Abstract
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.
Highlights
Juglans nigra L. (Juglandaceae), commonly referred to as black walnut or eastern black walnut, is a native tree species of eastern North America (Figure 1A)
The six pairs of primers designed based on the G. morbida genome produced a single amplicon (Table 2), and one pair corresponded to a polymorphic region (MAP) when the sequences of eight experimental isolates were compared (Table 1, isolates with two asterisks)
This study helped to explain the genetic structure of G. morbida and its multi-factorial and disjunct distribution in the United States (USA)
Summary
Juglans nigra L. (Juglandaceae), commonly referred to as black walnut or eastern black walnut, is a native tree species of eastern North America (Figure 1A). Its wood is highly prized for use in cabinetry, gunstocks, veneer, and other finished wood products, and the nuts are an important nutritional source for wildlife [1,2,3] This species was widely planted in the western United States (USA) as an ornamental and nut-bearing tree [1,2] during European colonization and the subsequent development of rural and urban landscapes. The WTB and TCD have been documented as causing J. nigra mortality throughout much of the western USA and more recently, in many locations within the native range of this species [4,11,12,13,14,15,16,17,18]. The anthropogenic movement of wood containing many haplotypes would explain the complex mixture of G. morbida haplotypes observed at most locations
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