Population structure of Geosmithia morbida, the causal agent of thousand cankers disease of walnut trees in the United States.
The ascomycete Geosmithia morbida and the walnut twig beetle Pityophthorus juglandis are associated with thousand cankers disease of Juglans (walnut) and Pterocarya (wingnut). The disease was first reported in the western United States (USA) on several Juglans species, but has been found more recently in the eastern USA in the native range of the highly susceptible Juglans nigra. We performed a comprehensive population genetic study of 209 G. morbida isolates collected from Juglans and Pterocarya from 17 geographic regions distributed across 12 U.S. states. The study was based on sequence typing of 27 single nucleotide polymorphisms from three genomic regions and genotyping with ten microsatellite primer pairs. Using multilocus sequence-typing data, 197 G. morbida isolates were placed into one of 57 haplotypes. In some instances, multiple haplotypes were recovered from isolates collected on the same tree. Twenty-four of the haplotypes (42%) were recovered from more than one isolate; the two most frequently occurring haplotypes (H02 and H03) represented 36% of all isolates. These two haplotypes were abundant in California, but were not recovered from Arizona or New Mexico. G. morbida population structure was best explained by four genetically distinct groups that clustered into three geographic regions. Most of the haplotypes isolated from the native range of J. major (Arizona and New Mexico) were found in those states only or present in distinct genetic clusters. There was no evidence of sexual reproduction or genetic recombination in any population. The scattered distribution of the genetic clusters indicated that G. morbida was likely disseminated to different regions at several times and from several sources. The large number of haplotypes observed and the genetic complexity of G. morbida indicate that it evolved in association with at least one Juglans spp. and the walnut twig beetle long before the first reports of the disease.
Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, Colorado, United States of America.
Marcelo M
MM
Zerillo
Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, Colorado, United States of America.
Jorge
J
Ibarra Caballero
USDA Forest Service Hardwood Tree Improvement and Regeneration Center, Department of Forestry and Natural Resources, Purdue University, West Lafayette, Indiana, United States of America.
Keith
K
Woeste
USDA Forest Service, Forest Health Protection, Albuquerque, New Mexico, United States of America.
Andrew D
AD
Graves
Department of Forestry and Natural Resources, Purdue University, West Lafayette, Indiana, United States of America.
Colleen
C
Hartel
Department of Botany and Plant Pathology, Oregon State University, Corvallis, Oregon, United States of America.
Jay W
JW
Pscheidt
Department of Forestry and Natural Resources, Purdue University, West Lafayette, Indiana, United States of America.
Jadelys
J
Tonos
Department of Biological Sciences, University of New Hampshire, Durham, New Hampshire, United States of America.
Kirk
K
Broders
Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, Colorado, United States of America.
Whitney
W
Cranshaw
USDA Forest Service, Pacific Southwest Research Station, Davis, California, United States of America.
Steven J
SJ
Seybold
Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, Colorado, United States of America.
Ned
N
Tisserat
D000818:Animals; D006239:Haplotypes; D031324:Juglans / Q000382:microbiology*; D010935:Plant Diseases / Q000382:microbiology*; D020641:Polymorphism, Single Nucleotide; D020037:Sordariales / Q000235:genetics*; D014481:United States