Effect of Nitrogen Concentrations of Nutrient Solution on the Occurrence and Development of Downy Mildew in Susceptible and Resistant Cucumber Cultivars

Abstract

The effect of nitrogen (N) concentrations of hydroponic nutrient solution on the occurrence and development of downy mildew (Pseudoperonospora cubensis) in susceptible and resistant cucumber (Cucumis sativus L.) cultivars was investigated. A susceptible cultivar 'Pretty' and a resistant one 'Poinsett' were grown in hydroponic systems with three N levels (NO₃-N : 4, 8 and 16 me·liter^-1) in the nutrient solution, with or without fruit to change the nutrient condition of plant. In both cultivars, higher N levels of the nutrient solution and complete blossom thinning treatment tended to increase the shoot dry weight and leaf N content. The cultivar-specific resistance, shoot dry weight and leaf N content of both cultivars did not affect the number of days to appearance of the disease symptom under the high humidity condition subsequent to the start of treatments. However, the total lesion area of both cultivars correlated more closely with shoot dry weight than with leaf N content, indicating that the plant growth rate is a predominant factor governing the expansion of lesion area. Despite the similarity in shoot dry weight and leaf N content between cultivars, the lesion area of leaves of 'Poinsett' expanded slower than that of 'Pretty' and this resistance was not influenced by leaf N content. These results suggest the difficulty in suppressing the development of downy mildew in both cucumber cultivars by controlling the N concentrations (NO₃-N : 4 to 16 me·liter^-1) of hydroponic nutrient solution and leaf N content.