Rice blast, caused by Pyricularia oryzae, is the most prevalent and widespread disease of rice in Japan. In this study, simple sequence repeat (SSR) markers were used to discriminate among three field populations of rice blast, which consisted of isolates collected from three respective plots with two cultivars during the growing seasons. In total, 302 isolates of P. oryzae genotypes were found with 8 SSR markers and used in population genetic analyses. Collected isolates were classified into three populations by field plot, and each population was separated into subpopulations according to the blast epidemic stage. Haplotypic diversity of the populations ranged from 0.535 to 0.904, revealing that genetic diversity in the field populations was high. All three populations were significantly differentiated according to pairwise F_ST and R_ST comparisons, but no differentiation was detected among subpopulations grouped by epidemic stages in the same plot. A migration event was detected by SSR genotyping of the isolates between neighboring plots. A phylogenic tree based on a minimum spanning tree algorithm also suggested migration of the fungus and showed that the direction of evolution differed among populations of the same cultivar located spatially distant from adjacent plants of different cultivars. From these results, SSR markers may enable us not only to discriminate field populations of P. oryzae that are genetically similar, but also to evaluate genetic processes such as gene flow, genetic drift, mutation and natural selection of the populations.

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Crabgrass blast, caused by Pyricularia grisea, is widespread in the Shonai district of Yamagata Prefecture and occurs every year in early July. Because the primary inoculum in snowy areas is not always known, here we looked for sources of primary inoculum in areas of the Shonai district that receive heavy snowfall. First, we tested for overwintering of the pathogen in crabgrass. Although ~0.1% of crabgrass seeds collected in the fall carried P. grisea, none of the seed collected after snowmelt the next spring carried the fungus. Nor did any nursery blast develop on seedlings grown from these seeds. The pathogen was reisolated from infected seeds and leaf blades after overwintering under low temperatures and dry conditions, but not from seeds and infected leaf blades and ear branches that had been covered with snow during the winter. On infected leaf blades, P. grisea survived for ~2 months in the field under snow, and for about 3 months without snow. These results suggest that P. grisea does not survive successfully on crabgrass plants under snow. Next, the timing of spore dispersal and the conditions required for infection were examined. Spores were dispersed in early July, but not in June in Shonai. For infection, 3–6 h of wetness and a density of 10 spores/ml were needed. Pot2 genotypes of P. grisea, found during early, middle and late periods of epidemics were analyzed for any changes in the genotypes in the populations. at multiple fixed points. As the disease progressed, the genotypic constitution of the populations diversified. Because the fungus is not successful in overwintering in Shonai, dispersed spores serve as the primary inoculum in a single event to initiate crabgrass blast in Shonai, and secondary infections are initiated from newly produced spores.

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In October 2011, hairy root symptoms were observed on commercially cultivated tomato (Lycopersicon esculentum) grown on coco slabs (a plant cultivation medium made with finely flaked coconut fibers) in large hydroponic installations equipped with recirculating drip irrigation systems in Hiroshima Prefecture, Japan. Abundant proliferative adventitious hairy roots appeared and thickly covered the slab surfaces, leading to a “root mat” symptom. The dense growth of roots was also observed inside the slabs. The causal bacterium, which was demonstrated by inoculation and reisolation to be pathogenic on tomato, was a gram-negative, aerobic rod with 2 to 4 peritrichous flagella. It formed opaque, milky or pale yellowish-colored, circular colonies, and had biochemical and physiological characters similar to those of Rhizobium species. Phylogenetic analysis based on 16S rDNA sequences also confirmed that it was a member of the genus Rhizobium. MLSA analyses based on the concatenated sequences of genes atpD, glnA, and recA revealed that the isolates are in the R. radiobacter clade. Thus, the pathogen was identified as Rhizobium radiobacter (Beijerinck and van Delden 1902) Young, Kuykendall, Martinez-Romero, Kerr and Sawada 2001. PCR analyses clarified that the pathogen harbored a cucumopine Ri plasmid. This is the first report on the occurrence of root mat (hairy root) of tomato caused by R. radiobacter (Ri) in Japan. Phylogenetic analyses revealed the existence of chromosomal polymorphisms among the present isolates, which proved to belong to genomovars G7 and G9 of R. radiobacter species complex. On the other hand, the cucumopine Ri plasmids harbored by the present isolates and the root mat pathogens isolated in Europe were highly homogeneous, which suggests that plasmids derived from the same origin might have been transferred to the indigenous bacteria in Europe and Japan, respectively.

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Wheat spikes, grown in a field that received only natural dew- and rainfall or in a plastic greenhouse that received water from a sprinkler system, were inoculated with a deoxynivalenol (DON)-producing Fusarium graminearum species complex to test the relationship between wetness duration after inoculation and subsequent DON concentration. When wetness duration, measured with a wetness sensor, exceeded 150 h, DON concentration increased with increasing durations of wetness. Wetness duration can thus serve as an index to predict the beginning of DON accumulation. In the field, accumulation of DON was related not only to rainfall but also to wetting by dewfall.

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Sensitivity to strobilurin fungicides was tested for 107 isolates of Colletotrichum gloeosporioides, obtained in 2009 and 2010 from mangoes with anthracnose in Okinawa Prefecture, Japan. When isolates were grown on potato dextrose agar containing 1000 ppm of salicylhydroxamic acid and various concentrations of azoxystrobin and kresoxim-methyl, two strains showed a minimal inhibitory concentration of >3000 ppm and were regarded as resistant. The control efficacy of azoxystrobin against two strains was low in a leaf bioassay, and the G143A point mutation in the cytochrome b gene, conferring resistance to strobilurins, was confirmed by PCR-RFLP using restriction enzyme Fnu4HI.

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