Pinus strobus L. is an iconic forest species native to eastern North America. Managing for high-quality P. strobus timber is complicated by Pissodes strobi Peck, which colonizes and kills terminal leaders, causing bole defects. Pinus strobus is also impacted by Caliciopsis canker. This emerging disease, caused by the native fungal pathogen Caliciopsis pinea Peck, causes shallow bole and branch cankers, dieback, and sapling mortality. Caliciopsis canker disease was detected in Michigan in 2016, but the distribution and severity of the disease in the state is not well known. Management strategies for P. strobi often reduce P. strobus growth. To evaluate trade-offs between growth and P. strobi caused defects, long-term P. strobus stands were established under four different regeneration tactics: monoculture, shelterwood, even-aged mixed-species stand with varying density, and even-aged mixed-species stand with varying hardwood competition (Chapter 1). Trees in these stands were assessed 17 to 19 years after they were planted. Mean annual growth rate of P. strobus was eight times greater in the monoculture than in the shelterwood, however incident rate of bole defects was 64% in the monoculture and 6% in the shelterwood. The even-aged mixed-species stands yielded markedly different results with P. strobus reaching the overstory at the variable density stand but suffering up to 90% mortality at the varying hardwood competition stand. A survey of 66 P. strobus stands across northern Michigan found defects in P. strobus boles in 85% of stands and 11% of surveyed P. strobus (Chapter 2). Pinus strobus crown class and size class were predictive of bole defects but stand density factors (i.e., basal area, trees per hectare, stems per hectare) did not affect the likelihood of defects. Caliciopsis canker disease was widespread throughout the surveyed area. Signs of Caliciopsis canker disease were found in 47% of stands and 7% of surveyed P. strobus. The disease was most frequently found on P. strobus saplings, suggesting Caliciopsis canker has greater impact on regeneration. Lower P. strobus basal area was associated with a higher likelihood of Caliciopsis canker signs and symptoms, likely because lower basal area was more conducive to regeneration. During the stand surveys, bark samples with Caliciopsis spp. ascocarps were collected from P. strobus stems and branches (Chapter 3). From these, 37 isolates of Caliciopsis spp. were isolated, 35 of which were identified as C. pinea by sequencing the ITSrDNA gene region. The other two isolates were a potentially undescribed species of Caliciopsis, C. sp. 1. Pathogenicity tests were conducted on P. strobus excised branches and live seedlings. On excised branches, both species produced cankers larger in area and deeper than the control. In live seedlings, C. pinea produced cankers larger in area than the control while C. sp. 1 produced deeper but not larger cankers than the control. Caliciopsis species were re-isolated from canker margins following both pathogenicity trials. This study confirmed C. pinea as a pathogen of P. strobus and found that it was more virulent than C. sp. 1. In live seedlings, C. sp. 1. was more effective in colonizing down into the sapwood rather than the living cambium suggesting it is a weak pathogen or saprophyte, rather than a primary disease agent. These studies provide new insight into two native damage agents of P. strobus. Quantitative data on trade-offs between growth, competition, and P. strobi damage could help managers weigh different silviculture options (Chapter 1). Surveys found both damage agents are widespread throughout northern Michigan and provide a baseline for future efforts to track damage over time (Chapter 2). Pathogenicity tests showed Caliciopsis pinea is the primary cause of Caliciopsis canker disease, but at least one other species may contribute to the pathogen load (Chapter 3). Future studies are needed to elucidate the epidemiology of this emerging disease and to better inform management decisions.
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