© 2008 Plant Management Network.
Ascosporic Fertilization of EpichloŽ typhina in Dactylis glomerata Seed Production Fields in Oregon and Implications for Choke Management
S. C. Alderman, National Forage Seed Production Research Center, USDA-ARS, Corvallis, OR 97331; and S. Rao, Department of Crop Science, Oregon State University, Corvallis 97331
Corresponding author: Stephen Alderman. firstname.lastname@example.org
Alderman, S. C., and Rao, S. 2008. Ascosporic fertilization of EpichloŽ typhina in Dactylis glomerata seed production fields in Oregon and implications for choke management. Online. Plant Health Progress doi:10.1094/PHP-2008-0421-01-BR.
EpichloŽ typhina, causal agent of choke, is a heterothallic, endophytic fungus that systemically infects the crown and foliage of Dactylis glomerata (orchardgrass). The sexual stage, which develops on reproductive tillers as 4 to 16 cm long, whitish, felt-like, cylindrical stromata, restricts inflorescence development, resulting in seed yield loss. Conidia produced on stromata function as spermatia. Fertilization is facilitated by flies (1), which transfer conidia while feeding, defacating, and ovipositing on the stromata. Conidia are not known to be wind disseminated (1). Following fertilization, stromata thicken and turn orange as perithecia develop and mature. Ascospores are forcibly discharged and are disseminated by wind. Although the process of infection of orchardgrass by E. typhina has not been fully established, ascospores are believed to be responsible for new plant infections.
About 90% of US orchardgrass seed production occurs in the Willamette Valley of Oregon. Choke, first observed in orchardgrass in the Willamette Valley in the mid-1990s, spread rapidly among orchardgrass fields. Up to 30% of plants within a field can be infected by the third or fourth year of production (2). Although fertilization of stromata by flies is a well established fact (1), recent exclusion experiments suggest that a mechanism other than insects may be responsible for fertilization in E. typhina (4).
The objective of this study was to determine if ascospores could serve as spermatia and fertilize stromata of E. typhina. Source material included 50 naturally E. typhina-infected orchardgrass plants from commercial fields. Plants were maintained via clonal propagation of tillers and vernalized in a growth chamber to initiate stromata. At tiller elongation and just prior to stromatal emergence, plants were transferred to the laboratory and each stroma was enclosed in a plastic tube (2.5 cm diameter ◊ 30 cm long). Aquarium pumps, 0.45-μm filters, and flasks with deionized water were used to supply each tube with clean, humidified air. For ascosporic inoculum, a fertile stroma was excised and placed on a wire mesh screen over a microscope slides in a covered petri dish containing moist tissue. After 20 to 40 min, ascospores ejected onto the glass slide were suspended in drops of DI water + surfactant (2 drops of Tween 20 per 100 ml). The drops were examined at 200◊ to confirm presence of ascospores and that no conidia were present. A 5-μl drop of ascospore suspension was placed at the top, base, and center of each of five replicate stromata. Five-μl drops of conidia (1 ◊ 102 conida/ml) or water were similarly transferred to stromata (five replicate stromata per treatment) as control inoculations. Conidia were collected from unfertilized stromata among a separate set of plants with an artist paint brush and suspended in 10-ml DI water + surfactant. There were 15 plants in the experiment, and the experiment was repeated twice.
A thickening of the stromata at the point of conidia or ascospore application was observed within 72 h in all stromata inoculated with ascospores or conidia in each of the three experimental runs. There was no thickening or evidence of fertilization in stromata inoculated with only water. Ascospore viability from stromata fertilized by ascospores or conidia in the third experimental run was verified in drops of water on glass slides.
To our knowledge, this is the first report of ascosporic fertilization in the genus EpichloŽ. The pattern of perithecia development in a stroma typically corresponds to the pattern of movement of conida-bearing flies on the stroma surface (1). In orchardgrass seed production fields in the Willamette Valley, thickening and subsequent perithecia development on stromata begin as discrete spots that enlarge and coalesce, resulting in perithecia distributed over the entire stroma surface. Ascosporic fertilization accounts for our observation of near 100% complete fertilization of stromata and for the rapid spread of choke in orchardgrass. However, we believe that flies are responsible for the fertilization of early emerging stromata, which then provide ascospores for subsequent fertilization and ascospore production among neighboring fields.
Currently there are no control options for choke in orchardgrass, and fungicides have proven to be ineffective (3). However, there may be greater efficacy of fungicides if applied after emergence but prior to fertilization of stromata, rather than applications after fertilization or post harvest to protect from infection. Alternatively, since the sole source of inoculum is infected plants, and choke can be visually detected prior to ascospore production (color change from white to orange), chemical or mechanical roguing of infected plants may be an alternative or additional means of reducing ascospore inoculum.
1. Bultman, T. L., White, J. F. Jr., Bowdish, T. I., Welch, A. M., and Johnston, J. J. 1995. Mutualistic transfer of EpichloŽ spermatia by Phorbia flies. Mycologia 87:182-189.
2. Pfender, W. F., and Alderman, S. C. 1999. Geographical distribution and incidence of orchardgrass choke, caused by EpichloŽ typhina, in Oregon. Plant Dis. 83:654-758.
3. Pfender, W. F., and Alderman, C. S. 2003. Evaluation of postharvest burning and fungicides to reduce the polyetic rate of increase of choke disease in orchardgrass seed production. Plant Dis. 87:375-379.
4. Rao, S., and Baumann, D. 2004. The interaction of a Botanophila fly species with an exotic EpichloŽ fungus in a cultivated grass: Fungivore or mutualist? Entomol. Exp. Appl. 112:99-105.