First report of Alternaria alternata causing leaf spot on the whorled sunflower (Heilianthus verticillatus) in the southeast United States

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: 11/22/2016
Publication Date: 11/28/2016
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 the whorled sunflower (Heilianthus verticillatus) in the southeast United States. Plant Disease. 101:632. doi:10.1094/PDIS-08-16-1216-PDN.

Interpretive Summary: The whorled sunflower is an endangered perennial that is native to limited areas in west Tennessee, east Alabama, and west Georgia. There is very little information about plant pathogens and the impact on whorled sunflower. Researchers at the University of Tennessee and USDA scientist at Charleston, SC, documented the first incidence of leaf spot on whorled sunflower. 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 whorled sunflower. Due to the endangered status of the plant, knowledge about plant pathogens is valuable for conservation efforts.

Technical Abstract: The whorled sunflower, Helianthus verticillatus (Small), is an endangered, herbaceous perennial plant endemic to limited small tracts of land in west Tennessee, east Alabama, and west Georgia. In October 2015, plants from wild populations in Georgia and Alabama exhibited small, circular brown, necrotic lesions on older leaves. Symptomatic tissue was collected, surface sterilized in 10% bleach solution for 1 min, rinsed three times in sterile water, blotted dry and placed onto ½ strength potato dextrose agar (PDA). After 14 days of incubation at 21°C in the dark, white-green fungal colonies were transferred to Alternaria sporulation medium. Club-shaped conidia (n = 20) in branched chains had transverse and longitudinal septa, and ranged from an average length of 17.96 µm (12.13 – 29.0 µm) and an average width of 10.58 µm (7.54 – 12.60 µm). The morphology of the conidia was similar to the description of Alternaria alternata. Genomic DNA was extracted from mycelium grown on potato dextrose agar (PDA) for 10 days using the Phire Plant Direct polymerase chain reaction (PCR) kit. The internally transcribed ribosomal spacer region (ITS) was amplified using ITS1 and ITS4 primers, and sequenced 563 bp amplicons were 100% identical with A. alternata (accession no. KU293578 in GenBank). Conidia for completion of Koch’s postulates were harvested from an Alabama isolate (AL1A) grown on 1/10 PDA flooded with sterile, distilled water, and adjusted to a 105 conidia/ml. Five disease-free plants were sprayed with one ml of the spore suspension and five plants with one ml of sterile, distilled water. All plants were enclosed in plastic bags for one week to provide high humidity and maintained under greenhouse conditions (~29°C). Small, circular necrotic lesions appeared on conidia-inoculated leaves after 8 days and none of the control plants exhibited any disease symptoms. The fungus was re-isolated from symptomatic tissue and the morphological, and molecular characteristics were identical to those from the original isolation of A. alternata. Although leaf spot caused by A. alternata on H. annuus L. is commonly encountered, to our knowledge, this is the first report of A. alternata causing leaf spot on H. verticillatus. Because of the endangered status of this plant, any information about pathogens affecting it will be fundamental in augmenting current management plans for plants in wild populations. This knowledge will also be beneficial to future ornamental production of H. verticillatus.