Bacterial Weapons of Fungal Destruction: Phyllosphere-Targeted Biological Control of Plant Diseases, with Emphasis on Sclerotinia Stem Rot and Blackleg Diseases in Canola (Brassica napus L.)

Abstract

Canola (oilseed rape) has worldwide production led by China, Canada, Australia and Europe. Sclerotinia stem rot, caused by Sclerotinia sclerotiorum, and blackleg, caused by Leptosphaeria maculans, are the two most economically important diseases of canola, e.g. in western Canada (yield loss 5–100%). Inefficient control resulting from cultural methods, resistant cultivars, and changes of pathogens into virulent strains, especially for blackleg, stress the need for an alternative, eco-friendly management, less dependent on chemical fungicides. Bacteria are excellent sources of biocontrol agents, owing to their multiple mechanisms of disease control. Compared to root pathogens, biological control of foliar pathogens has been less exploited for the phyllosphere, harsh conditions making it difficult for the biocontrol agent to establish, grow and survive. This chapter discusses these unfavourable chemical and physical conditions encountered by bacteria, and their mechanisms of adaptation, particularly how bacteria mediate phyllopshere biological control, with emphasis on sclerotinia stem rot and blackleg disease of canola. Understanding the pathogen life cycle, the growth stage at which the host is susceptible to infection, and the mechanism of host infection would help in the effective application of the bacterial antagonist. Time of application is highly critical, mediating disease control through, e.g. antibiosis, pre-emptive colonization, antifungal volatile production, and induction of systemic resistance in the host plant. If bacterial biocontrol agents are ever to become realistic alternatives to chemical pesticides, then it is essential that we unravel their control at the molecular level, including both biotic and abiotic factors regulating their expression.