The international trade in ornamental plants has increased significantly in recent years and represents an important pathway for the spread of exotic pests and pathogens from other countries to the United States (Rossman, 2009). There are several historical examples of pathogen and pest introductions through the ornamental trade, some of which have caused widespread and catastrophic epidemics not only on ornamentals, but also on agricultural and forest crops in the United States (Liebhold et al., 2012; Parke and Grunwald, 2012).
In 2012, ≈71% of 1.2 billion live ornamental plants that entered the United States came through the Miami Plant Inspection Station in Florida (U.S. Department of Agriculture–Agricultural Quarantine Activity System). It is estimated that a substantial percentage of exotic pests and pathogens arriving at U.S. ports of entry along with ornamental plant imports is missed because plants may be infected but may not express symptoms; fungicides may suppress disease temporarily; pots or potting media may be infested but go unnoticed; pathogens are particularly easy to miss when infecting roots; and symptoms may not be recognized by plant inspectors (Liebhold et al., 2012). As a consequence, new pests and diseases are frequently introduced into Florida (Buck and Ono, 2012; Momol, 2006) and become distributed throughout Florida and the United States through infected plant materials. Plant pathogens such as Puccinia hemerocallidis, the causal agent of daylily rust, and Ralstonia solanacearum race 3, biovar 2, the causal agent of geranium bacterial wilt, entered the United States through Florida in 2000 (Buck and Ono, 2012) and 2003 (Momol, 2006), respectively.
The genus Cordyline consists of woody monocotyledonous flowering plants in the family Asparagaceae, subfamily Lomandroideae, and has ≈20 species, including Cordyline fruticosa L. A. Chev. (Hawaiian Ti) and C. australis G. Forst. Endl. (cabbage tree) (U.S. Department of Agriculture, Agricultural Research Service, 2011; 5 Mar. 2014 <http://www.ars-grin.gov/cgi-bin/npgs/html/taxon.pl?401621>). This genus is native to China, Taiwan, Myanmar, Indonesia, New Zealand, eastern Australia, and Hawaii. Cordyline fruticosa is the most frequently imported ornamental species into the United States from Guatemala (Agri-Starts, Inc., Apopka, FL, personal communication). It is a long-lived broadleaf evergreen plant that features thin lance-shaped leaves that emerge pinkish red but mature to deep green or variegated foliage. In the United States, C. fruticosa plants are grown outdoors in southern Florida, southwestern United States, and Hawaii but indoors in temperate climates.
Cordyline species are susceptible to several diseases in the United States and Florida. These include fungal diseases such as Botrytis blight, Cercospora leaf spot, fusarium leaf spot, fusarium stem and root rot, Phyllosticta leaf spot, Phytophthora leaf spot, Sclerotium southern blight as well as bacterial diseases such as Erwinia leaf spot and stem rot (Daughtrey and Chase, 1992; Moorman, 2014). However, new plant pathogens continue to be introduced. For example, we recently found that lucky bamboo (Dracaena sanderiana Sander ex Mast.) cuttings imported from China carried a latent infection of Colletotrichum dracaenophilum, a new species in Florida (Sharma et al., 2014). Similarly, other potential harmful exotic pests and pathogens currently not in the United States could potentially be imported with Cordyline plant materials.
In Apr. 2013, Hawaiian Ti ‘Tipsy Pink’ plants were observed with anthracnose-like symptoms in retail stores in Gainesville, FL (Fig. 1). Plant stems had brown, sunken lesions surrounded by a dark brown border, which later became necrotic and spread to the entire stalk. Sticky masses of conidia were found in fruiting bodies (acervuli) on symptomatic tissue and appeared to belong to a Colletotrichum species. Over time, several leaves wilted and dropped off the plants, and the plants died. Phoulivong et al. (2010) reported a new species of Colletotrichum, C. cordylinicola, that was associated with an anthranose on the leaves of C. fruticosa and Eugenia javanica in Thailand and Laos, respectively. To our knowledge, no Colletotrichum species has been associated with C. fruticosa in the United States.
Colletotrichum spp. are broad host range pathogens; many species can infect a given host, and a single species can infect diverse hosts, leading to serious cross-infection problems in ornamental nursery production (Freeman et al., 1998; Sanders and Korsten, 2003). With the introduction of new Colletotrichum spp., it is important to establish whether they are host-specific or have a wide host range. This will have important implications for disease management when susceptible ornamental plants are grown in the same greenhouse.
The objectives of this study were to isolate the suspect Colletotrichum spp. from Hawaiian Ti plants, perform Koch’s postulates with the isolate, genetically identify the isolate and compare it with previously studied Colletotrichum strains, and determine the variation in aggressiveness of the isolate on different Cordyline and Dracaena species and cultivars.
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