Emerging pathogens inhibit early season invasive grass litter decomposition

Brett R. Lane, Nicholas J. Heiss, Philip F. Harmon and Erica M. Goss, Plant Pathology Department, University of Florida, Gainesville, FL, Christopher M. Wojan, Department of Biology, Indiana University, Bloomington, IN, Amy Kendig and Luke Flory, Agronomy Department, University of Florida, Gainesville, FL, Robert Holt, Department of Biology, University of Florida, Gainesville, FL, Keith Clay, Ecology and Evolutionary Biology, Tulane University, New Orleans, LA, Erica M. Goss, Emerging Pathogens Institute, University of Florida, Gainesville, FL

Background/Question/Methods
Accumulation of plant litter drives significant changes in ecological dynamics through chemical, physical, and biological effects, including alterations to nutrient cycling, allelopathic inhibition of germination, and the physical inhibition of seedling recruitment. The invasive C4 annual grass, Microstegium vimineum, produces of large volumes of litter, which facilitates spread through transportation of cleistogamous seeds and suppression of native seedling recruitment. In 2010, epidemics of a disease caused by fungal pathogens in the genus Bipolaris were described on M. vimineum. The effect of foliar disease on litter dynamics and decomposition is largely unknown. We hypothesized that the establishment of a foliar pathogen prior to litter senescence would slow decomposition of the infected litter. This in turn may amplify the effects of litter on native species recruitment and survival.
To test this hypothesis, M. vimineum litter was collected from Big Oaks National Wildlife Refuge, Indiana, in October 2019 from six healthy and five infected sites. The litter was placed into mesh bags and returned to an un-infected common garden site at BONWR in December 2019. Bags were collected bi-monthly for a year. Decomposition was determined by the loss of dry weight. Subsamples were taken to estimate changes in stem and leaf weight.
Results/Conclusions
Foliar disease played a significant role in early season litter dynamics by delaying the decomposition of infected litter. In April, the healthy litter had decomposed 3.57% more than the infected litter. In June, there was no difference between infected and healthy litter overall, however, healthy leaf material was 22.8% more decomposed than infected leaf material, with leaf material making up a small fraction of the litter. Between June and August, infected litter decomposed faster than healthy litter, leading to no significant differences between healthy and infected samples.
To our knowledge, this is the first report of a foliar disease of a grass species significantly affecting the decomposition of litter material. These findings are consistent with previous studies that have tied the delayed decomposition of forage grasses colonized by endophytic fungi to changes in litter chemical composition. A consequence of the extended persistence of infected leaf tissue may be an increase in primary fungal inoculum in the subsequent growing season. The increased prevalence of fungal inoculum leads to the emergence of epidemics earlier in the season, further altering competitive interactions between native and invasive species.