Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/2008
Publication Date: 12/1/2008
Citation: Freshour, L., Agarwal, S., Sorochan, J., Zale, J., Ownley, B., Gwinn, K., Castlebury, L.A., Carris, L.M. 2008. First report of Puccinia emaculata on switchgrass (Panicum virgatum L.) in Tennessee. Plant Disease. 92:1710. Interpretive Summary: Switchgrass is a fast-growing plant that is being considered for large-scale production as a source of biofuels. However, knowledge of the plant pathogens that cause diseases of this plant is limited. In growing a wide range of switchgrass accessions and cultivars in Tennessee, a disease was discovered to which all cultivars were susceptible although some were more susceptible than others. This disease is caused by a rust fungus that was identified as Puccinia emaculata using morphological characteristics. In addition this fungus was sequenced to determine an accurate identification. This rust fungus was not known previously to occur in Tennessee. This research will be used by plant pathologists who are developing strategies to control the disease caused by this rust fungus.
Technical Abstract: In the spring of 2007, switchgrass accessions and cultivars Alamo, Kanlow, SL-93-2001, and NSL 2001-1 (lowland), Blackwell (upland), Grenville, Falcon, and Miami (unknown ploidy levels) were sown at the East Tennessee Research and Extension Center in Knoxville, Tennessee for evaluation and controlled hybridization. In July and August, urediniospore pustules were observed on the upper leaf surfaces of all switchgrass cultivars, with some found to be highly susceptible using the rust rating system developed for switchgrass. Accordingly, Blackwell, Grenville and Falcon were highly susceptible and allowed for abundant sporulation on the leaves, sheaths, and stems without chlorosis or necrosis. The accession, Miami, was susceptible and uredinia appeared on leaves, sheaths, and stems with light necrosis. Alamo and Kanlow were moderately resistant with light sporulation and necrosis on the leaves. Oklahoma cultivars, SL-93-2001-1 and NSL 2001-1, were resistant with mild sporulation and necrosis on the leaves. In the fall, abundant teliospores were isolated from Grenville, Falcon and Blackwell, with fewer teliospores isolated from Alamo. Teliospores dimensions are reported as the mean ± S.D. Teliospores were brown, two-celled, ellipsoid to oblong, 33.6 µm ± 4.8 long with the an apical cell width of 17.5 µ ± 1.2 and basal cell width of 15.9 µm ± 2.5. Pedicels were brown, 28.5 µm ± 7.4. Telipspore length with pedicel was 61.9 µm ± 8.0. This rust is identified as Puccinia emaculata Schwein., which has been reported to infect upland and lowland populations of switchgrass. A 2109 bp fragment containing the internal transcribed spacer (ITS) 1, 5.8S, ITS 2, and D1/D2 region of the large subunit ribosomal DNA was sequenced for a specimen on ‘Falcon’ (GenBank number, BPI number). No sequences of P. emaculata are available for comparison. The alternate hosts of this rust are species of the Euphorbiaceae family (2) which are ubiquitous in this area. This is the first report of P. emaculata on switchgrass in Tennessee. This rust may become problematic given the large acreage of switchgrass planned in the Tennessee Switchgrass Initiative, and long-standing fields of this perennial may compound the problem.