Distribution of Pythium porphyrae, the causal agent of red rot disease of Porphyra spp., in seafloor sediment was investigated in the Ariake Sea, Japan. A total of 170 samples of each 200 ml of sediment was collected from the seafloor at a total of 13 sites across the sea from 1998 to 2002. Each sample was filtered through two layers of nylon mesh with pore sizes of 100 and 15 μm. The residue on 15 μm mesh was assayed by a soil plating technique using a semiselective medium for P. porphyrae and by polymerase chain reaction (PCR) using species-specific primers. P. porphyrae were detected in 6 out of 13 sites and 2 out of 10 sites surveyed by soil plating and PCR, respectively. The representative isolate of P. porphyrae from the sediment was identical to the Porphyra thallus isolate from the same sea based on pathogenicity to the thallus, morphology, and rDNA internal transcribed spacer sequences. Recovery of P. porphyrae propagules in the sediment was up to 60 CFU per 100 ml of the fresh sample and was consistently higher in May than in the other months. The results suggest that P. porphyrae is distributed in the seafloor sediment in a wide area of the Ariake Sea.
The effects of initiation timing of drought stress on the carbohydrate content and growth of Japanese apricot (Prunus mume Sieb. et Zucc.) 'Nanko' tree were investigated. Three-year-old trees were subjected to drought stress for three months in May ('Spring') and August ('Summer'), respectively. Irrigation was started immediately after drought stress treatment had finished. The starch concentration in the roots of stressed trees increased in both 'Spring' and 'Summer' two months after starting drought stress treatment and rapidly decreased just after starting irrigation in 'Spring'; however, it still temporarily increased in 'Summer'. These different results were attributed to seasonal changes in the carbohydrate demand. The dry matter weight of each tree organ in February of the following year was suppressed by drought stress, particularly in 'Spring'. This result indicates that, if a tree suffers drought stress in spring, vegetative growth is suppressed more seriously than in summer and tree growth, declined by suffering from severe and long-term drought stress, will hardly recover even the tree is reirrigated. The total starch in all organs of the drought-stressed trees in February of the following year was significantly less than that in well-irrigated trees. These results indicate that severe drought stress for three months inhibits not only current tree growth but also the accumulation of carbohydrates that is necessary for the following year's growth. Hence, irrigation to avoid a fatal growth decline due to severe drought stress is needed during the current growth stage, especially in the early growth stage in spring.
Japanese apricot scab is an agriculturally important fungal disease caused by Cladosporium carpophilum, which is presented on fruits as black spots of approximately 1-3 mm. A quinone-outside inhibitor (QoI) fungicide, kresoxim-methyl, has been used to effectively control this disease since 2005. Another QoI fungicide, azoxystrobin, has been in use since 2011. However, resistance to these fungicides was suspected in some Japanese apricot orchards in Wakayama Prefecture in 2012. Therefore, we investigated the susceptibility of C. carpophilum isolates from 19 orchards to kresoxim-methyl and azoxystrobin. First, a culture test was performed where the 41 isolates tested with kresoxim-methyl were divided into nine isolates with an EC 50 value of 0.02 ppm to 0.81 ppm and 32 isolates with an EC 50 value greater than 100 ppm. The 22 isolates tested with azoxystrobin were divided into five isolates with an EC 50 value of 0.09 ppm to 1.99 ppm and 17 isolates with an EC 50 value of 69.2 ppm or more. Thus, the frequency distribution of EC 50 for kresoxim-methyl and azoxystrobin had two distinct peaks, considered to be high susceptibility and low susceptibility, respectively. Thereafter, the inhibitory effects of three isolates with low susceptibility in the culture test and one control isolate with high susceptibility were investigated using a bioassay. The protective value for the control isolate was 100 for the two fungicides, whereas, for the three isolates with low susceptibility, the protective values for kresoxim-methyl and azoxystrobin were 0-41.6 and 0-58.8, respectively. Thus, we observed a measurable decrease in the protective values.These results strongly indicate the development of Japanese apricot scab resistance to QoI fungicides in Wakayama Prefecture.
To estimate the infection period of a root rot pathogen of ginger (Pythium myriotylum Drechsler) under an open-field cultivation condition, an inoculation test was conducted under field conditions for two cultivation seasons. The pathogen infection was confirmed by the LAMP method and symptoms were observed from late May 2016 to mid-September and 2017. This demonstrates that the pathogen can infect ginger tubers for more than four months from early to mature stages of the plant under field conditions.
In September 2019, Pythium rot of ornamental cabbage (Brassica oleracea L. Acephala Group) occurred in Kinokawa City, Wakayama Prefecture, Japan. The causal agents were identified as Pythium aphanidermatum and P. myriotylum based on cultural characteristics and molecular analysis of ribosomal DNA internal transcribed spacer (rDNA-ITS) regions. An inoculation test confirmed that oomycetes were the causal agents of the disease. This is the first report of Pythium rot in ornamental cabbage in Japan.
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