Horizontal transmission and expression of Epichloë typhina in orchardgrass (Dactylis glomerata)

Authors: MERLET, LEA - Oregon State University; Dombrowski, James; Bushman, Shaun; Gilmore, Barbara; Rivedal, Hannah; Martin, Ruth

Submitted to: European Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2022
Publication Date: 5/5/2022
Citation: Merlet, L., Dombrowski, J.E., Bushman, B.S., Gilmore, B.S., Rivedal, H.M., Martin, R.C. 2022. Horizontal transmission and expression of Epichloë typhina in orchardgrass (Dactylis glomerata). European Journal of Plant Pathology. 163:415–428. https://doi.org/10.1007/s10658-022-02485-y.
DOI: https://doi.org/10.1007/s10658-022-02485-y

Interpretive Summary: Choke disease was first discovered in orchardgrass seed production fields in the Willamette Valley of Oregon in 1997 and by 2003 it was found in 90% of the orchardgrass fields surveyed. The fungus that causes this disease, Epichloë typhina, grows mainly internally during plant vegetative growth. Then, just before the flower stalk elongates, the fungus emerges on the outer surface of leaf sheaths and grows rapidly, surrounding the leaf sheaths and forming a cattail-like structure that “chokes” off the flower stalk and prevents seed production. This disease can cause up to 30% losses in seed yields. Currently, there are no available control methods for this disease, and it is unclear how Epichloë typhina/orchardgrass infections occur in the field. In this study, the potential for infection during harvest was investigated by placing trap (uninfected) plants in fields displaying high levels of choke disease. The infection rate in these trap plants varied between 41% in 2018 and 9% in 2019. The effects of exposure timing (before or after swathing), trap plant cutting prior to exposure or at different times post-exposure, and flower induction on fungal establishment in trap plants were examined. Spread within infected plants, which also contributes to the incidence of choke within fields was also examined. Not too surprisingly, daughter tillers arising from an infected tiller were usually infected, while uninfected tillers generally gave rise to uninfected daughter tillers. Interestingly, the tillering rate of infected tillers was greater than that of uninfected tillers, which could lead to gradual increases in choke over time.

Technical Abstract: Choke disease, which is caused by Epichloë typhina, was first reported in orchardgrass seed production fields in the Willamette Valley, Oregon in 1997 and quickly spread throughout the valley. The fungus is innocuous most of the year, but just prior to flower head emergence, a rapid and dense growth of fungus within and among leaf sheaths forms a sexual reproductive structure (stromata) that destroys, or “chokes”, the host flower. This disease can cause up to 30% losses in seed yield. Unlike many Epichloë species that are transmitted through the seed, it is unclear how Epichloë typhina/orchardgrass infections occur in the field, and currently there are no available control measures for this disease. A better understanding of choke spread to and within fields is an essential step towards developing control measures. In the present study, trap plants placed in the field during harvest were infected at rates of 41% in 2018 and 9% in 2019. The effects of artificial aphid honeydew, field exposure timing (before or after swathing), trap plant cutting prior to exposure or at different times post-exposure, and vernalization on fungal infection/establishment in orchardgrass were also evaluated. Spread of fungi within tillering plants was examined. Not too surprisingly, daughter tillers arising from an infected tiller were usually infected, while uninfected tillers generally gave rise to uninfected daughter tillers. Interestingly, the tillering rate of infected tillers was greater than that of uninfected tillers, which could contribute to a gradual increase in choke in fields over time.