Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/17/2003
Publication Date: 2/28/2004
Citation: Pfender, W.F. Role of phenology in host susceptibility and within-plant spread of stem rust during reproductive development of perennial ryegrass. Phytopathology.2004.94:308-316. Interpretive Summary: Stem rust is the most damaging disease that occurs in seed-production fields of perennial ryegrass. The factors controlling rate of disease increase must be understood in order to limit and optimize fungicide use in managing the disease. This research determined that plants of all developmental stages are equally susceptible to infection, but that the youngest leaves and stems on any plant are more susceptible than the older parts. An additional result was the finding that the disease can spread from a single infection on the leaf sheath to produce a very large diseased area on the developing flower and flower stem. This process of disease increase may account for most of the final disease level in a production field. By demonstrating the existence of this phenomenon, and quantifying its development based on plant growth, the research provides a basis for improved prediction and management of stem rust epidemic development.
Technical Abstract: Perennial ryegrass (Lolium perenne) was inoculated with urediniospores of Puccinia graminis subsp. graminicola at four stages of reproductive tiller development. All developmental stages, from expansion of the penultimate leaf through anthesis, were equally susceptible to infection. However, there was a negative correlation of susceptibility with age of host tissue, the highest disease severities occurring on expanding or newly expanded leaves or inflorescences. Within-plant spread of the disease from primary lesions on the flag leaf sheath to the elongating stem of the inflorescence was observed and verified. This process can produce an elongated strip of contiguous secondary infections on the inflorescence head and stem, a characteristic sign of stem rust, from a single primary infection on the leaf sheath. Infections are initiated on the stem slightly distal to the overlying sheath lesion and erupt one latent period later, by which time tiller elongation has moved the stem infection site upward. Repeated infections occurring by this process as the stem elongates produce contiguous lesions. Expansion rate and final extent of the lesioned area on stems were correlated with elongation rate of the inflorescence. Microscopic observation of plant tissue sections revealed that the fungus sporulates on the inner surface of the sheath, and germinating urediniospores are attached to the stem surface beneath the covering sheath, starting 1 to 2 cm distal to the location of the sheath infection site.