Worldwide, the United States ranks third in the production of dry bulb onions after China and India, with production at 3.2 million tonnes with a value of $950 million in 2014 (FAO, 2017). The Vidalia onion industry is an important component of Georgia’s agriculture and economy. In 2014, more than 12,000 acres were harvested in Georgia with an estimated farm gate value of $138 million (Wolfe and Stubbs, 2015). In terms of revenue generated among vegetables, onions ranked first in Georgia with 13.6% of total vegetable farm gate value in the state (Wolfe and Stubbs, 2015). Vidalia onions are short-day onions that bulb when day lengths reach 11–12 h during late winter and early spring. Short-day onions are also noted for having high water content and low solids, including sulfur, that make them both mild flavored and more susceptible to diseases (Boyhan and Torrance, 2002; Maw et al., 1997b).
Several fungal and bacterial pathogens can attack Vidalia onions and can cause such diseases as BNR, purple blotch (Alternaria porri), pink root (Phoma terrestris), and stemphylium leaf blight (Stemphylium vesicarium) all of which can cause severe damage to the crop. Of all of the diseases, BNR is the most important in postharvest-stored onions. In some years, 70% of the total stored crop can be damaged by the pathogen that causes BNR (Sanders et al., 2008). Under favorable conditions, this pathogen invades the dead or dying tissue of the onion bulb, and then it grows downward through the neck into the bulbs (Pappelis et al., 1974).
Postharvest diseases may start before or after harvest. A number of postharvest diseases such as gray mold (Botrytis cinerea) of grape (Vitus sp.), brown rot (Monilinia fructicola) of peach (Prunus persica), and yeasty rot (Geotrichum candidum) of tomato (Solanum lycopersicum) infect the crop in the field shortly before harvest. However, symptoms of these diseases might not be visible in the field, but symptoms may become visible when the pathogen grows under storage conditions. Pre- and postharvest applications of labeled fungicides are used to control these diseases (Coates and Johnson, 1997). Postharvest application of fungicides can inhibit fungal activity of a number of microorganisms, which results in improved shelf life of amla or indian gooseberry [Phyllanthus emblica (Ram et al., 2011)]. Presently, few postharvest “reduced risk” fungicides, as designated by the U.S. Environmental Protection Agency (EPA), have been registered for horticultural crops in the United States (Schirra et al., 2011).
Fludioxonil (Syngenta, Greensboro, NC) is one of the fungicides registered for postharvest treatment of various diseases and is marketed as Scholar. The active ingredient belongs to the phenylpyrrole family. Fludioxonil is classified by the EPA as a “reduced risk” fungicide (Fenoll et al., 2009). According to the Fungicide Resistance Action Committee (FRAC) classification based on the mode of action, it is classified as a group 12 fungicide, which are involved in disrupting signal transduction (Rosenberger, 2009). It has a wide spectrum of activity and is registered for postharvest use on stone fruit (Prunus sp.), apple (Malus domestica), pear (Pyrus communis), kiwifruit (Actinidia deliciosa), yam (Diosorea sp.), and others (Tedford, 2004). It is not currently labeled for use on onion. It is a nonsystemic fungicide, which inhibits the cell growth of fungi by inhibiting osmotic signal transduction (Fenoll et al., 2009). Because of its long residual and ability to inhibit sporulation of targeted pathogens, it helps to enable fruit storage for longer periods without spreading diseases in the packinghouse or during shipment. Its unique mode of action makes it effective in disease control even against the fungal isolates that are resistant to other fungicides (Tedford, 2004).
Fluopyram/pyrimethanil (Bayer CropScience, Research Triangle Park, NC) is a broad-spectrum fungicide combination that is sold as Luna Tranquility (Bayer CropScience, 2016). It is classified as a group 7/9 fungicide according to the FRAC codes. Fluopyram/pyrimethanil is a succinate dehydrogenase inhibitor that is labeled for preharvest use on onions. It should not be applied within 7 d of harvest for onions.
Boscalid/pyraclostrobin (BASF, Research Triangle Park, NC) is a preharvest fungicide marketed in a formulation as Pristine and used to control different fungal diseases in various crops, including onions (BASF Corporation, 2017). Boscalid is a new broad-spectrum fungicide belonging to the carboxamide class and pyraclostrobin belongs to the quinine outside inhibitor class (Xiao and Boal, 2009). These fungicides have different modes of action, but they both inhibit respiration (Kim and Xiao, 2010). Boscalid/pyraclostrobin should not be applied within 7 d of harvest.
Copper-based materials have broad effectiveness against a number of pathogens. Copper hydroxide (Griffin, Valdosta, GA) is a chemical compound marketed as Kocide, which acts both as a fungicide and bactericide, and is labeled for use on onions. It is used as a preharvest fungicide to prevent diseases in a number of fruits and vegetables. Copper hydroxide converts into ionic copper as the active ingredient and can be used up until the day of harvest (DuPont, 2009). The mode of action of copper-based compounds is an M according to the FRAC classification with multisite contact activity (Horton, 2016). They can denature proteins and enzymes (Babadoost, 2012). They prevent diseases as a nonsystemic protectant (Petit et al., 2012). Copper sulfate pentahydrate (Quadrual, Ridgefield, CT) is another copper-based compound marketed as ClearBlue 104, which is used to suppress microorganisms. It is also effective against algae, fungi, and bacteria (Quadrual, 2011).
Another important postharvest treatment required for long-term storage of onion bulbs is curing (Maw et al., 1997a). Curing is a drying process intended to dry down the necks (Bayat et al., 2010) and outer scales of the bulbs (Maw et al., 2004) to prevent the loss of moisture and attack by pathogenic organisms during storage. From harvest to storage, curing can occur at any stage, whenever the conditions around the bulb become favorable to remove moisture from the bulb (Maw et al., 2004).
There are two ways of curing onion bulbs: artificial and natural. Natural curing takes place under the sun and wind after the harvest when bulbs are left in the field. It is the least expensive way of curing and can be helpful in enhancing onion quality by allowing downward movement of nutrients from tops into the bulb (Maw et al., 1997a). Drying onions by forcing heated air around the bulbs is another way of curing. Standard conditions for this type of curing are blowing the dry, hot (up to 100 °F) air around the onions with an airflow of 365–1030 ft3/min per ton of onions (Boyhan et al., 2001). The duration of heat curing varies according to the harvest maturity of the bulbs. For early harvested onions, longer curing is required than for those harvested at the optimal time. Early harvested onions benefit from heat curing of 72 h, whereas optimally mature onions require only 48 h (Maw et al., 1997b).
The objectives of this study were to evaluate postharvest treatments, time in storage, and poststorage shelf life on marketability of Vidalia onions.
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