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Effect of Sodium Tetrathiocarbonate, Metalaxyl, and Fosetyl-Al on Development and Control of Phytophthora Root Rot of Citrus. M. E. Matheron, Extension Plant Pathologist, University of Arizona, Yuma Agricultural Center, Yuma 85364. J. C. Matejka, Research Specialist, University of Arizona, Yuma Agricultural Center, Yuma 85364. Plant Dis. 75:264-268. Accepted for publication 5 September 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0264.

The activity of sodium tetrathiocarbonate (STTC) on sporulation and growth of several species of Phytophthora in vitro has recently been demonstrated. This study was initiated to evaluate and compare the effect of root and soil treatments with STTC, metalaxyl, and fosetyl-Al on the subsequent development of Phytophthora root rot on citrus. Growth of rough lemon (a citrus rootstock) seedlings inoculated with sporangia of P. citrophthora and P. parasitica in the presence of STTC at 245 ?g/ml or metalaxyl at 10?g/ml was enhanced significantly compared with untreated plants. Shoot growth of rough lemon seedlings in soil naturally infested with P. parasitica that was treated 1 wk before planting with either STTC at 4,900 ?g/ml or metalaxyl at 10 ?g/ml was equivalent to that obtained in sterilized orchard soil. STTC, when applied as a soil drench at 2,450 and 4,900 ?g/ml, was lethal to P. citrophthora and P. parasitica on colonized leaf disks of lemon buried in soil, whereas metalaxyl at 10 ?g/ml or fosetyl-Al at 3,000 ?g/ml did not appreciably affect the viability of the pathogens. Sporangium production on leaf disks of lemon colonized by P. citrophthora and P. parasitica and buried in soil was reduced at least 90% compared with the untreated control 3 and 6 days after treatment of soil with 2,450 μg/ml of STTC, 10 ?g/ml of metalaxyl, and 3,000 μg/ml of fosetyl-Al. Compared with untreated soil, the number of lesions formed on pear fruit bait incubated with citrus orchard soil containing P. citrophthora or P. parasitica and treated with STTC at 24 μg/ml was reduced 76 and 74%, respectively, whereas no lesions developed when soil was treated with metalaxyl at 10 μg/ml or fosetyl-Al at 3,000 μg/ml. These investigations demonstrate the potential utility of STTC as a fungicide for control of Phytophthora root rot of citrus.