Shigeo Naito

Characterization of mycorrhizal fungi isolated from the threatened Cypripedium macranthos in a northern island of Japan: two phylogenetically distinct fungi associated with the orchid

We isolated Rhizoctonia-like fungi from populations of the threatened orchid Cypripedium macranthos. In ultrastructural observations of the septa, the isolates had a flattened imperforate parenthesome consisting of two electron-dense membranes bordered by an internal electron-lucent zone, identical to the septal ultrastructure of Rhizoctonia repens (teleomorph Tulasnella), a mycorrhizal fungus of many orchid species. However, hyphae of the isolates did not fuse with those of known tester strains of R. repens and grew less than half as fast as those of R. repens. In phylogenetic analyses, sequences for rDNA and internal transcribed spacer (ITS) regions of the isolates were distinct from those of the taxonomically identified species of Tulasnella. On the basis of the ITS sequences, the isolates clustered into two groups that corresponded exactly with the clades demonstrated for other Cypripedium spp. from Eurasia and North America despite the geographical separation, suggesting high specificity in the Cypripedium–fungus association. In addition, the two phylogenetic groups corresponded to two different plant clones at different developmental stages. The fungi from one clone constituted one group and did not belong to the other fungal group isolated from the other clone. The possibility of switching to a new mycorrhizal partner during the orchid’s lifetime is discussed.

Comparison of Chinese and Japanese A1 isolates of Phytophthora infestans

The mating type, glucose-6-phosphate isomerase (Gpi) and peptidase (Pep) genotypes, RG57 fingerprint, and mitochondrial DNA (mtDNA) haplotype of Chinese isolates of Phytophthora infestans collected in Hebei and Gansu in 1996 were compared with those of Japanese isolates collected during 1997–2000. The Chinese isolates were divided into four genotypes, one of which was identical to the dominant Japanese genotype, A1-A (mating type A1; Gpi 100/100; Pep 100/100; RG57 100010001100110100011001110: 1–25, 14a, and 24a; and mtDNA haplotype IIa). Comparison of the genotypes with reported data revealed that some completely and partially identical genotypes occur in Russia and parts of Europe. The other two A1 genotypes and one A2 genotype were also detected in Gansu (Gpi 100/100, Pep 100/100, and mtDNA haplotype Ia), which were regarded as unique to this region.

Ecological studies on teleomorphic and anamorphic stages in Rhizoctonia fungi

The species concept of Rhizoctonia was poorly defined until outlined by Parmeter and Ogoshi in the 1970s. The genus Rhizoctonia contains taxonomically diverse species of fungi with teleomorphs associated with the genera Botryobasidium, Ceratobasidium, Sebacinia, Thanatephorus, Tulasnella, Uthatobasidium, or Waitea. Rhizoctonia solani (teleomorph = Thanatephorus cucumeris) is the most widely studied species because of its importance as a pathogen of plants and is a species complex that consists of 13 anastomosis groups (AG). Six of these AG have been further separated into ecologically and genetically distinct subgroups. Although the taxonomic classification and identification of Rhizoctonia species and AG have recently advanced based on molecular techniques, less information is available on the ecological function of basidiospores of Thanatephorus and their relative contribution to the genetic diversity of fungal populations. The reason(s) for this lack of information may be partly attributed to the difficulty in artificial production of sexual fruiting structures (hymenia) under laboratory conditions and our inability to readily recognize hymenia formation on plants in the field. In this article, information is presented on hymenia formation, basidiospore dispersal, germination, infection of leaves, survival in soil, and their contribution to the genetic diversity of populations of R. solani AG-2-2 IV (a pathogen that causes root rot and foliar blight of sugar beet). Additional information is presented on the role of the fungus gnat, Pnyxia scabiei, in destruction of sclerotia of R. solani in fields of sugar beet exhibiting decline of Rhizoctonia disease and the occurrence of two newly described AG subgroups of R. solani from soybean (AG-2-3) and tobacco (AG-2-1).

