Chitosan inhibited growth of Botrytis cinerea in liquid culture and suppressed grey mould on detached grapevine leaves and bunch rot in commercial winegrapes. Germination of B. cinerea was completely inhibited in malt extract broth containing chitosan at concentrations greater than 0·125 g L ⁻¹ . However, treated conidia were able to infect detached Chardonnay leaves and pathogenicity was not affected, even after incubation for 24 h in chitosan at 10 g L ⁻¹ . When added after conidial germination, chitosan inhibited B. cinerea growth and induced morphological changes suggestive of possible curative activity. The effective concentration of chitosan that reduced mycelial growth by 50% (EC ₅₀ ) was 0·06 g L ⁻¹ . As a foliar treatment, chitosan protected detached Chardonnay leaves against B. cinerea and reduced lesion diameter by up to 85% compared with untreated controls. Peroxidase and phenylalanine ammonia‐lyase activities were also induced in treated leaves. In vineyard studies, Chardonnay winegrapes exhibited 7·4% botrytis bunch rot severity at harvest in 2007 after treatment with an integrated programme that included chitosan sprays from bunch closure until 2 weeks preharvest, compared with 15·5% in untreated controls and 5·9% with fungicide treatment. In the following season, botrytis bunch rot severity was 44% in untreated Chardonnay at harvest and the integrated programme (21%) was less effective than fungicides (13·8%). However, in Sauvignon blanc winegrapes, the integrated and the fungicide programme each reduced botrytis bunch rot severity to 4% and were significantly different from the untreated control (11·5%). This study provides evidence that suppression of botrytis in winegrapes by chitosan involves direct and indirect modes of action.

Plants have evolved a highly sophisticated immune system to resist pathogen attack comprising both preformed and inducible mechanisms. Over the last 50 years, various biological and chemical inducers have been used to artificially trigger the defense response in plants, thereby promoting an induced resistance (IR) to subsequent pathogen attack. IR has proven effective for disease control in laboratory and glasshouse conditions but has seldom equalled the level of protection offered by synthetic pesticides in the field. However, renewed interest in IR for crop protection is being driven by legislation to reduce the use of synthetic chemicals in agriculture. Inducers can contribute to integrated crop management strategies when used in combination with fungicides, bactericides, and with other biological control options. Integrating inducers in this way can reduce chemical inputs without loss of efficacy. Moreover, advances in our understanding of plant defense are informing the development of new inducers and guiding new strategies for their implementation in sustainable crop protection. This review will discuss the use of IR in selected cropping systems and describe opportunities for optimizing its potential, including the development of more effective inducers and their integration with conventional and cultural control options.