Resistance to Gremmeniella abietina (European race, large tree type) in introduced Pinus contorta and native Pinus sylvestris in Sweden

To study the resistance of Pinus contorta Dougl. ex Loud. var. latifolia Engelm. ex Watts to large tree type (LTT) Gremmeniella abietina (Lagerberg) Morelet of the European race, seedlings of P. contorta and Pinus sylvestris L. were planted in gaps of an infected 40-year-old P. sylvestris stand in 2005 and assessed for symptoms in 2006-2007. In 2007, 45% and 32% of the P. contorta and P. sylvestris seedlings were infected, respectively. However, mortality was lower and recovery was better in the P. contorta seedlings. The mean infection length on surviving seedlings was significantly shorter in P. contorta ( 4 cm) compared with P. sylvestris ( 10 cm), and 47% of the P. contorta seedlings had developed new leader shoots in 2007 compared with 19% of the P. sylvestris seedlings. Histopathological examinations of the transition zone of infected shoots showed that P. contorta, as well as P. sylvestris, produced ligno-suberized boundaries extending from healthy needle bases transversally across the shoots. Together with areas of phenol-filled cells, these boundaries completed the compartmentalization of the invaded outermost parts of a majority of the infected seedlings. Our results indicate that P. contorta is more resistant to LTT G. abietina than P. sylvestris.