Because l-methionine has been reported to have an inhibitory effect on disease, here we tested l-methionine for a direct antifungal effect on Fusarium oxysporum f. sp. lycopersici (FOL), cause of Fusarium wilt, and an inhibitory effect on the disease. l-Methionine had no observable effect on mycelial growth or bud cell germination of FOL Treatment of tomato leaves with 100 μg/mL l-methionine reduced disease severity caused by FOL. Two days after inoculation, total RNA was extracted from roots and analyzed by quantitative PCR for expression of several defense-related genes; expression of multiple PR genes increased after l-methionine treatment, suggesting that l-methionine treatment of tomato plants may induce resistance to Fusarium wilt by a priming effect.
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.