New perspectives on thousand cankers disease of walnut
M. T. KASSON (1), G. J. Griffin (2), D. M. Geiser (3), E. S. O"Neal (3), J. Juzwik (4), S. Reed (5), N. Tisserat (6), R. M. Turcotte (7), D. K. Martin (7), D. D. Davis (3), K. A. Fenstermacher (3), K. O'Donnell (8). (1) West Virginia University, Morgantown, WV, U.S.A.; (2) Virginia Tech, Blacksburg, VA, U.S.A.; (3) Pennsylvania State University, University Park, PA, U.S.A.; (4) USDA Forest Service, St. Paul, MN, U.S.A.; (5) University of Missouri, Columbia, MO, U.S.A
Thousand Cankers Disease (TCD) is a new disease of black walnut (<i>Juglans nigra</i>) caused by the fungus <i>Geosmithia morbida</i>, a symbiont of the walnut twig beetle (WTB, <i>Pityophthorus juglandis</i>). Members of the <i>Fusarium solani </i>species complex (FSSC) have also been implicated in TCD in Colorado, but little is known regarding their phylogenetic diversity and potential role in TCD. To resolve their diversity, we sampled 127 cankers and 30 beetles from 12 tree species both inside and outside confirmed TCD locations, including 105 isolates from <i>Juglans</i> spp., 75 of which were from confirmed TCD locations. A 3-locus DNA sequence dataset was constructed to assess the phylogenetic diversity of FSSC from various substrates found both inside and outside TCD areas. Multilocus typing identified high genetic diversity spanning 15 FSSC species, eight of which were recovered from >1 tree host. Isolates from five phylogenetic species were exclusive to black walnut including FSSC 9, 15 and 25, together with two putatively novel FSSC species. Overall, 99/105 of the FSSC typed from <i>Juglans</i> sp. represented 36 newly discovered haplotypes that were distributed among 13 FSSC species. Sixteen of the haplotypes were exclusive to areas with TCD, 18 to areas without TCD; only two haplotypes were found in both areas. Although certain FSSC species were abundant across the landscape on multiple hosts, these data suggest that haplotypes associated with TCD may be novel and exclusive to those areas.
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