Anthracnose canker, caused by Neofabraea malicorticis, induces tree cankers that can kill newly planted cider apple trees west of the Cascade Mountains in the U.S. Pacific Northwest (PNW) (Garton et al., 2018a; Rahe, 1997a). N. malicorticis can infect intact bark tissue, and most infections occur through the lenticels (Cordley, 1900; Kienholz, 1939). Acervuli (asexual fruiting bodies) form on mature cankers in midsummer to late-autumn, producing conidia (asexual spores) that cause new N. malicorticis infections (Creemers, 2014) in autumn and winter during mild, moist weather (Davidson and Byther, 1992; Rahe, 2010). Apothecia (sexual fruiting bodies) may develop on overwintering cankers, and ascospores (sexual spores) may cause new N. malicorticis infections in the spring (Powell et al., 1970; Rahe, 1997a). Canker progression is slow or ceases in the winter, is more rapid in the spring, and by early summer, cankers are fully developed (Creemers, 2014; Davidson and Byther, 1992). Although N. malicorticis is the primary pathogen that induces anthracnose canker, additional species, including Phlyctema vagabunda (synonym Neofabraea alba) and Neofabraea kienholzii have also been implicated as the cause of this disease (Zang et al., 2011).
Populations of N. malicorticis have been reported throughout North America, Africa (Zimbabwe), Oceania (Australia and New Zealand), and Europe (European and Mediterranean Plant Protection Organization, 2017), with disease severity observed to be highest in the maritime PNW. Environmental conditions of the maritime PNW favor growth and spore dissemination of N. malicorticis year-round (Hortová et al., 2014; Kienholz, 1939; Miller, 1932; Senula, 1985; Spotts and Peters, 1982). The temperature in northwest Washington, for example, is 14 °C on average during the growing season (April–October) and 6 °C during the dormant season (November–March) (Washington State University, 2017). Relative humidity is 73% to 82% on average during the growing season and 82% to 91% during the dormant season, while 76 mm precipitation occurs on average during the growing season and 101 mm occurs during the dormant season. Solar radiation in the area also tends to be moderate, with 506 MJ·m−2 during the growing season on average, and 157 MJ·m−2 during the dormant season.
Cider apple growers in western Washington use several measures to control anthracnose canker, including excising cankers from infected wood and applying fungicides before autumn rains (Pscheidt and Ocamb, 2017). Zinc [Fungicide Resistance Action Committee (FRAC) code M03], captan (FRAC code M04), and copper-based products (FRAC code M01) are the common fungicides applied in PNW cider apple orchards. However, Garton et al. (2018b) found new N. malicorticis infections of susceptible hosts to occur after applying many of these common fungicide treatments on cider apple trees in northwest Washington. Further, in commercial orchards, these fungicide treatments have not provided significant disease control, as orchards continue to be abandoned or torn out within 4 years of symptom appearance (Garton et al., 2018a; Rahe, 1997a).
Research on fungicide efficacy for control of anthracnose canker on apple trees in general is limited and contradictory. In an orchard study in western Washington, Byther (1986) found a 50% reduction in the number of new canker infections to develop on trees when zinc and basic copper sulfate were applied in mid-October and again in mid-February. In an in vitro study in British Columbia, Canada, Rahe (1997a) found captan to be moderately toxic to N. malicorticis, whereas copper-based fungicides were nontoxic to the fungus. Rahe (1997b) also investigated efficacy of thiophanate-methyl (FRAC code 1) and thiram (FRAC code M3) for control of anthracnose canker on trees in British Columbia and found the fungicides to be ineffective when applied biweekly from mid-August through late October. Research in central Washington has tested fungicide efficacy to control four Neofabraea species (N. alba, N. malicorticis, N. perennans, and N. kienholzii) that cause bull’s-eye rot on pome fruit (Rosaceae), and found thiabendazole (FRAC code 1), thiophanate-methyl, pyrimethanil (FRAC code 9), and pyraclostrobin plus boscalid (FRAC codes 11 and 7, respectively) to control all Neofabraea species, whereas zinc did not provide effective control of N. malicorticis but copper sulfate did (Spotts et al., 2009). Creemers (2014) recommended the use of chemistries such as quinone outside inhibitors (FRAC code 11) for controlling anthracnose canker, but Aguilar et al. (2018) reported such chemistries as ineffective in controlling diseases incited by Neofabraea species. The objective of this study was to better understand the efficacy of current fungicides recommended for control of anthracnose canker, specifically, zinc, basic copper sulfate, captan, thiophanate-methyl, and pyraclostrobin plus boscalid, on cider apple trees in the maritime PNW.
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