Role of phenology in host susceptibility and within-plant spread of stem rust during reproductive development of perennial ryegrass.

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 measured as number of pustules per total inoculated plant area. However, within each stage from boot through anthesis, 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. A single primary infection on the leaf sheath can produce an elongated strip of contiguous secondary infections on the inflorescence head and stem, which is a characteristic sign of stem rust. Position and timing of eruption of the secondary pustules on the stem can be explained by assuming that infections occur 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 occur as the stem elongates, producing contiguous lesions. Expansion rate and final lesion area on the stem 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 under the covering sheath starting 1 to 2 cm distal to the location of the sheath infection site.