First Report of Stem and Root Rot of Tomato Caused by Phytophthora capsici in South Africa

Abstract

Tomato plants, grown in open hydroponic systems under shadecloth and plastic near Barberton and Pretoria in South Africa and Srelebi Phikwe in Botswana, developed symptoms of wilting with brown-to-black cankers on the lower stems, blackening of the vascular tissues, and root rot. Pathogens isolated from affected tissues were identified as Phytophthora capsici Leonian (1) and Pythium aphanidermatum (Edson) Fitzp. (2).They occurred separately or together. Pythium aphanidermatum has previously been recorded on tomato in South Africa. P. capsici isolates were papillate, caducous, grew at >36°C, had tapered sporangial bases, and a maximum sporangial length of >70 μm. Koch's postulates were confirmed by inoculating 4-week-old tomato seedlings (cv. Floradade) grown at 22 to 30°C in a steam-pasteurized mixture of sawdust compost, pine bark, and vermiculite (3:2:1). Plugs from V8 juice agar cultures of P. capsici were placed on wounds made on the stems of 10 seedlings. Ten wounded uninoculated plants served as controls. Water-soaked lesions were visible on the stems of all inoculated plants after 2 days. Control plants remained healthy. After 4 days, lesions turned dark brown with affected plants wilted or dead. Reisolation yielded P. capsici. The experiment was repeated with similar results. To our knowledge, this is the first report of P. capsici on tomatoes in South Africa. References: (1) A. H. Thompson et al. S. Afr. J. Bot. 60:257, 1994.(2) W. Dick. Keys to Pythium. University of Reading Press, Reading, U.K., 1990.