First Record of Thousand Cankers Disease Fungal Pathogen Geosmithia morbida and Walnut Twig Beetle Pityophthorus juglandis on Juglans regia in Europe
Authors: Montecchio L. , Fanchin G. , Simonato M. , Faccoli M. 
Published: August 2014

Thousand cankers disease (TCD) is a disease complex caused by the fungus Geosmithia morbida Kolařik (Ascomycota, Hypocreales) and its vector Pityophthorus juglandis Blackman 1928 (Coleoptera, Scolytinae; walnut twig beetle, WTB). Since the mid-1990s, the disease was responsible for widespread mortality of many walnut species in the United States (4). After the first detection of TCD on black walnut (Juglans nigra L.) in Italy (3), an extensive survey was activated in cooperation with the Regional Phytosanitary Service. In May 2014, early TCD symptoms (4) were observed on English walnuts (J. regia L.). Canopies showed yellowing, wilting, and dieback of the youngest twigs, and a number of small brown cankers. Longitudinal and radial sections sampled through the cankers revealed gray to brown discoloration of both phloem and bark, and the presence of bark beetle galleries. Xylem discoloration was never observed. From one ~20-year-old European walnut growing in a garden neighboring an infected black walnut plantation (Santorso, Vicenza, 45°72′ N, 11°40′ E), a number of 1- to 2.5-cm-diameter twigs showing cankers up to 2 cm long surrounding bark beetle holes were collected. Whitish mycelium producing verticillate conidiophores was detected inside the insect galleries. From the necrotic margin of eight cankers previously surface-sterilized with 3% sodium hypochlorite, two 4-mm-wide chips per canker were placed on potato dextrose agar and incubated at 28 ± 1°C in the dark. Slow growing lobate, plane, yellowish-ocher colonies with hyaline mycelium appeared in 5 days. After subculturing to the same medium, growth features, mycelium, conidiophores, and conidia with morphological characteristics matching Kolarik's description of G. morbida (2) were observed. The ITS region of rDNA from the fungus strain LM14GM001-JR was amplified by using ITS1F and ITS4 primers and sequenced obtaining a 387-bp gene fragment. BLAST analysis showed 99% identity to the G. morbida strain U19 (GenBank Accession No. KF808301.1) for 384 bp, and 99% identity to the G. morbida strain LM13GM001-JN previously isolated from J. nigra in Italy (3). From the same samples, two emerging beetles were collected and identified as P. juglandis both morphologically (5) and genetically by DNA extraction following a standard salting out protocol. The barcode region of the mitochondrial gene cytochrome oxidase I was then amplified by using universal primers (1) and sequenced to obtain a 614-bp fragment of the gene. BLAST analysis showed 100% identity to P. juglandis based on comparison with KJ451422. A few other English walnuts with both the fungus and WTB were also found close to other infected black walnut plantations. To our knowledge, this is the first record of G. morbida and P. juglandis on J. regia in Europe, where the tree is cultivated for both fruit and timber production, as well as a traditional landscape tree. Voucher specimens are stored in the TeSAF herbarium and in the DAFNAE insect collection.

References: (1) O. Folmer et al. Mol. Marine Biol. Biotechnol. 3:294, 1994. (2) M. Kolarik et al. Mycologia 103:325, 2011. (3) L. Montecchio and M. Faccoli. Plant Dis. 98:696, 2014. (4) S. J. Seybold et al. USDA Forest Service, NA-PR-02-10, 2013. (5) S. L. Wood. Great Basin Naturalist Memoirs 6:1123, 1982.