], and phenylalanine ammonia lyase [PAL]) also was measured.