First Report of Acidovorax avenae subsp. avenae Causing Bacterial Leaf Stripe of Canna sp. in the Southeastern United States

Canna sp. are herbaceous perennials belonging to the family Cannaceae that are grown for their large, paddle-shaped leaves and their attractive panicles of brightly colored flowers. An outbreak of suspected bacterial leaf stripe on Canna sp. was found in several nurseries located in Florida (cvs. Cannova Rose and Cannova Bronze Scarlet) and Texas (cv. Cannova Colors) in the spring of 2017. Symptoms included dark-brown or black spots with a yellow halo that coalesced into necrotic stripes along leaf veins, especially on immature leaves. Infection often resulted in damage to the entire blade. Approximately 25% of each affected cultivar was damaged and unsalable, and by the time plants reached salable size, 50% of specific cultivars was lost. Bacterial colonies that were white to cream-colored were consistently recovered on nutrient agar (NA) from symptomatic canna leaf lesions from both Florida and Texas samples. Hypersensitive response on tobacco was induced within 24 h. To confirm the identity of the causal bacterium, molecular analyses were conducted for two representative isolates from the two states (isolate 170340 from Florida and isolate 170682 from Texas) targeting 16S rDNA with universal primers (27F/1495R) and specific primers (Oaf1/Oar1) for Acidovorax avenae (Fontana et al. 2013). Both sequences from 16S rDNA using universal primers (GenBank accession no. MF993324 for isolate 170340 and MF993325 for isolate 170682) had 99 to 100% similarity with three isolates of A. avenae causing bacterial leaf blight of tea in China (KY363297, KY363296, and KF498647; Geng et al. 2017). Sequences obtained from specific primers (MF993326 for isolate 170340 and MF993327 for isolate 170682) shared 100% similarity with an isolate of A. avenae subsp. avenae causing bacterial stripe of hog millet in Korea (JQ743877; Myung et al. 2012). Pathogenicity tests of the two isolates were performed using two plants of 3- to 4-month-old cultivars (Cannova Rose and Cannova Colors) for each isolate, and the experiment was repeated. Inoculations were done by spraying foliage of healthy canna plants with a bacterial suspension (10⁸ CFU/ml) from a 24-h-old culture grown on NA. Plants were placed in a greenhouse, where temperatures ranged from 24°C (night) to 27°C (day) with 95 ± 5% relative humidity. Symptoms developed 15 days after inoculation on all inoculated plants, closely resembling the original ones described above. A. avenae subsp. avenae was reisolated from symptomatic leaf spots for each treatment and was confirmed via molecular analysis. Noninoculated control plants were sprayed with sterile deionized water and remained symptomless. A. avenae subsp. avenae was recently reported causing leaf stripe on Strelitzia nicolai in Florida (Seijo and Peres 2011). In 2014, Oregon State University reported isolating A. avenae subsp. avenae from Canna cultivar South Pacific Scarlet (http://plant-clinic.bpp.oregonstate.edu/content/14-1080b/). To our knowledge, this is the first report of A. avenae subsp. avenae causing bacterial stripe of a seed-propagated Canna in the Southeastern United States. High disease incidence reported in Florida and Texas poses a great potential threat to affect marketability of canna.