Relationship Between Silver Thiosulfate and Premature Plant Death of Seed-Propagated Geraniums Caused by Pythium ultimum. M. K. Hausbeck, Former Graduate Research Assistant, Department of Horticulture, Michigan State University, East Lansing 48824-1312. C. T. Stephens, and R. D. Heins. Associate Professor, Department of Botany and Plant Pathology, and Professor, Department of Horticulture, Michigan State University, East Lansing 48824-1312. Plant Dis. 73:627-630. Accepted for publication 15 February 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0627.

Topically sprayed silver thiosulfate (STS), a petal abscission preventative, increased premature plant death of seed-propagated geraniums (Pelargonium × hortorum ‘Ringo Scarlet’) grown in a soilless root medium infested with low, medium, and high levels of Pythium ultimum. Premature death occurred as quickly as 4 days after STS application. The area under the disease progress curve (AUDPC) was used to express the accumulated premature plant death within 30 days after STS application. According to the AUDPC data, premature plant death resulted when STS was applied to plants grown in the highly infested medium for a minimum of 7 days or in the medium infested with a low or medium level of inoculum for 14 days. A trend toward higher AUDPC values was observed, the longer plants were grown in the highly infested medium (up to 14 days) prior to STS application. Reapplication of STS to surviving plants 30 days after the first application caused a new surge of premature plant death 10–15 days later.