Effect of concentration, composition, and osmotic pressure of nutrient solution on the occurrence and development of cucumber downy mildew

Susceptible and resistant cucumber cultivars (Cucumis sativus L.) were grow n with different nutrient solutions to examine the relationship between the characteristics of plant (shoot dry weight, osmotic pressure of petiole sa p, and leaf mineral content) and the occurrence and development of cucumber downy mildew (Pseudoperonospora cubensis). Exp.1. The administration of different solutions (1/8, 1/2, 1(control), 3/2 unit of Enshi-shoho, nitrate-N at 20 or 2 me . liter(-1) and potassium at 12 or 1 me . liter(-1)) revealed that more days to appearance of symptom wa s required by the resistant 'Poinsett' than the susceptible 'Pretty', indep endent of the leaf mineral contents. The total lesion area in 'Pretty' was bigger than that in 'Poinsett', but both of them were highly correlated wit h shoot dry weight but not to the leaf N or K content. When both cultivars were grown under a low K regime, the osmotic pressure of the petiole sap an d leaf P, Ca and Mg content were the highest of all treatments; the rate of lesion area in total leaf area, especially in 'Pretty' was significantly l ower than that in control. Exp.2. The effect of double concentration of P, Ca and Mg (8, 16, 8 me lite r(-1)) and osmotic pressure (4 or 8 x 10(4)Pa) of the nutrient solution wer e investigated. Doubling the osmotic pressure of the nutrient solution ( 2 units or adding NaCl to 1 unit of Enshi- shoho solution) increased the osmo tic pressure of petiole sap, and suppressed the total lesion area in both c ultivars. Hence, the total lesion areas of both cultivars were negatively c orrelated with the osmotic pressure of petiole sap. Especially doubling the Enshi- shoho to 8 x 10(4)Pa delayed the appearance of symptom and suppress ed the total lesion area of 'Pretty' but yet shoot dry weight was unaffecte d. These results indicate that increasing the osmotic pressure of nutrient sol ution was more effective in suppressing expansion of lesion area of cucumbe r downy mildew than the increasing leaf mineral contents.