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Chemical Control of Stem and Root Rot of Cowpea Caused by Phytophthora vignae . W. G.D. FERNANDO, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331. R. G. LINDERMAN, USDA-ARS, Horticultural Crops Research Laboratory, Corvallis, OR 97330. Plant Dis. 78:967-971. Accepted for publication 11 July 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0967.

The fungicides metalaxyl (Subdue), fosetyl-AI (Aliette SOW), etridiazole (Banrot), and mancozeb (Manzate 200) significantly reduced in vitro mycelial growth of Phylophthora vignae isolates P001 and P006 at most concentrations tested. Metalaxyl was the most effective compound at low concentrations in both in vitro and pot experiments in reducing mycelial growth (1.68 mg a.i./L and 2.45 mg a.i./L for P001 and P006, respectively), oogonia formation (1.03 and 1.07 mg a.i./L for P00I and P006, respectively), and disease severity (10 mg a.i./L), and in increasing plant dry weight (10 mg a.i./L). High concentrations of metalaxyl (50 and 100 mg a.i./L) were phytotoxic and reduced plant and root dry weights. In contrast, fosetyl-AI was effective only at high concentrations. Mancozeb was effective both as a foliar spray and as a soil drench. Etridiazole was effective at low concentrations (5.75 mg a.i./L) in reducing radial growth of the fungus and decreased disease severity and increased plant dry weight at 50 mg a.i./L. Growth in vitro of strain DF-3101 of the biocontrol bacterium Brevibacterium linens was not affected by metalaxyl and etridiazole, but growth was partially inhibited by mancozeb and completely inhibited by 1,250 mg a.i./L of fosetyl-AI. Capacity for production of diffusible inhibitors by strain DF-3101 was not decreased significantly by prior exposure to 5 mg a.i./L of metalaxyl or 10 mg a.i./L of etridiazole but was decreased by 10 mg a.i./L of metalaxyl and 50 mg a.i./L of mancozeb on potato-dextrose agar; production of volatile inhibitors was not affected significantly by any chemical. All four fungicides tested were effective against P. vignae and could be used alternatively to reduce the development of fungicide resistance. Furthermore, all fungicides except fosetyl-AI could be applied at low concentrations in combination with bacterial biocontrol agents.

Keyword(s): toxicity