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Basal Stem and Root Rot of Christmas Cactus Caused by Phytophthora parasitica. S. A. Alfieri, Jr.,, Plant Pathologist, Division of Plant Industry, Florida Department of Agriculture and Consumer Services, Gainesville 32601; J. W. Miller, Plant Pathologist, Division of Plant Industry, Florida Department of Agriculture and Consumer Services, Gainesville 32601. Phytopathology 61:804-806. Accepted for publication 11 February 1971. DOI: 10.1094/Phyto-61-804.

A new and potentially serious disease has recently been observed affecting Christmas cactus (Zygocactus truncatus). The first symptoms occur as a wilting of stems which exhibit a lifeless, dull, gray-green color. The basal portion of the affected stems, usually at or just below the soil line, reveals a water-soaked but rather firm necrotic area with a faded reddish border. Infected roots are water-soaked, brown, soft, and necrotic. Repeated isolations from affected stems and roots of naturally infected plants, and results of controlled inoculations with a single isolate, have shown that Phytophthora parasitica is the causal agent. Among four fungicides tested for disease control as a soil drench prior to inoculation. Dithane M-45 (coordination product of zinc ion and manganese ethylene bisdithiocarbamate) provided complete control, and Dexon [p-(dimethylamino) benzenediazo sodium sulfonate], very good control; whereas Terrazole (5-ethoxy-3-trichloromethyl-1,4-thiadiazole) and Demosan (1,4-dichloro-2,5-dimethoxybenzene) provided the least effective disease control.