White mold, caused by the necrotrophic fungal pathogen Sclerotinia sclerotiorum, causes stem rot, crown rot, wilt, and death of many common annual bedding plants, including zinnia, petunia, verbena, snap dragon, and salvia (Boland and Hall, 1994; Farr and Rossman, 2017; Grabowski and Malvick, 2015). Infested flower beds often suffer significant plant loss each year in part due to the ability of the pathogen to survive for at least 8 years in soil (Bolton et al., 2006). Infection occurs in cool moist conditions and plant death from white mold occurs in mid to late summer (Abawi and Grogan, 1979; Bolton et al., 2006). At this stage of the growing season, replacement plants for summer flowering annuals are typically no longer available. Garden managers are left with unsightly patches of dead plants or bare soil.
Identifying ornamental plants with resistance to white mold would allow growers to avoid disease problems in beds known to be infested with S. sclerotiorum. Over 400 plant species from more than 75 families are known to be susceptible to S. sclerotiorum (Boland and Hall, 1994). Gleason et al. (2009) list over 30 genera of herbaceous perennials and Daughtrey et al. (1995) list 10 common flowering potted plants that are susceptible to S. sclerotiorum. New susceptible host plants are regularly identified (Chang et al., 1997; Garibaldi et al., 2008a, 2008b, 2008c, 2001; Grabowski and Malvick, 2015; Gulya et al., 2006; Strauss and Dillard, 2009). Grabowski and Malvick (2015) tested four genera of annual bedding plants with no reported susceptibility to white mold for potential resistance to S. sclerotiorum. Although moderate resistance was found, plants from all four genera became infected by S. sclerotiorum to some degree. To identify plants with reliable resistance to white mold, new taxa of ornamental plants need to be evaluated for resistance.
Ornamental tropical plants are often grown in annual flower beds to add color and texture. Canna, caladium, and elephant ear have a long history of cultivation. Canna originates from South America and was first introduced to European gardeners as a foliage plant in 1595 (Khoshoo and Mukherjee, 1970; Prince, 2011). Caladium originates from tropical regions of Central and South America (Germplasm Resources Information Network, 2017). Breeding of hybrid garden caladium has occurred for over 150 years (Deng et al., 2007). Elephant ear, also known as taro, originates from Southeast Asia and is believed to be one of the oldest domesticated crops (Quero-Garcia et al., 2010). Plants from these genera are readily available to landscape managers in a variety of colors to meet designers’ needs. Cultivation and care of these plants in gardens is well described (Iversen, 1999), allowing for easy adoption of plants should they prove resistant to white mold.
Perhaps because they are of tropical origin, plants in these genera have few pest problems in temperate climates (Horst, 2001; Iversen, 1999) and no recorded history of infection by S. sclerotiorum (Boland and Hall, 1994; Farr and Rossman, 2017). In addition, all are monocots. Although some monocots are listed as hosts to S. sclerotiorum, economic damage due to this pathogen primarily occurs in dicotyledonous plants (Boland and Hall, 1994; Bolton et al., 2006; Farr and Rossman, 2017).
Although canna, caladium, and elephant ear are grown as annuals in temperate climates, each produces a belowground storage organ (rhizome, tuber, corm) that can be dug up at the end of the season, stored for the winter, and planted the following season. Belowground storage organs can be used to propagate these plants as well (Iversen, 1999). In addition to evaluating resistance to infection of aboveground plant parts, it is important to determine the susceptibility of corms, tubers, and rhizomes. White mold is known to spread in storage on crops such as carrot (Koike, 2007). If corms, tubers, and rhizomes of tropical ornamentals were infected, similar spread in storage would be likely.
The objective of this study was to evaluate three commonly grown tropical ornamentals, canna, caladium, and elephant ear for potential resistance to S. sclerotiorum.
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