Intumescences are a physiological disorder that develops sporadically on the leaf tissue of many plant species, including some varieties of tomato (Solanum lycopersicum ‘Maxifort’; Rud, 2009), sweetpotato (Wetzstein and Frett, 1984), and cuphea (Cuphea sp.; Jaworski et al., 1988). This disorder is often described as abnormal, translucent outgrowths on the leaf surface with a gall or wart-like appearance (J.K. Craver, C.T. Miller, K.A. Williams, and D.L. Boyle, in press; Morrow and Tibbitts, 1988; Wetzstein and Frett, 1984). This disorder was first observed and named as intumescence by Sorauer (1899) and was found at that time to develop on numerous plant species (La Rue, 1932). Although the term intumescence is commonly used to describe this disorder, other common and interchangeably used nomenclature in the published literature includes excrescences, neoplasms, galls, genetic tumors, leaf lesions, enations, and oedemata (Pinkard et al., 2006).
Intumescences can have a substantial impact on both the economic and aesthetic value of affected plants (Balge et al., 1969; Rangarajan and Tibbitts, 1994). Many of the species most susceptible to this disorder are grown solely for ornamental purposes, and intumescences can greatly impair the overall aesthetics of the crop. Additionally, severe cases of intumescence development can result in impaired photosynthesis (Pinkard et al., 2006; Roloff and Scherm, 2004). Thus, this physiological disorder presents a substantial problem for growers attempting to produce crops that may be prone to its development.
The causative factors related to intumescence development remain somewhat elusive. One key finding has been that this disorder predominantly occurs on plants being produced in controlled environments (Jaworski et al., 1988; Lang and Tibbitts, 1983; Petitte and Ormrod, 1986; Wetzstein and Frett, 1984). For the most part, no pathogen has been found to be involved in intumescence development, which leads most to agree that this is a physiological disorder (Rangarajan and Tibbitts, 1994). Some of the most commonly proposed causative factors include air contamination (Kirkham and Keeney, 1974; Lang and Tibbitts, 1983), hormones and hormone concentrations (Kirkham and Keeney, 1974; Lang and Tibbitts, 1983; Morrow and Tibbitts, 1988; Petitte and Ormrod, 1986; White, 1951), humidity (Douglas, 1907; Eisa and Dobrenz, 1971), temperature (Balge et al., 1969; Eisa and Dobrenz, 1971), and light (Lang and Tibbitts, 1983; Morrow and Tibbitts, 1987; Rud, 2009; Wheeler, 2010). Excess water has also been cited as a potential causative factor and is commonly found when referring to the development of oedema on geranium (Pelargonium sp.; Balge et al., 1969; Metwally et al., 1970, 1971).
Light quality is one of the strongest candidates as a causative factor or, conversely, as a potential preventive measure for intumescence development. Ultraviolet (UV) light, in particular, is thought to be connected to the disorder because many greenhouse-glazing materials block ultraviolet light wavelengths (100 to 400 nm) and intumescences occur in protected culture. Lang and Tibbitts (1983) found that ultraviolet light, specifically UVB, effectively prevented intumescence development on tomato (var. hirsutum and var. esculentum ‘Oxheart’). Similar results have been observed by Rud (2009) on tomato ‘Maxifort’ and by Morrow and Tibbitts (1987) on tomato var. hirsutum. However, the exact mechanism by which ultraviolet light effectively inhibits intumescence development remains uncertain.
Ornamental sweetpotato is an annual ornamental crop commonly produced in greenhouses during the spring season; it is a popular species because of its trailing habit and striking foliage colors with several new cultivar introductions in recent years. However, the species is prone to intumescence development when produced in greenhouses, as indicated in a recent patent application for a new variety (Yencho et al., 2012).
Intumescences on sweetpotato have not been extensively studied, and very few published research articles discuss the development of this disorder on the crop. One paper, “Intumeszenze fogliari di ‘Ipomoea batatas,’” was published by A. Trotter in 1904. This research was cited by Wetzstein and Frett (1984) as they evaluated intumescence anatomy on sweetpotato leaves based on light and electron microscopy. However, very little is known about specific causative factors that may contribute to the occurrence of this disorder on ornamental sweetpotato. Therefore, the goal of this study was to assess the effect of UVB radiation in the prevention of intumescence development on two cultivars of ornamental sweetpotato.
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Rud, N.A. 2009 Environmental factors influencing the physiological disorders of edema on ivy geranium (Pelargonium peltatum) and intumescences on tomato (Solanum lycopersicum). Master’s thesis, Kansas State Univ., Manhattan, KS
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Wheeler, R. 2010 Physiological disorders in closed environment-grown crops for space life support. 38th COSPAR Scientific Assembly
Yencho, G.C., Pecota, K.V. & Reeber, M.K. 2012 Sweetpotato plant named ‘NCORNSP013GNLC’. Patent application number 20120246772, issued 27 Sept. 2012