Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/25/2006
Publication Date: 7/17/2006
Citation: Cowger, C., Silva-Rojas, H. 2006. Frequency of phaeosphaeria nodorum, the sexual stage of stagonospora nodorum, on winter wheat in north carolina, usa.. Phytopathology. 96:860-866. Interpretive Summary: Debate has occurred over the epidemiological role of the sexual stage of the pathogen that causes Stagonospora nodorum blotch of wheat. Researchers have hitherto been unable to find the sexual stage in the eastern U.S. We found sexual fruiting bodies, or ascocarps, in wheat debris collected in Kinston, NC, each month from May through October of 2003, and also in a Kinston debris sample from August 2004. Ascocarps comprised a significantly higher percentage of fruiting bodies from wheat spikes than of those from lower stems and leaves.
Technical Abstract: Ascocarps of Phaeosphaeria nodorum, which causes Stagonospora nodorum blotch of wheat, have not been found by other researchers in the northeastern or southeastern U.S. despite extensive searches. We sampled tissues from living wheat plants or wheat debris in Kinston, North Carolina, each month except June from May to October 2003. Altogether, over 1,000 fruiting bodies were examined microscopically and tallied as P. nodorum pycnidia or ascocarps, “unknown,” or “other fungi.” P. nodorum ascocarps were present each month after May at a frequency of 0.8%-5.4%, and comprised a significantly higher percentage of fruiting bodies from wheat spikes than of those from lower stems and leaves. Also, a sample of 333 fruiting bodies taken from Kinston wheat debris in August 2004 yielded percentages of ascocarps in spikes and basal tissues that were similar to those from August 2003. Analysis of the nucleotide sequences of internally transcribed spacer (ITS) regions of 18 genetically distinct North Carolina isolates from 2003 suggested that all were P. nodorum, not the morphologically similar P. avenaria f. sp. triticea. Neither the 2003 isolates nor a sample of 77 isolates derived from 2004 Kinston leaf samples gave reason to suspect a mating-type imbalance in the larger population (P ' 0.3592). We conclude that in the North Carolina P. nodorum population, sexual reproduction plays a role in initiation of new epidemics and the creation of adaptively useful genetic variability, although its relative importance in structuring this population is still uncertain.