White mold, also called Sclerotinia stem rot, is caused by the fungus Sclerotinia sclerotiorum. This necrotrophic pathogen can infect over 400 species of plants from 75 families, including monocotyledons and dicotyledons and herbaceous and woody ornamentals (Boland and Hall, 1994). Gleason et al. (2009) listed over 30 genera of herbaceous perennials and Daughtrey et al. (1995) reported 10 genera of common flowering potted plants that are susceptible to S. sclerotiorum. Boland and Hall (1994) list over 50 susceptible genera in asteraceae alone. These lists are outdated because multiple new publications describe infection of new host plants by S. sclerotiorum (Chang et al., 1997; Garibaldi et al., 2001, 2008a, 2008b, 2008c; Gulya et al., 2006; Strauss and Dillard, 2009).
Annual bedding plants are widely used and highly valued in landscapes in private homes, public parks, and around commercial properties. The U.S. Department of Agriculture Agricultural Statistics Service reports that the value for wholesale annual bedding and garden plants totaled $1.36 billion in 2012.
Many of the most popular annual bedding plants are susceptible to white mold, including petunia, zinnia, verbena, snapdragon, and salvia (Boland and Hall, 1994). Infection of annual bedding plants can cause significant plant death in ornamental beds in home and public landscapes. Although infection may start early in the season, plant death from white mold often occurs midsummer, when replacement plants are unavailable.
Infestation of an annual flower bed with S. sclerotiorum is particularly problematic because the fungus produces long-term resting structures called sclerotia (Bolton et al., 2006; Grau and Radke, 1984). Once introduced, the fungus can persist in soil and plant debris for decades with repeated infection each year. The close plant spacing and overhead irrigation used in most annual plantings create highly favorable environmental conditions for sporulation and infection by S. sclerotiorum (Bolton et al., 2006; Grau and Radke, 1984).
Although white mold causes significant levels of stem rot, crown rot, wilt, and death of many common herbaceous ornamental plants (Bolton et al., 2006), relatively little research has been done on management of white mold in ornamental crops. Annual bedding plants are not commonly evaluated for resistance to white mold. There are several commonly used genera and species of annual bedding plants for which susceptibility or resistance to white mold has not been reported. Identification of genera or species with resistance to S. sclerotiorum would provide an effective means of managing white mold, requiring little extra labor or expenses. The objective of this study was to evaluate widely available annual bedding plant varieties with no reported susceptibility to white mold for potential resistance to this disease.
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