(Oudem) J.J. Davis
SPOTS mainly on leaves, occasionally on culms, variable, subcircular, elliptic to oblong, sometimes irregular, 1-40 x 0.5-8 mm, at first pale yellowish-ochraceous, brownish, later greyish white, with an indefinite or a narrow ochraceous to dark brown, sometimes dull violet border, occasionally irregularly zonate.
FERTILE HYPHAE (fh) subcuticular or immediately below the outer epidermal wall, horizontally spread, colourless, septate, branched, 1.5-5 µm wide, frequently forming ropes of parallel hyphae; single cells, groups of cells or almost all cells of long hyphae are often inflated and form small to moderately large stromatic aggregations; ropes and aggregations are often confluent in severe or old infections and form irregular, diffuse stromatic layers "pseudoacervuli".
Conidiogenous cells (cc) minute, peg-like, subcylindric-conical form on the fertile hyphae. The cells perforate the cuticule and produce superficial conidia.
CONIDIA (c) solitary, subcylindric, ellipsoid-ovoid, fusiform, straight, curved, 0-3-septate, colourless; hilum neither thickened nor darkened.
PLANT HOST AND DISTRIBUTION. Rhynchosporium secalis affects different plant species of the family Poaceae (Braun 1998).
The fungus occurs in the whole world (Braun 1998; Brooks 1953).
NOTES. The teleomorph of Rhynchosporium secalis is unknown.
Rhynchosporium secalis does not form typical acervuli with definite shape and size. Its pseudoacervuli are produced from ropes and stromatic aggregations of the fertile hyphae. According to Braun (1998), R. secalis is intermediate between hyphomycetes and acervular coelomycetes, although it should be referred to the hyphomycetes.
The most economically important plant hosts of R. secalis are Hordeum vulgare L. and Secale cereale L. (Brooks 1953). Although the fungus commonly occurs, it does not cause significant looses in the yield of these plants.
Rhynchosporium secalis prefers cooler and wetter regions (Holliday 1989). Depending on environmental conditions, it persists on plant debris for up to 12 months (Smith et al. 1988). Additionally, R. secalis spreads by infected seeds. In the growing period, conidia of R. secalis are splash dispersed.
Conidia germinate readily on the leaf surface and may produce one or more germ tubes from each cell (Smith et al. 1988). Appressoria may be produced at the end of the germ tubes, but penetration is not dependent on them. The fungus penetrates its plant host directly, i. e., through epidermal cells, and not through stomata. This results in the collapse of mesophyll cells, an event which is evident externally as water soaking and scalding of tissues.
The optimum temperature range for growth, conidial germination, infection, and sporulation is 15-20oC. With a wetness period lasting 24 h, there is a high risk of infection at any temperature from 6 to 24oC, whereas with a period of 12 h high risk is confined to 16 to 24oC.
Braun U. 1998. A monograph of Cercosporella, Ramularia and allied genera. Vol. 2. IHW-Verlag.
Brooks F. T. 1953. Plant diseases. Geoffrey Cumberlege. Oxford University Press. London, New York, Toronto.
Holliday P. 1989. A dictionary of plant pathology. Cambridge University Press. Cambridge, New York, New Rochelle, Melbourne, Sydney.
Smith I. M., Dunez J., Lelliott R. A., Phillips D. H., Archer S. A. 1988. European handbook of plant diseases. Blackwell Scientific Publications.