Gray blight caused by Pestalotiopsis longiseta is one of the most important diseases of tea plant. Thiophanate-methyl (benzimidazole) and azoxystrobin (strobilurin, QoI) fungicides have been used to control gray blight in Kagoshima Prefecture. However, benzimidazole-resistant strains of P. longiseta have been found in tea fields in Kagoshima Prefecture since 1983 and strobilurin-resistant strains since 2008. Therefore, we examined samples of P. longiseta from several green-tea-cultivation regions in Kagoshima Prefecture in 2008 and 2009 for their sensitivities to strobilurin and benzimidazole. As a result, 31.7% of the strains in 2008 and 33.6% in 2009 were strobilurin-resistant. The rate was especially high in the Nansatsu region and low in the Aira and Hokusatsu regions. On the other hand, 80.9% of the strains were benzimidazole-resistant in 2009, found widely in separated fields. Strains that were highly resistant to benzimadazole were frequently found in the same fields as moderately resistant strains. Highly resistant strains to both fungicides occurred in as much as 20.0% of the tea fields. Azoxystrobin was not effective against QoI-resistant strains. In addition, the control effect of azoxystrobin on P. longiseta was decreased with the increase in the rate of QoI-resistant strains in the tea fields. Just a one-time application of QoI-fungicide remarkably increased the percentage of QoI-resistant strains in the tea field. This was the case for thiophanate-methyl.
We selected 12 simple sequence repeat (SSR) markers useful for differentiating Japanese isolates of the rice blast fungus, Pyricularia oryzae, and developed three multiplex sets, each containing four SSRs, for fluorescence detection. Genetic diversity and population structure of P. oryzae field isolates in western Japan were studied based on these markers. For a total of 293 isolates collected from 16 prefectures in 2001, a high level of genetic diversity was observed, with 265 different haplotypes and an average of 14.3 (range 2-36) alleles per locus. Estimates of population differentiation varied greatly among 10 prefectures, with pairwise FST ranging from 0.002 to 0.275. A Mantel test revealed significant positive correlation (r2=0.224, P=0.004) between geographical distance (69-539 km) and pairwise FST, suggesting that the level of gene flow is remarkably high between neighboring prefectures, probably occurring via dispersal of spores and movement of infected rice seed. In contrast, a relatively high level of genetic differentiation was observed between prefectures located in geographically distant regions, suggesting that spore dispersal and seed movement over a relatively long distance is restricted. The SSR markers selected in this study should be useful for studying the population genetics of the rice blast fungus.
Pepper mild mottle virus (PMMoV) in the soil can be detected by double antibody sandwich enzyme linked immunosorbent assay (DAS-ELISA) (Ikegashira et al., 2004) with great sensitivity. A PMMoV-disease risk assessment system, based on DAS-ELISA measurements of the virus in field soils, was developed to obtain a better understanding of PMMoV-contamination levels in pepper fields. When the DAS-ELISA was used on soils collected from five locations in the field, the greatest value correlated significantly with the incidence of PMMoV disease. To estimate the threshold of disease incidence in consecutive cultivations, we used DAS-ELISA for soils collected from 72 pepper fields after cultivation. The viruliferous soils had an absorbance over 0.1 in the DAS-ELISA, correlating with the disease incidence during the next cultivation, whereas most of the nonviruliferous soils were below this value. Taken together, our results showed that measuring the virus in soils from pepper fields using the DAS-ELISA system, just before cultivation, is useful for assessing the risk of disease from PMMoV.