Disease suppression by cultural and biological means and mechanisms involved in the process of disease reduction have been the subject of numerous studies. Passive heating of greenhouses, soil solarization and soil applications of biochar, the biocontrol agent Trichoderma harzianum or chemicals such as benzothiadiazole have all been associated with induced resistance (IR). To study the mode of action in each of these cases, treatments were spatially separated from the site of infection. The results of these tests strongly suggest that none of these treatments directly affect the examined pathogens, but rather that the observed disease control is due to systemic IR. Passive heating of tomato (Solanum lycopersicum) greenhouses revealed a negative relationship between disease severity and high (>21°C) soil temperatures. Under controlled conditions, soil heating was found to induce resistance to Botrytis cinerea infection of tomato leaves, despite the fact that the temperature of the leaves was not affected by the treatment. Priming of salicylic acid- and ethylene-related gene expression was observed in B. cinerea-infected leaves harvested from tomato plants grown in T. harzianum-treated soils; these plants expressed IR to B. cinerea. Benzothiadiazole was found to trigger IR to gray mold independent of salicylic acid and to trigger strong up-regulation of two genes involved in defense against B. cinerea, Pti5 and PI2. Pathogenesis-related genes were up-regulated in plants grown in solarized soil. In a study of tomato wild-types and mutants with modified salicylic acid, ethylene or jasmonic acid (JA) metabolism, IR was not observed in a JA-deficient mutant, def1, indicating that biochar-mediated IR in the tomato-B. cinerea pathosystem involves the JA pathway. In summary, IR associated with cultural methods may be due to a direct effect on the plant or an indirect effect based on the stimulation of beneficial microorganisms in the rhizosphere. © 2018 International Society for Horticultural Science. All rights reserved.
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