Disease management in organic greenhouse horticulture: solutions, new insights and bottlenecks

Plant diseases pose a major threat to horticultural crops grown in greenhouses. High relative humidity (RH) and the presence of a film of water on susceptible plant organs for long periods of time provide favorable conditions for the germination of conidia and infection by many pathogens: bacteria, fungi and fungus-like microorganisms. Conditions of excess nitrogen and unbalanced availability of N may be found in organic cropping systems. The unbalanced growth of plants may not only promote susceptibility, but also restrict air movement in the greenhouse, promoting epidemics. There are a limited number of control agents available for use in organic cropping systems and, in some cases, their efficacy is limited. The roles of calcium and potassium in restricting disease have been described. Cultural methods that restrict RH and condensation on susceptible plant organs have been shown to suppress disease. Plant spacing, passive heating of unheated greenhouses, polyethylene soil mulch, soil solarization and the addition of Trichoderma, biochar or compost to the soil can reduce plants' susceptibility to disease or induce resistance, as described. Integrated management that is based on sanitation, cultural means, sprayed control agents and weather forecasts is suggested as a potential effective system for keeping plants healthy in organic greenhouse cropping systems.