In the late summer of 1996, a root rot and vine decline was noticed on greenhouse grown melons (Cucumis melo L.) at the National Agriculture and Water Research Center, Riyadh, Saudi Arabia. Melons were grown on sandy clay soil and peat moss was spot applied as organic manure. The pH of the soil was 7.8. Disease symptoms first appeared at early stage of fruit maturity of the crop. Diseased vines exhibited stunting, yellowing, and, at a late stage, complete collapse of the leaf canopy. The roots showed discoloration, discrete lesions on all root systems, and loss of secondary and tertiary feeder roots. Numerous perithecia were observed on the secondary and tertiary roots only. Each ascus contained one large spherical ascospore. The fungus was isolated on potato dextrose agar and identified as a Monosporascus sp. (2). A pure culture was confirmed as Monosporascus cannonballus Pollack & Uecker. IMI (373485(01)) by IMI, Surrey, U.K. Pathogenicity was confirmed in a temperature-controlled growth chamber on the melon cv. Red Queen(F1). A medium that consisted of 3 liters of sand, 275 g of dried ground oat hulls, and 450 ml of distilled sterile water (1) was inoculated and incubated for 40 days. The colonized medium (100 ml) was mixed with a pasturized potting mix (sand/peat moss; 1:1) in 17-cm-diameter plastic pots. Five seeds were planted in each pot and, 15 days after germination, seedlings were thinned to one per pot. Noninfested pots were inoculated with sterile sand/oat hull medium. The pots were placed in chambers maintained at 30/20°C, day/night. At various intervals, the onset and severity of symptom expression were recorded. The isolate caused stunting and root necrosis. The fungus was reisolated from symptomatic plants. This report extends the known range of M. cannonballus to include Saudi Arabia. Other reports of this fungus are from India, Israel, Japan, Libya, Mexico, Spain, Taiwan, Tunisia, and the U.S.
References: (1) J. C. Mertely et al. Plant Dis. 77:667, 1993. (2) F. G. Pollack and F. A. Uecker. Mycologia 66:346, 1974.
