Eighty-five Pyricularia isolates were collected from 29 host species of Gramineae, Bambusideae and Zingiberaceae plants sampled in Brazil, Uganda, Ivory Coast, India, Nepal, China, Indonesia and Japan. These isolates were compared on the basis of pathogenicity, mating ability and restriction fragment length polymorphisms with single-copy DNA probes. Based on the pathogenicity to eight differential gramineous plants, these isolates were classified into seven pathotypes : finger millet type, foxtail millet type, common millet type, rice type, crabgrass type, Italian ryegrass/ weeping lovegrass type, and non-cereal/grass type. Genetic variation among these isolates was assessed by RFLP analysis with two restriction enzymes and nine single-copy DNA probes isolated from a finger millet strain. An UPGMA dendrogram based on the RFLPs revealed that the 86 isolates could be classified into seven major groups. Isolates from cereal crops (finger millet, foxtail millet, common millet, wheat and rice) and a grass, Brachiaria plantaginea, were clustered into a single group. They were further divided into six subgroups corresponding to the pathotypes. Among cereal crop isolates only an isolate from pearl millet was located into a different group. The remaining isolates were clustered into five groups designated as the crabgrass group, the buffelgrass and jungle rice group, the rice cutgrass, knotroot bristlegrass and Setaria tomentosa group, the bamboo and bamboo grass group and the Zingiber mioga group. The isolates from cereal crops were generally capable of mating with finger millet strains and constituted a closed mating compatibility group. These results suggested that the isolates from cereal crops form a single group with a common ancestor although they are pathogenic to taxonomically diverse plants. A combined analysis of the pathogenicity and genetic similarity suggested that the transmission of M. grisea isolates occurs in natural agroecosystems between finger millet and Eleusine africana, goosegrass or Bambusa arundinacea, between foxtail millet and green bristlegrass, and between rice and tall fescue, Italian ryegrass, sweet vernalgrass, reed canarygrass or Oryza longistaminata.

An outbreak of downy mildew disease of onion, caused by Peronospora destructor, in Japan in 2016 necessitated a reevaluation of the primary inoculum sources to optimize disease management. Detection of the P. destructor pathogen in plants with asymptomatic infection and in soil would guide the application of fungicides according to the extent of infection before disease development. Here, we detected P. destructor in both plants and soil using newly developed primer sets (Pd ITS and Pd ITS 614) by both conventional and real-time PCR. Validation by real-time PCR with Pd ITS 614 showed that P. destructor DNA was amplified from symptomless seedlings at 3.7 × 102 to 1.0 × 100 conidium cells/50 mg leaf tissue, suggesting the detection of asymptomatic infection. Real-time PCR with Pd ITS amplified pathogen DNA from field soils at 1.6 × 103 to 8.3 × 101 oospore cells/g of soil. This real-time PCR assay provides a useful tool for identifying and quantifying inoculum sources, which may be the foundation of the design of integrated disease management strategies.

The suppressive potentials of Bacillus and Paenibacillus strains isolated from the tomato phyllosphere were investigated to obtain new biocontrol candidates against Fusarium crown and root rot of tomato. The suppressive activities of 20 bacterial strains belonging to these genera were examined using seedlings and potted tomato plants, and two Paenibacillus strains (12HD2 and 42NP7) were selected as biocontrol candidates against the disease. These two strains suppressed the disease in the field experiment. Scanning electron microscopy revealed that the treated bacterial cells colonized the root surface, and when the roots of the seedlings were treated with strain 42NP7 cells, the cell population was maintained on the roots for at least for 4 weeks. Although the bacterial strains had no direct antifungal activity against the causal pathogen in vitro, an increase was observed in the antifungal activities of acetone extracts from tomato roots treated with the cells of both bacterial strains. Furthermore, RT-PCR analysis verified that the expression of defense-related genes was induced in both the roots and leaves of seedlings treated with the bacterial cells. Thus, the root-colonized cells of the two Paenibacillus strains were considered to induce resistance in tomato plants, which resulted in the suppression of the disease.

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