Strategies for successful infection of host plants are highly diverse in fungal pathogens, which range from biotrophs to necrotrophs. As more microbial genomes have been sequenced, more fungal genes have been identified as being involved in pathogenesis, as exemplified by those for biosynthesis of toxic secondary metabolites such as host-specific toxins. Filamentous fungi produce a diverse array of secondary metabolites—small molecules that are not necessary for normal growth or development. The role of host-specific toxins in plant–fungus interaction as well as the biochemistry and molecular basis of toxin biosynthesis are discussed. The availability of fungal genomic sequences has revealed a remarkably large number of biosynthetic gene clusters for secondary metabolites, e.g., polyketides and nonribosomal peptides including cyclic peptides, extremely large classes of natural products of fungal origin. The origin and evolutionary processes for these gene clusters are largely unknown. Analysis of the arrangement and sequences of genes in the clusters should shed light on how the clusters and abilities to produce toxic secondary metabolites evolved.
In Miyagi Prefecture, 2–3 fungicide applications after heading are encouraged to suppress Fusarium head blight (FHB) and mycotoxin contamination in wheat, but farmers want to reduce fungicide applications to decrease costs and environmental impacts. To obtain the data needed to rationally reduce fungicide applications, we applied fungicide 0–2 times starting at early flowering in 2007 to 2013 on the popular wheat cultivar Shiranekomugi having a medium level of FHB resistance to test for correlations between climatic conditions after heading and the levels of FHB and deoxynivalenol (DON) in wheat in fields at the Miyagi Prefecture Furukawa Agricultural Experimental Station. FHB severity was positively correlated with both number of rainy days and minimum air temperature for 10 days after the early flowering stage, and DON concentration was positively correlated with total precipitation during 11 to 20 days after early flowering with and without fungicide applications. Fungicide applications reduced FHB severity and DON concentrations in wheat grains collected from plots with either 1 or 2 fungicide applications did not exceed the temporary standard in Japan of 1.1 ppm DON. The results suggest that 1 application for FHB control at early flowering can reduce DON in wheat grains to below the Japanese standard level for Shiranekomugi in average climatic conditions in Miyagi Prefecture.
Satsuma dwarf virus (SDV) causes serious damage to citrus production by reducing the quality and yield of fruit. SDV in young shoots of citrus trees is usually detected using an enzyme-linked immunosorbent assay (ELISA); however, such tests can only be performed during several weeks in the spring and are time-consuming. To overcome these issues, we developed an all-season, rapid detection method using an immunochromatographic assay (ICA) kit. The results obtained using this system showed that petals, stigmas, and ovaries sampled from SDV-infected plants during the spring gave positive results that were comparable to those obtained using shoots. In the summer and autumn, the virus was detected from the flavedo of fruits of SDV-infected plants at a high frequency and more frequently detected in the flavedo of fruits from inside the tree crown than in fruits from the exterior of the crown. In the winter, flavedo from fruits stored at ambient temperature for 3 months also tested positive. The frequency of detection was sometimes lower in fruits obtained in the summer or stored at ambient temperature for 1–3 months. We therefore recommend that more samples should be taken from fruits for SDV diagnosis using ICA. SDV diagnosis of citrus scions in the winter is more effective using young leaves and flower buds that have been sprouted by soaking the cut shoot under controlled conditions such as a growth chamber at approximately 27°C. These results showed that all-season diagnosis of SDV is feasible using samples from young shoots, flower buds, fruits in the summer, stored fruits in the winter, and from samples sprouted from scions during the winter.
Strains of Ralstonia solanacearum from eggplant and tomato fields in Tochigi Prefecture separated into four distinct groups (I, II, IV and one unknown) based on differences in pathogenicity to four species of Solanum plants. In repetitive sequence-based polymerase chain reaction (rep-PCR) DNA fingerprinting to discriminate the strains belonging to each pathogenicity group, two characteristic bands were identified. One was universally amplified from all tested strains, and the other was amplified only from strains in groups I, II and V. Based on the DNA sequences of the bands, we designed two more PCR primer sets (RsUp-F and RsUp-R, and RsDwn-F and RsDwn-R) that discriminated the strains in group I, II and V from those in III and IV.