Genetic differentiation and spatial structure of Geosmithia morbida, the causal agent of thousand cankers disease in black walnut (Juglans nigra)

Abstract

The main objectives of this study were to evaluate genetic composition of Geosmithia morbida populations in the native range of black walnut and provide a better understanding regarding demography of the pathogen. The fungus G. morbida, and the walnut twig beetle, Pityophthorus juglandis, have been associated with a disease complex of black walnut (Juglans nigra) known as thousand cankers disease (TCD). The disease is manifested as branch dieback and canopy loss, eventually resulting in tree death. In 2010, the disease was detected in black walnut in Tennessee, and subsequently in Virginia and Pennsylvania in 2011 and North Carolina in 2012. These were the first incidences of TCD east of Colorado, where the disease has been established for more than a decade on indigenous walnut species. A genetic diversity and population structure study of 62 G. morbida isolates from Tennessee, Pennsylvania, North Carolina and Oregon was completed using 15 polymorphic microsatellite loci. The results revealed high haploid genetic diversity among seven G. morbida populations with evidence of gene flow, and significant differentiation among two identified genetic clusters. There was a significant correlation between geographic and genetic distance. Understanding the genetic composition and demography of G. morbida can provide valuable insight into recognizing factors affecting the persistence and spread of an invasive pathogen, disease progression, and future infestation predictions. Overall, these data support the hypotheses of two separate, highly diverse pathogen introductions into the native range of black walnut.

Thousand Cankers Disease Complex: A Forest Health Issue that Threatens Juglans Species across the U.S.

“…This report further indicated high genetic diversity of the pathogen, thus confirming the hypothesis of multiple introductions from multiple sources [3,39]. Both Zerillo et al [3] and Hadziabdic et al [40] proposed that anthropogenic movement of timber from multiple TCD infested areas, coupled with high susceptibility of eastern black walnut to disease pressures [7] would support the hypothesis that the pathogen has been recently introduced into the eastern U.S.-the native range of eastern black walnut. This scenario has been observed in other forest pathogen systems, particularly the expansion of non-native pathogens into native forest communities [42,43].…”

“…This scenario has been observed in other forest pathogen systems, particularly the expansion of non-native pathogens into native forest communities [42,43]. These novel introductions pose a major risk due to absence of co-evolutionary encounters and evolutionary potential of the pathogen, and lack of detection due to cryptic symptoms, thus allowing migration and admixture of populations to occur and therefore potentially increase genetic diversity [3,40,44,45].…”

“…Recently, English walnut, an important species for California walnut production, has also exhibited serious TCD disease symptoms [39]. The origin of G. morbida is currently unknown, but its spread has been widely documented to coincide with the range and spread of WTB [2,3,8,12,16,40]. Two different hypotheses were provided by Zerillo et al [3] regarding G. morbida point of origin that coincide with native range and distribution of either Arizona or California walnut trees.…”

“…Two different hypotheses were provided by Zerillo et al [3] regarding G. morbida point of origin that coincide with native range and distribution of either Arizona or California walnut trees. Although research progress has been made in elucidating these questions and additional work remains in progress, high genetic diversity and the complexity of the pathogen suggest that G. morbida has been in association with at least one the walnut species and its vector for a long period of time [3,40]. Indeed, these latter studies both examined population structure and genetic diversity of the pathogen, and their results indicate high genetic diversity and the presence of between two [40] and four [3] genetic clusters, which corresponded to geographical distribution among G. morbida isolates in the U.S. Zerillo et al [3] found no evidence of sexual reproduction, and described strong evidential support to indicate that G. morbida has evolved in close association with WTB and at least one walnut species.…”

Transcriptome and secretome of two Pythium species during infection and saprophytic growth

“…Considering the substrate specificity of bark beetles and Geosmithia associates to their respective plant hosts [56], and the highly diverse and complex genetic structure in G. morbida that we and others [10,26,27] have observed, it is unlikely that the current TCD epidemic was a result of a new association between the WTB and G. morbida.…”