Since use of methyl bromide for soil fumigation has been limited by the Montreal Protocol, there is an increasing need to find an alternative method to manage soilborne pathogens and pests in open-field vegetable production. One of the proposed alternative methods is the use of grafting rootstock that is resistant to soilborne diseases (Kubota et al., 2008). Although vegetable grafting has been widely used in Asia and Europe to manage soilborne diseases, the use is still limited in the United States (Lee et al., 2010; Louws et al., 2010). One issue limiting the use is the large number of seedlings needed for large-scale open-field production, and another issue is the high cost of grafted seedlings compared with conventional seeds or seedlings. However, vegetable grafting as an alternative to soil fumigation with methyl bromide is now attracting growing interest in the United States (Colla, 2010).
In vegetable grafting, the position of the grafted union must be high enough to prevent the vulnerable scion from coming into direct contact and exposure with the soil, especially when the grafted plants are transplanted by machines in large-scale open-field production. For tomato, while grafting above the rootstock cotyledons could be a solution to assure adequate distance between grafted union and soil line, potential grow-out of axillary shoots from rootstock cotyledons can be problematic because it requires additional labor cost to manually prune the axillary shoots in the field as well as at the propagation stage (Bausher, 2011). Therefore, the use of chemicals to inhibit the cotyledonary axillary shoot growth from rootstock may allow grafting above cotyledons and reduce the additional management cost of grafted tomato production in the field.
Fatty alcohol and fatty acid methyl ester with carbon chain lengths from 8 to 12, emulsified with appropriate surfactants, have been reported to kill the rapidly dividing meristematic tissues such as apical and axillary buds without damaging mature leaf or stem tissues in a variety of plants (Cathey et al., 1966; Steffens et al., 1967; Tso, 1964; Tso et al., 1965). The mechanism of selective killing of the meristematic tissue in tobacco (Nicotiana tabacum) is reportedly due to the property of the well-developed cuticular layer over mature tissue that acts a penetration barrier to these chemicals compared with young tissue (Nelson and Reid, 1971). Once penetrating the meristematic tissue, these chemicals disrupt the plasma membranes and cause desiccation of the tissue (Wheeler et al., 1991). In tobacco production, commercially available fatty alcohols, usually mixtures of octanol (C8 fatty alcohol), decanol (C10 fatty alcohol), and sometimes dodecanol (C12 fatty alcohol) formulated with surfactants, have been used to control axillary shoot (sucker) growth after topping (removal of the flower heads) (Steffens, 1980). Topping and sucker control of tobacco improves yield and the concentration of nicotine of leaf to be harvested, by reducing the competition of nutrients (Moore, 2012). Also a commercial fatty alcohol compound used on tobacco was recently demonstrated to be a part of new grafting method for watermelon (Citrullus lanatus), controlling rootstock shoot regrowth while preparing seedlings (Daley, 2014; Daley and Hassell, 2014). For tomato, Maw (1977) demonstrated effective use of fatty alcohol for controlling axillary shoot on tomato plants grown in greenhouse, although the fatty alcohol had to be applied exclusively to the axillary shoots otherwise it caused chemical damage on leaves and stems. These studies suggest that fatty alcohol could control growth of cotyledonary axillary shoot for tomato rootstocks; however, there is no information on the effective concentration and application methods of fatty alcohol for young tomato or tomato rootstock seedlings of 2 to 3 weeks after germination.
The objective of this preliminary study was to determine the effects of fatty alcohol applied at various concentrations on cotyledonary axils (nonextended buds or extended shoots) and possible chemical damage of tomato seedlings, to evaluate the possibility of using fatty alcohol to control cotyledonary axillary shoot from rootstock in tomato grafting.
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