First report of walnut blight caused by WB strains of <i>Xanthomonas arboricola</i> pv. <i>juglandis</i> in Japan.

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  • <i>Xanthomonas arboricola</i> pv. <i>juglandis</i> WB系統によるクルミ黒斑細菌病のわが国における初発生
  • Xanthomonas arboricola pv. juglandis WB系統によるクルミ黒斑細菌病のわが国における初発生
  • Xanthomonas arboricola pv. juglandis WB ケイトウ ニ ヨル クルミ コク ハン サイキンビョウ ノ ワガクニ ニ オケル ハツ ハッセイ

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Abstract

<p>Since 2014, brown to black spots have been observed on leaves, catkins, and fruits of commercially cultivated walnut (Juglans regia) in most walnut-growing regions of Nagano Prefecture, Japan. The causal bacterium, which was demonstrated by inoculation and reisolation to be pathogenic on walnut, was a gram-negative, aerobic rod with one polar flagellum. It formed yellow, slimy colonies and had biochemical and physiological characters similar to those of Xanthomonas species. On the basis of PCR assays targeting 16S rRNA gene and qumA, a multilocus sequence analysis (MLSA) using the concatenated sequences of seven housekeeping genes (atpD, dnaK, efp, fyuA, glnA, gyrB and rpoD), and the aforementioned phenotypes, we identified the pathogen as Xanthomonas arboricola Vauterin, Hoste, Kersters and Swings 1995. Moreover, the results of the MLSA, additional PCR tests to clarify the composition of type III effector (T3E) genes, biochemical and physiological characterization using API 20NE, and inoculation tests clearly showed that this pathogen corresponds to X. arboricola pv. juglandis (Xaj). In addition, because Xaj strains comprise two lineages (walnut blight [WB] strains harboring xopAH, and vertical oozing canker [VOC] strains harboring xopB), we used PCR tests to investigate which T3E gene the pathogen possesses and clarified that it harbors xopAH. We also found that even in MLSA trees, the pathogen clusters together with WB strains and that the symptoms observed in Nagano orchards agree with those caused by WB strains. As a result, the pathogen was determined to correspond to WB strains of Xaj. To our knowledge, this report is the first in Japan of walnut blight caused by Xaj. Furthermore, although it had been shown that WB strains of Xaj worldwide are genetically heterogeneous, all the present isolates used proved to be highly homogeneous. This result suggests that one clone of WB strains has been expanding its distribution to most walnut-growing regions in Nagano Prefecture so far, leading to the present situation.</p>

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