Diplodia Shoot Blight Damage to Understory Red Pine Seedlings

The fungal pathogen Diplodia pinea has caused substantial damage to red pine (Pinus resinosa Ait.) seedlings in nurseries, but the impact of Diplodia shoot blight on naturally occurring seedlings has not been thoroughly investigated. In 2008, incidence and severity of Diplodia shoot blight symptoms on living red pine seedlings and shoot blight-associated mortality of seedlings were quantified in understories of mature plantations at four sites in Wisconsin. Blighted shoots were examined for characteristic fruiting structures of the pathogen and a species-specific polymerase chain reaction (PCR) assay confirmed D. pinea as present on these shoots. Mean seedling mortality ranged from 13 to 31% and shoot blight incidence on living seedlings was ≥ 94% at each site. Mean damage severity ratings on a scale from 0 to 3 ranged from 2.16 to 2.98, with evidence of previous blighting of leaders on ≥ 54% of seedlings. These results document Diplodia shoot blight damage to naturally occurring understory trees and the potential for this disease to impede efforts to naturally regenerate red pine stands.