First report of Alternaria alternata causing leaf spot on Ruth's golden aster (Pityopsis ruthii) in Tennessee

Authors: EDWARDS, TYLER - University Of Tennessee; TRIGIANO, ROBERT - University Of Tennessee; Wadl, Phillip; OWNLEY, BONNIE - University Of Tennessee; HADZIABDIC, DENITA - University Of Tennessee

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/18/2016
Publication Date: 9/18/2016
Citation: Edwards, T.P., Trigiano, R.N., Wadl, P.A., Ownley, B.H., Hadziabdic, D. 2016. First report of Alternaria alternata causing leaf spot on Ruth's golden aster (Pityopsis ruthii) in Tennessee. Plant Disease. 101:383. doi:10.1094/PDIS-08-16-1214-PDN.

Interpretive Summary: Ruth's golden aster is an endangered perennial that only grows on small sections of the Hiwassee and Ocoee Rivers, in southeastern Tennessee. There is very little information about plant pathogens and the impact on Ruth's golden aster. Researchers at the University of Tennessee and a USDA scientist at Charleston, SC, documented the first incidence of leaf spot on Ruth's golden aster. The fungal organism causing the leaf spot was identified using both microscopy and molecular tools. Prior to this study, there were no known leaf spot diseases for Ruth's golden aster. Due to the endangered status of the plant, knowledge about plant pathogens is valuable for conservation efforts.

Technical Abstract: Ruth’s golden aster, Pityopsis ruthii (Small), is an endangered, herbaceous perennial plant that is only endemic to small sections of the Hiwassee and Ocoee Rivers, in Polk County, Tennessee. In July 2015, a greenhouse grown plant exhibited symptoms of disease that included elongated brown lesions on leaves and leaf dieback. Symptomatic tissue was surface sterilized in a 10% bleach solution for 1 min, rinsed three times in sterile water, blotted dry and then plated onto 1/10 potato dextrose agar (PDA). Within 2 days, white mycelium was present and subcultured onto fresh 1/10 PDA. After 14 days of incubation at 21°C in the dark, white and green mycelium covered the entire surface of agar. Club-shaped conidia (n=20) that contained both longitudinal and transverse septa were produced in linear and branched chains and measured an average length of 17.09 µm (11.86 – 23.91 µm) and an average width of 9.88 µm (6.27 – 12.6 µm ). The morphology of the conidia matched the description for Alternaria alternata. A aqueous suspension of 2x105 spores/ml was made using isolate PUK2 from the original symptomatic plant grown on the Alternaria sporulation medium. Ten P. ruthii plants were inoculated with approximately 0.5 ml of the spore suspension and five additional plants sprayed with sterile, distilled water as controls. Inoculated and control plants were placed into plastic bags for 7 days to maintain high humidity, after which the bags were removed, and thereafter plants were maintained under greenhouse conditions (~29°C). Fourteen days post inoculation, elongated brown lesions were evident on leaves and leaf dieback occurred on the five of the spore-inoculated plants. None of the control plants exhibited disease symptoms. Alternaria alternata was re-isolated from lesions as described previously, and had identical morphology compared to the initial isolation. Genomic DNA was extracted from mycelium grown on PDA for 10 days using the Phire Plant Direct polymerase chain reacton (PCR) kit. The internally transcribed ribosomal spacer region (ITS) was amplified using ITS1 and ITS4 primers. The amplicon was 569 bp and was 100% identical to A. alternata (GenBank accession No.KU293578.1). The ITS sequence from Alternaria alternata isolated from P. ruthii was submitted to Genbank as accession No. KX611152. To our knowledge, this is the first report of A. alternata causing disease on the endangered species P. ruthii. Information concerning diseases is valuable for the ongoing conservation efforts and will aid in commercial propagation of the species.