In recent years there has been a rapid expansion of cider apple production in Washington state, with many new plantings occurring west of the Cascades where there is an average reported yield of 36,000 lb/acre and crop value of $12,960/acre, assuming a fruit price of $0.36/lb (Galinato et al., 2014; Northwest Cider Association, 2016). A major obstacle to long-term orchard productivity and continued expansion in western Washington is the widespread occurrence of anthracnose canker on apple, caused by Neofabraea malicorticis (Rahe, 2010; Zang et al., 2011). In the absence of effective management of N. malicorticis, anthracnose canker can spread within an entire western Washington orchard in 1 or 2 years, and within 3 or 4 years can kill newly planted trees and structurally weaken established trees (Davidson and Byther, 1992; Garton et al., 2018; Rahe, 2010). Information on cultivar susceptibility and host resistance to anthracnose canker is limited and contradictory (Borecki and Czynczyk, 1985; Braun, 1997), and currently all cider apple cultivars are considered susceptible (British Columbia Ministry of Agriculture, 2016; Pscheidt and Ocamb, 2017).
Neofabraea malicorticis can directly infect intact bark tissue, with most infections occurring through the lenticels (Kienholz, 1939). Stem and trunk infections appear to occur primarily in the autumn but can take place throughout the winter and early spring during mild, moist conditions (Davidson and Byther, 1992; Rahe, 2010). Cankers develop during the autumn and to a lesser extent in the winter. In the following spring, cankers resume development, reaching full size in the summer, and pycnidia that form on the canker margin are the source of inoculum for new infections (Barss, 1925; Creemers, 2014; Rahe, 1997). In midsummer to late autumn, conidia (asexual) are disseminated by rain and wind to other parts of the tree as well as to surrounding trees and fruit, causing new infections (Creemers, 2014). Cankers that overwinter may produce ascospores (sexual) in the spring, which can be carried over substantial distances and may also incite new infections (Powell et al., 1970; Rahe, 1997).
Current recommendations for managing anthracnose canker include 1) excising cankers, and 2) applying fungicides to minimize the occurrence of new infections (Pscheidt and Ocamb, 2017). The reported efficacy of canker excision in managing disease incidence and severity (appearance of new cankers, and expansion of existing cankers) on trees is limited and variable. Byther (1986) reported a 45% reduction in the occurrence of new cankers 1 year after canker excision (CE) treatment, while Rahe (2010) observed no reduction in the number of new cankers. The reported efficacy of applying cauterization (CAU) and/or fungicidal treatments to cankers or excised areas is also quite variable. Studies of pathogens that incite similar canker diseases on fruit trees, such as bacterial canker (Pseudomonas syringae), phytophthora canker (Phytophthora citricola), and botryosphaeria canker (Botryosphaeria sp.), have demonstrated that CAU or chemical treatments applied directly to cankers or to the excised area following CE can reduce or prevent infections (Brown-Rytlewski and McManus, 2000; El-Hamalawi and Menge, 1994; Fawcett, 1915; Hawkins, 1976). Studies on anthracnose canker specifically have reported that the effect of CAU treatments in limiting disease progression does not last more than 1 year after application (Byther, 1986; Rahe, 2010). Chemical treatments to control Neofabraea species are predominantly copper-based fungicides (Barss and Mote, 1931; Childs, 1927; Henriquez et al., 2006; Spotts et al., 2009). Byther (1986) found that basic copper sulfate reduced the number of new cankers on trees by 50% when an application was made once in mid-October followed by a second dormant application in mid-February. In contrast, an in vitro study found that copper-based fungicides were ineffective to control N. malicorticis (Rahe, 1997). Early 20th century recommendations for managing anthracnose canker included applying Bordeaux mixture (BM) (basic copper sulfate and calcium hydroxide) after CE (Cordley, 1900; Zeller, 1926). There are no published data on the efficacy of CE plus BM or other copper-based products to control anthracnose canker.
Anthracnose canker is an economic threat to cider apple production in western Washington and growers are in need of an effective management plan. Cider apple growers in the region are currently using several physical and fungicidal treatments alone and in combination in an attempt to control anthracnose canker during the winter, although there is no evidence of treatment efficacy. The objective of this study was to assess the efficacy of various treatments on existing cankers on cider apple trees in western Washington, and to assist growers to eliminate the application of ineffective and costly treatments.
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