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Impact of white pine blister rust on resistant and previously immune cultivated Ribes and neighboring Eastern white pine in New Hampshire
I. A. MUNCK (1), P. Tanguay (2), K. Lombard (3), J. Weimer (3), S. Villani (4), K. Cox (4). (1) USDA Forest Service, Durham, NH, U.S.A.; (2) Canadian Forest Service, Québec, QC, Canada; (3) New Hampshire Division of Forests and Lands, Concord, NH, U.S.A.; (4) Cornell University, Geneva, NY, U.S.A.

White pine blister rust (WPBR) has been a threat to both forest resources and agricultural commodities since its introduction to North America in the early 1900s. The objective of this study was to determine the impact of WPBR following the recent breakdown in immunity. During 2013, 255 plants of 19 <i>Ribes</i> cultivars and 445 neighboring white pines (<i>Pinus strobus</i>) in 43 sites were evaluated. Disease severity defined as percentage of leaf area affected was assessed for 18 leaves per <i>Ribes</i> plant. Infected leaves of each cultivar were collected for PCR analyses. Immune <i>Ribes</i> from Canadian Clonal Genebank were inoculated with samples of immune black currants from NH. At each site, WPBR incidence for the 12 nearest white pines within 300 m of cultivated <i>Ribes</i> was recorded. Incidence of WPBR ranged from 0% to 60% and 0 to 100% for immune and resistant <i>Ribes</i> cultivars, respectively. Mean WPBR severity on resistant <i>Ribes</i> was limited to <6% of leaf area compared to 14% for previously immune <i>Ribes</i>. All infected <i>Ribes</i> cultivars were PCR positive for <i>Cronartium ribicola</i>. Immune <i>Ribes</i> from the Canadian Clonal Genebank were successfully infected with samples from NH. It was more likely to find infected pines neighboring infected immune black currants (probability=0.18) than neighboring infected resistant <i>Ribes</i> (0.09), or WPBR-free <i>Ribes</i> (0.02). The breakdown in of WPBR immunity poses a threat to the white pine resource and to cultivated <i>Ribes</i> production.

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