Diseased samples of globe mallow, Sphaeralcea grossulariaefolia and S. munroana, were submitted by an ornamental seed producer in Wyoming to our Extension Plant Pathology Laboratory in July 1997. Dark brown, amphigenous telia surrounded by chlorotic halos were present on both foliage and stems. Mean teliospore dimensions observed were 30.8 × 44.8 μm. The teliospores germinated readily on water agar at 20°C and formed basidiospores within 24 h. Aecia and uredinia were not found. Based on characteristics mentioned above, this fungus was identified as Puccinia sherardiana Körn (1). This microcyclic rust was previously described on 12 other Sphaeralcea spp. plus other plant species in the Malvaceae family (1,2). Stem and foliar symptoms were reproduced in a greenhouse on 8-week-old plants of S. grossulariaefolia and S. munroana. These plants were inoculated with teliospores removed from the original diseased plant material. Immediately after inoculation, plants were misted and placed in plastic bags and incubated for 36 h at 100% relative humidity and 20°C. Plants continued growth with natural lighting and with day and night temperatures of 20 and 15°C, respectively. Symptoms developed within 12 days with initial telia rupturing the host epidermis 13 days after inoculation. Telia were examined microscopically to complete Koch's postulates.
References: (1) J. C. Arthur. Manual of Plant Rusts in United States and Canada. Hafner Pub., 1962. (2) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. American Phytopatholical Society, St. Paul, MN, 1989.