Washington State is the largest producer of apple in the United States accounting for ≈60% of total production as of 2013 (NASS, 2012, 2015). Demand for Washington apples in the international market is high, and ≈30% of the total crop is exported (Globalwise Inc., 2014). Because of the availability of advanced fruit storage facilities in Washington State, the state’s apple crop can be stored in cold rooms at 0 °C and controlled atmospheric (CA) condition for 9–12 months postharvest (Kupferman, 2003). However, fungal pathogens causing postharvest decay of apple can lead to a significant loss of fruit in storage resulting in economic losses (Kim and Xiao, 2006, 2008).
Phacidiopycnis washingtonensis and S. pyriputrescens causing speck rot and Sphaeropsis rot of apple, respectively, are two postharvest fungal pathogens recently determined to be of concern to the Washington apple industry (Kim and Xiao, 2006; Kim et al., 2013, 2014; Sikdar et al., 2014; Xiao et al., 2004, 2005, 2009). These pathogens incite latent infection of fruit in the orchard, and symptoms become apparent after 2–3 months of fruit storage in CA conditions (Kupferman, 2003) at −1 to 4 °C, decreased oxygen (1% to 2%), and increased carbon dioxide (0.5% to 1%). The total incidence of decay due to these two pathogens is ≈4% to 5% annually (Kim and Xiao, 2008). These pathogens have been reported in the United States, Germany (Weber, 2011) and, very recently, Chile (Diaz et al., 2016). Several countries, including China and Australia, have raised quarantine concerns regarding these fungal species. Because of the interception of infected fruit in shipments bound for China, the General Administration of Quality Supervision, Inspection, and Quarantine of People’s Republic of China (AQSIQ) imposed a complete ban on import of U.S. apples between 2012 and 2014 (Sikdar and Mazzola, 2015). The current work was undertaken as a result of an agreement between the USDA’s Animal and Plant Health Inspection Service and AQSIQ to conduct “proof-of-concept” research in support of the systems approach which allowed the Chinese market to reopen to Washington apples.
Phacidiopycnis washingtonensis and S. pyriputrescens both infect manchurian crabapple fruit [Malus mandshurica (Maxim.) Kom. ex Juz.] and branches (Sikdar et al., 2013, 2014; Xiao and Boal, 2005; Xiao et al., 2009, 2014). Infected crabapple fruit often decay in the trees, turning into dark black fruit mummies. These fungi are also capable of causing cankers and dieback of crabapple twigs and branches. Pycnidia are produced on fruit mummies and the margins of cankers, thus serving as a potential source of inoculum in the orchard. Historically, Washington State has used manchurian crabapple as a pollinizer in apple orchards because of its timing of flowering and fruit set (Church and Williams, 1983; Crassweller et al., 1980; Williams and Church, 1983). Manchurian crabapple produces large and sharp spurs on its branches which can cause damage to farm equipment if pruned branches and twigs are left on the orchard floor. Hence, pruning of manchurian crabapple had been avoided to reduce potential economic losses caused by damage to farm machinery and potential injury to laborers. Because of the absence of pruning, crabapple pollinizers commonly occur in the orchard as bushy trees possessing pendant limbs often bearing dead and dying wood, diseased twigs, and fruit mummies, thereby providing a ready and abundant source of inoculum for fruit infection.
The objective of this research was to determine whether pruning of manchurian crabapple, removal of fruit mummies from the trees, and postharvest drench application of fungicide could effectively control the postharvest incidence of speck rot and Sphaeropsis rot over the course of the fruit storage season.
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