Effect of Salinity on Root Rot of Cucumis melo L. Caused by Phytophthora melonis

Cultivars of Cucumis melo L. are important economic crops planted in both saline and non-saline soils in Iran. Root rot on C. melo caused by Phytophthora melonis is one of the most devastating soil-borne diseases causing great loss. C. melo crops cultivated in saline soil adjacent to Maharloo Lake (salt lake) in Fars Province have been associated with diseases caused by Phytophthora species for many years. In this study, effect of salinity on Phytophthora root rot on C. melo under hydroponic system was investigated: Four-weekold plants of three cultivars, namely, Shahde-Shiraz, Dastanbo-Khorasan, and Kharbozeh-Mashhad grown in Nukaya solution were subjected to salinity stress for one week. A week later, all plants were inoculated with zoospore suspension of P. melonios. After 48 hours, inoculated solution was replaced by fresh nutrient solution and postinoculation salt-stressed treatment was applied to some plants. Based on shoot dry weight and concentration of Na+, K+, and Cl-, cultivars Shahde-Shiraz and Dastanbo-Khorasan were sensitive and resistant to salinity and also with the highest and lowest colonization of roots by P. melonis, respectively. Interaction of salinity and infection by P. melonis reduced shoot dry weight in the salt-tolerant cultivar more than salt-sensitive plants. Salinity increased root colonization by P. melonis compared to non-saline condition. The increase in root colonization due to salinity was not significantly different in Shahde- Shiraz and Kharbozeh-Mashhad cultivars. In Dastanbo-Khorasan, due to its higher resistance to P. melonis, salinity resulted in significant increase in root colonization, indicating reduction of root resistance due to salinity stress.