The epidemiology of Puccinia emaculata (rust) in switchgrass and evaluation of the mycoparasite Sphaerellopsis filum as a potential biological control organism for switchgrass rust.

Switchgrass (Panicum virgatum L.) is a warm-season, perennial grass, whose native range includes the entire United States and north into Canada, excluding areas along the pacific coast. Recently, symptoms and signs of rust disease (Puccinia emaculata) have been observed on agronomic switchgrass, which include chlorosis of leaf tissue, necrosis, lodging, and plant death.

To evaluate disease progress of switchgrass rust, in four fields, individual leaves of twenty-five switchgrass plants were rated once per week for fifteen weeks over two growing seasons for disease severity. Rust was first observed on Julian day 166 and 152 in 2010 and 2011, respectively. Ninety-five percent of switchgrass plants were at the 5-7 leaf growth stage before rust was first observed. Disease severity progressed logistically after detection; the rate of increase in disease severity lessened in late August to early September. The log phase of disease progression occurred from mid-June to mid-August. Leaf mortality was first observed in mid-to-late June. Greater than five percent of leaf surfaces were covered with uredia by early-to-mid October. Data collected in this study indicates when rust epidemics begin and subside on switchgrass in East Tennessee. If fungicide sprays become a viable management strategy, this data will be useful in timing those applications.

Growth and pycnidial production of Sphaerellopsis filum was highest on V8 juice agar, which was used to maintain cultures. To evaluate the mycoparasite’s ability to impede urediospore production and viability, uredia of P. emaculata on detached switchgrass leaves were inoculated with conidia of S. filum. Pycnidia formed in uredia at 12-14 days after inoculation. The mycoparasite significantly reduced the number of urediospores per uredium by an average of 246 spores when compared to untreated uredia.

When germination of urediospores was compared between healthy or those parasitized by S. filum, percent germination was 73% and 42%, respectively. Germ tubes of urediospores from healthy uredia averaged 96.9 um in length, whereas those from parasitized uredia averaged 32.3 um at three hours. As the mycoparasite reduced urediospore production, germination, and germ tube length, further investigation into its use as a potential biological control agent for P. emaculata is warranted.