Anthracnose canker (Figure 1), caused by the fungal pathogen Neofabraea malicorticis (synonym Cryptosporiopsis curvispora), and potentially Phlyctema vagabunda (synonym Neofabraea alba), is a major disease limiting apple production in western Washington, western British Columbia, and the Columbia Gorge (Spotts et al. 2009; Zang et al. 2011). The relatively mild temperatures combined with high humidity and frequent rains that occur during the autumn, winter, and spring in this region promote infection and disease development. Anthracnose canker is rare, or absent, on apple trees in the dry interior areas of the Pacific Northwest. Spores of the fungus infect healthy bark tissue, and the pathogen grows in the cambium beneath the bark for a period of time before killing the bark to form a visible canker. In the absence of effective management, the disease can readily spread in a short period of time, killing young trees, and structurally weakening established trees. Spores produced on the dead canker bark can cause additional cankers in infected trees, as well as surrounding trees, and also cause a postharvest fruit rot (known as bull’s-eye rot). The key to effectively managing anthracnose canker is to inspect apple trees regularly and apply treatments within the appropriate timeframe.
Pathogen and Disease Cycle
In the maritime Pacific Northwest, stem and trunk infections appear to occur primarily in the autumn, but infection can occur throughout the winter and early spring when weather is mild and moist. Initial disease symptoms include the appearance of a reddish-purple lesion on the tree bark (Figure 2A and 2B) (Davidson and Byther 1992).
Cankers can attain full size in one year, and can range from
1 to 10 inches long. By mid-summer to late autumn, acervuli (asexual fruiting bodies) are formed on mature cankers, producing conidia (asexual spores) that are held together in a water-soluble matrix. Rain dissolves the matrix, and the spores 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). The acervuli first appear as cream-colored pustules on the center of the canker surface and later on the canker margin, and as acervuli age they become dark in color. Germination of conidia can occur under high humidity and temperatures between 30°F and 84°F (Cordley 1900; Spotts and Peters 1982). On cankers that are allowed to overwinter, the pathogen may produce apothecia (sexual fruiting bodies) in the old acervuli and release ascospores (sexual spores) into the air that can be carried over substantial distances.
Ascospore production begins by the end of March or early April and can continue throughout the summer and into the autumn when there is high humidity and temperatures between 40°F and 55°F (Jurkemikova and Rahe 1998). While, the capacity of ascopores to incite infection is uncertain, it is presumed that airborne ascospores are the cause of initial infection (Rahe 1997). The pathogen can survive as mycelium in cankered limbs or in fruit left lying under the tree, and can produce spores that incite new infection during cool, moist weather.
Eradication of anthracnose canker is not guaranteed once the infection has become established. Managing the disease requires an integrated plan that includes sanitation, removing cankers during dry weather year-round, and applying fungicides to limit or prevent infection during the dormant and growing seasons.