Effect of osmotic pressure and calcium concentration of nutrient solution on lesion development in downy mildew-susceptible and -resistant cucumber [Cucumis sativus] cultivars

Cucumber plants were grown on nutrient solutions having different osmotic pressures and Ca concentrations to test their effects on the occurrence and development of downy mildew, Pseudo-peronospora cubensis ( Berk. & Curt. ) Rost. Exp.1. The leaf incidence of downy mildew per inoculated leaves of plants irrigated with 4 times strength of standard Enshi-shoho solution (H treatment) was lower than those irrigated with 1/8 strength solution (L treatment) in susceptible cultivar Pretty. There were no differences in the days to appearance of symptom and the number of lesions per leaf between these two treatments. Eleven days after inoculation, leaf osmotic pressure in H and L-H treatments (concentration was changed one day after inoculation) were higher than that in L treatment. The lesion area per spot in L treatment was significantly large, whereas those in H and L-H treatments remained unchanged. Exp.2. 'Pretty' and 'Poinsett' (resistant) were grown in nutrient solutions with different osmotic pressures based on total salt concentrations (1, 1.5 and 2 strength) or by adding CaCl2 to 1 strength solution (Ca 16, 24, 32 me / liter sup(-1) ). The rate of lesion area per whole plant decreased with increasing osmotic pressure of petiole sap in both cultivars; no relationship was found between rate of lesion area and mineral concentrations of petiole sap. In 'Pretty', many hyphae and sporangia were present in the lesions in all treatments, but the density of sporangia and density and lengths of hyphae in 2 strength and in Ca 32 me tend to be lower than those in the control. Although few hyphae were observed in 'Poinsett' lesion, the density of sporangia in double strength and in Ca 32 me tended to be lower than that in the control. These results suggest that high osmotic pressure in the plant was effective in suppressing lesion development, mainly by inhibiting hypha growth, regardless of the composition of nutrient solution. It seemed that 'Poinsett' has a stronger resistance to invasion of downy mildew than 'Pretty'.