Abstract
An outbreak of a foliar disease on New Guinea impatiens (Impatiens hawkeri Bull) was observed in a nursery in Dade County, Florida. Symptoms on leaves were water-soaked spots that increased rapidly in size and became light to dark brown necrotic areas. A Rhizoctonia-like fungus was isolated from infected leaves and stems on corn meal agar subcultured on potato-dextrose agar and identified as Thanatephorus cucumeris (Frank) Donk (anamorph Rhizoctonia solani Kuehn). Six plants were randomly inoculated with agar blocks containing the isolate. Control plants received agar blocks without the pathogen. Plants were placed in a modified humidity chamber (polyethylene bags) for 48 hours and then transferred to a greenhouse at 27 C. The symptoms appeared as water soaked spots, 10mm in diameter, that enlarged to 25mm or more and turned dark brown. A mycelial web grew over the leaves, killing them, and spreading from leaf to leaf. Small brown sclerotia and mycelium were found on leaves and stems, typical of that found on nursery plants. Thanatephorus cucumeris was consistently re-isolated from inoculated plants with no symptoms observed on uninoculated plants, fulfilling Koch's postulates.
