Control of Disease Caused by Pythium ultimum in Seed-Propagated Geraniums Sprayed or Not Sprayed with Silver Thiosulfate. M. K. Hausbeck, Research Technician, Pennsylvania State University, University Park 16802. C. T. Stephens, and R. D. Heins. Associate Professor, Department of Botany and Plant Pathology, and Associate Professor, Department of Horticulture, Michigan State University, East Lansing 48824. Plant Dis. 72:764-768. Accepted for publication 25 March 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0764.

Drenches of fenaminosulf, ethazol, and metalaxyl were evaluated for their effectiveness in preventing death of plants, plant stunting, and delay of flowering caused by Pythium ultimum in seed-propagated geraniums (Pelargonium × hortorum) sprayed or not sprayed with silver thiosulfate (STS). Silver thiosulfate, a petal abscission preventative, increases the incidence of plant death caused by P. ultimum. Fungicides were applied according to the following drenching schedules to plants grown in P. ultimum-infested or noninfested soilless root medium: 1) at seeding, 2) at seeding and transplanting, 3) at seeding, transplanting, and 1 wk after transplanting, 4) at transplanting, 5) 1 wk before STS application, and 6) on the day of STS application. Three metalaxyl drenching schedules (2, 3, and 4) prevented death of plants sprayed or not sprayed with STS and reduced plant stunting. Schedules 2 and 4 also reduced delay of flowering. Plant death was not prevented by any ethazol drenching schedule compared to the control when STS was not applied. Plant death after STS application was reduced, but not eliminated, by all ethazol drenching schedules except schedule 2. Plant stunting was reduced with ethazol drenching schedules 2, 3, and 4 and delay of flowering was decreased with drenching schedules 2 and 3. Fenaminosulf drenching schedules 3 and 4 prevented plant death when STS was not applied. Following STS application, plant death was reduced by all fenaminosulf drenching schedules, with the exception of schedule 1. Fenaminosulf did not reduce delay of plant flowering or consistently reduce plant stunting.