Detection of Geosmithia morbida in several insect species
Thousand cankers disease (EPPO A2 List) of Juglans spp. is caused by the fungus Geosmithia morbida and its known insect vector is Pityophthorus juglandis (Coleoptera: Curculionidae: Scolytinae- walnut twig beetle). The disease was initially described in the USA in 2009 and reported for the first time in the EPPO region, in Italy, in 2013 (EPPO RS 2014/001). In the USA, several surveys have been carried out since 2010 to monitor the distribution of G. morbida, P. juglandis and to study the possible presence of G. morbida in other insect species. During recent studies, G. morbida was isolated from Xylosandrus crassiusculus and Xyleborinus saxesenii collected in Ohio, as well as from Stenomimus pallidus collected in Indiana. Initially, it was thought that thousand cankers disease was caused by the unique association of G. morbida and P. juglandis. However, the above detections of G. morbida in other insect species led to the hypothesis that the association G. morbida/P. juglandis was not unique and that G. morbida was more widespread in the USA than initially thought.
From 2015 to 2017, another study was carried out in Illinois, Indiana, Minnesota and North Carolina (Eastern USA) on ambrosia beetles, bark beetles and other weevils to detect the possible presence of G. morbida. Traps of different types were placed in the 4 above states (e.g. near sawmills, in areas where J. nigra trees are abundant) and the captured beetles were tested for the presence of G. morbida. Fungal colonies were obtained by spreading macerated insect suspensions on artificial growing media, and suspect G. morbida colonies were subsequently tested by several PCR methods. As a result, G. morbida was detected in 18 coleopteran insect species* belonging to different subfamilies (Bostrichinae, Cerambycinae, Cossoninae, Dryophthorinae, Molytinae, Scolytinae) indicating that the fungus can be carried out by a broad range of insect species. In addition, G. morbida was detected in several insect species collected in Illinois and Minnesota, where thousand cankers disease has not been observed. During this study, X. crassiusculus was the most commonly found species on J. nigra trees (or their vicinity). However, it is stressed that for the moment, only P. juglandis is known to transmit the fungus to healthy J. nigra trees. The potential role of these insects in disease transmission remains to be further studied.
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* List of insect species in which G. morbida was detected: in alphabetical order and in brackets (total number of tested insects / number of G. morbida-positive) : Ambrosiodmus obliquus (5/1), Ambrosiophilus atratus (17/4), Cnestus mutilatus (23/11), Conotrachelus retentus (5/3), Dryophthorus americanus (1/1), Himatium errans (27/4), Monarthrum fasciatum (7/3), Monarthrum mali (33/9), Neoclytus acuminatus (20/4), Pityophthorus juglandis (3/1), Pseudopityophthorus minutissimus (30/18), Stenomimus pallidus (20/8), Stenoscelis brevis (1/1), Xyleborinus saxesenii (198/77), Xyleborus californicus (5/1), Xylobiops basilaris (32/18), Xylosandrus crassiusculus (735/250), Xylosandrus germanus (34/12).
Sources
Moore M, Juzwik J, Miller F, Roberts L, Ginzel MD (2019) Detection of Geosmithia morbida on numerous insect species in four Eastern States. Plant Health Progress, 1–7. DOI: https://doi.org/10.1094/PHP-02-19-0016-RS