First Report of Soybean Rust Caused by Phakopsora pachyrhizi in Iowa and its Statewide Occurrence.

On 25 September 2007, soybean leaves with lesions typical of soybean rust were found in two commercial fields located in Dallas County, Iowa. Growth stage of the infected soybean plants was near physiological maturity. Diagnosis of Phakopsora pachyrhizi on leaves was confirmed by morphological observation of uredinia and urediniospores and conventional PCR conducted by the Iowa State University Plant and Insect Diagnostic Clinic using P. pachyrhizi-specific primers Ppm1 and Ppa2 as described (1). Water blanks were used as negative controls in PCR testing. Leaves were collected from additional counties throughout Iowa and examined microscopically. Soybean rust was identified on leaves from 14 counties, almost all the counties sampled, ranging from far western to far eastern Iowa. The northernmost detection was at 42.9°N in Hancock County, which also is the northernmost detection of soybean rust in the continental United States so far. In a commercial field in Fremont County, in the southwestern corner of Iowa along the Missouri River, disease incidence was approximately 20% and disease severity was 5%. Observed disease incidence was 1 to 2% and severity was less than 1% from all other samples. Most uredinia were scattered on the leaves as single pustules or clustered in groups of three to five pustules. Pustules on some leaf samples were sporulating, depending on weather conditions at the time when samples were collected. Fresh urediniospores collected from the leaf samples were placed on glass slides with free water on the surface and incubated in a dew chamber under darkness for 8 h to test for germination. Germination rates ranged from 80 to 90%. Rust spores also were used to inoculate detached soybean trifoliate leaves, which were kept in a dew chamber under darkness for 12 h with water-soaked cotton wrapped around the petioles. Typical soybean rust pustules developed within 10 to 14 days after incubation. The detections of soybean rust in Iowa were consistent with the predictions of an aerobiological spore dispersal model, which predicted spore showers in central to western Iowa in August. Above normal wet weather in the Great Plains and Iowa may have favored the statewide disease occurrence. To our knowledge, this is the first report of soybean rust in Iowa. Reference: (1) R. D. Frederick et al. Phytopathology 92:217, 2002.