Citrus black spot is a fungal disease caused by G. citricarpa Kiely [anamorph Phyllosticta citricarpa (McAlpine) van der Aa]. This disease was first described in Australia in the 1890s and has since been found in the humid subtropical regions of Sub-Saharan Africa, Asia, and South America (Paul et al., 2005). Europe has classified G. citricarpa as an A1 quarantine disease (absent from the region) and fruit with CBS are not allowed into the European market (Paul et al., 2005). Almost all commercial citrus cultivars are susceptible to CBS, with sweet oranges, especially mid- to late-harvested cultivars such as Valencia, being the most susceptible. Although CBS does not affect the internal fruit quality, external peel blemishes can render the fruit unmarketable and cause extensive premature fruit drop (Hincapie et al., 2014).
In North America, CBS was first discovered on ‘Valencia’ oranges in Southwest Florida in Mar. 2010 (Schubert et al., 2012). As of this writing, the disease has been found in three counties: Collier, Hendry, and Polk. The infected groves in these counties have been designated restricted quarantine areas (USDA-APHIS, 2012). However, for the Polk County discovery, no subsequent CBS detections have occurred since the initial find in Nov. 2012. The USDA’s pest risk assessment concluded that infected fruit are not a likely vehicle to spread CBS and infected fruit can be shipped to all U.S. states provided grade standards are met and the fruit have been treated according to specific protocols, including washing, brushing, surface disinfesting, imazalil, and/or thiabendazole application and waxing (USDA-APHIS, 2010, 2012). However, previous reports indicated that postharvest thiabendazole or imazalil treatment had no significant inhibition on CBS development (Agostini et al., 2006; Lucon et al., 2010). Infected fruit may develop lesions after packing and shipping that could eventually exceed USDA grade standards, causing economic losses from rejection at destination markets (Canale et al., 2011).
Hot water treatments have been widely evaluated and used to control postharvest decay, reduce physiological disorders, and improve storage quality of a variety of horticultural products (Fallik, 2004). It is an environmentally friendly procedure with increasing acceptability in commercial packinghouses. Ritenour et al. (2003) found that dipping ‘Ruby Red’ and ‘Marsh’ grapefruit from Florida in 59 °C water for 10 s resulted in an approximate 90% reduction in stem-end rot (Lasiodiplodia theobromae) incidence. Porat et al. (2000a, 2000b) found that a short-duration, hot water brushing (56 °C for 20 s) not only reduced surface microorganism and natural decay on citrus fruit, but also induced defensive mechanisms in fruit against decay organisms. Pavoncello et al. (2001) reported that hot water treatment of ‘Star Ruby’ at 62 °C for 20 s increased resistance against green mold (Penicillium digitatum) as well as induced the accumulation of heat-shock proteins, chitinases, and β-1,3-glucanase in grapefruit peel tissue. Fallik (2004) concluded that the reduction in disease development on fruit treated with hot water was mainly due to the induction of plant disease-resistance compounds as well as the reduction in microorganism population on the fruit surface.
In addition, heated solutions have been reported to enhance fungicide effectiveness for postharvest decay control, allowing lower fungicide concentrations to be used on fruit. For example, D’Aquino et al. (2006) showed that dipping citrus fruit in heated (50 °C) imazalil or pyrimethanil solution required 8-or 16-fold lower fungicide concentration, respectively, than treatments at 20 °C for control of P. digitatum or Penicillium italicum. There is currently no report of hot water treatment or heated fungicide treatment to delay CBS lesion development on citrus fruit after harvest.
The objective of this study was to investigate whether hot water or heated fungicide treatments can reduce CBS lesion development on ‘Valencia’ orange fruit. The effects of heat treatments on mycelial growth of G. citricarpa in vitro and the potential changes in fruit quality caused by hot water treatments were also investigated.
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