Abstract
A comparison of Nectria canker (caused by Neonectria ditissima) incidence was made between contemporary stands and data sets collected in the 1930s. Cankers were noted on 2,520 black and 1,704 yellow birch growing in 31 study areas used during seven separate surveys in Connecticut between 1995 and 2005. Canker incidence was compared with a survey of 6,938 black and 4,117 yellow birch completed during 1934-1935. Canker incidence in the mid-1930s was much greater than recent incidence for most diameter, crown, and age classes. Birch that grew in open conditions had higher canker incidence than understory birch. In unmanaged mature stands, canker incidence in 2000 was higher for birch older than 90 years (24%) than for birch 20-29 years old (1%) and 30-49 years old (2%). Because the younger birch are probably progeny of intermingled older birch with cankers, is unlikely that the lower incidence of cankers observed on the younger birch was related to local soil conditions, a genetically resistant population, or lack of inoculum. Incidence in more recent surveys was much higher for 20-29-year-old (12%) and 30-49-year-old (32%) birch in clearcuts <50 years old than for birch of similar ages in unmanaged forests. Canker incidence of birch in clearcuts was similar to older birch in mature stands and birch during the mid-1930s (13%) that became established following clearcutting or farm abandonment. Canker incidence in birch might be reduced by regeneration prescriptions that minimize exposure of birch saplings to “open-field” environmental conditions during early stand development and by removing infected stems during the early thinning operations.
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