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Ecology and Epidemiology

Suppression of Phytophthora parasitica on Catharanthus roseus with Aluminum. D. M. Benson, Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616; Phytopathology 83:1303-1308. Accepted for publication 20 August 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-1303.

The relationship of exchangeable aluminum, liming rate, and control of preemergence damping-off of Catharanthus roseus caused by Phytophthora parasitica was investigated. A peat/vermiculite medium at pH 4.1 was limed at 0, 3, or 6 g of dolomitic limestone per 1,000 cm3 of medium and misted for 3 days prior to seeding with Catharanthus roseus in 81-cell plug trays. Seeds were covered with additional medium infested with Phytophthora parasitica on colonized rice particles. Aluminum sulfate at rates of 17250 meq of Al per 100 cm3 of medium was drenched onto the surface of the medium. Samples were collected beginning 2 days after seeding to determine pH, exchangeable Al, and population density of P. parasitica. Stand counts were made beginning 9 days after seeding. Phytophthora damping-off was controlled in unlimed medium drenched with 17 meq of Al per 100 cm3 of medium, in medium limed at 3 g/1,000 cm3 when drenched with 41 meq of A1 per 100 cm3 of medium, and in medium limed at 6 g/1,000 cm3 when drenched with 100 meq of A1 per 1,000 cm3 of medium. Four days after seeding, exchangeable aluminum levels in treatments that controlled damping-off were 5.7, 1.2, and 0.5 meq/100 g of medium, respectively, in the unlimed medium and media limed at 3 or 6 g/1,000 cm3. At 3 or 6 g of lime per 1,000 cm3, but not in unlimed medium, pH was 0.5 units lower in treatments in which damping-off was controlled compared to treatments in which disease was not controlled. Populations of P. parasitica were suppressed in media in which damping-off was controlled. At the highest rate of aluminum drenches, populations were suppressed over a 16-day period, but at lower rates of aluminum, where control was observed, populations were suppressed only 79 days after seeding. Production of sporangia of P. parasitica in vitro was inhibited by 1.3 meq of A1 per 100 g of medium at pH 5. Aluminum may control Phytophthora damping-off by suppression of the P. parasitica population during the critical period of seed germination and emergence.

Additional keywords: calmodulin.