Environmental factors influencing sporocarp formation in Typhula ishikariensis

Environmental factors influencing sporocarp formation in Typhula ishikariensis were studied under controlled conditions. Sporocarp formation in T. ishikariensis was divided into two stages: stipe elongation from the sclerotium and fertile head development at the tip of the stipe. Factors required for each stage differed. At the stipe elongation stage, low temperature (10°/5°C; day/night) and high humidity were important, but light was not required. In contrast, at the fertile head stage, light and moderate day length (8 h/day) were essential. Fertile heads developed at 46 µEm−2 s−1; and high intensity (137 µEm−2 s−1) did not suppress development. Moreover, adding unsterilized soil to the sea sand medium accelerated sporocarp formation. These findings imply that the sclerotium of T. ishikariensis recognizes several physical factors for sporocarp formation. Sporocarps of T. ishikariensis developed within 4 weeks after incubation under optimal conditions. The sporocarp produced basidiospores, and differential mating incompatibility was confirmed among monokaryons derived from basidiospores produced under artificial conditions. This method should be useful for obtaining monokaryons for genetic studies of T. ishikariensis.

Ratio of rice reflectance for estimating leaf blast severity with a multispectral radiometer

Abstract Rice reflectance was measured to determine the spectral regions most sensitive to leaf blast infection with a multispectral radiometer. As disease severity increased, reflectance also increased in the 400–500 nm (blue), 570–700 nm (red), and 900–2000 nm regions but decreased in the 500–570 nm and 700–900 nm regions. The increased reflectance in the blue and red regions may be attributed to decreased chlorophyll and carotenoid contents in response to the blast infection. The maximum and minimum reflectance differences occurred at 680 nm and 760 nm for the nondiseased and diseased rice, respectively. The spectral location of maximum sensitivity was 675 nm regardless of disease severity. Rice reflectance ratios were evaluated as indicators of leaf blast severity. Two ratios, R550/R675 (reflectance at 550 nm divided by reflectance at 675 nm), and R570/R675 quantified the significant disease severity. These wavelengths were selected based on the sensitivity minima and maxima. The ratios of nondiseased rice plants varied depending on growth stage. The variation in ratios must be considered when they are used to estimate leaf blast severity.

Precise, simple screening for resistance in potato varieties to common scab using paper pots

Resistance to common scab pathogen Streptomyces turgidiscabies of seven potato varieties was compared in the field with a newly developed paper pot method. Seedlings raised in soil in paper pots containing inocula at 1 × 103 to 107 cfu/g soil were transplanted into a scab-free field and grown for 3 months. The disease severity of the seven varieties in the field trials differed in iteration and from year to year, even though their resistance levels were approximately similar at the expected levels. With the paper pot method, the seven varieties had different resistance levels, which were almost completely consistent with the results of the field trials, at more than 1 × 105 cfu/g soil. Significant differences in disease severity between resistant and susceptible varieties were observed (P = 0.05) for 2 years, and the resistance level of the varieties was elucidated.

Identification and pathogenicity of Pythium species causing damping-off of kidney bean

Five Pythium species (Pythium irregulare, P. mamillatum, P. myriotylum, P. spinosum and P. ultimum var. ultimum) were isolated from the hypocotyls and roots of kidney bean plants with damping-off from a commercial field and from experimental plots that have undergone either continuous cropping with kidney bean or rotational cropping with arable crops. In inoculation tests, all five Pythium species were pathogenic to kidney bean. This is the first report of damping-off of kidney bean caused by Pythium species; we named this disease damping-off of kidney bean.

New virulent race of Phialophora gregata f. sp. adzukicola associated with continuous cultivation of adzuki bean cultivar Acc259

Adzuki bean cultivar Acc259, which is resistant to races 1 and 2 of Phialophora gregata f. sp. adzukicola, was used as a breeding resource for resistance to brown stem rot (BSR). During the third year after two successive cultivations of Acc259, a severe outbreak of BSR occurred in an experimental plot at the Tokachi Agricultural Experiment Station, Hokkaido, Japan. The isolates obtained from diseased plants were virulent to Erimo-shozu (susceptible to all races) and Acc259 but avirulent to Kita-no-otome (resistant to race 1 but susceptible to race 2). The existence of a new race of P. gregata f. sp. adzukicola, designated race 3, was determined; and its frequency in the plot soil was shown to increase from 16.7% before planting Acc259 to 100% after the third year. Of 140 isolates from the commercial production area that were formerly identified as race 1, 13 were actually race 3 and were restricted to certain limited fields.

New multilocus genotypes of Phytophthora infestans in Japan

Twelve isolates of Japanese Phytophthora infestans, which differed from the major genotypes US-1, JP-1, JP-2, and JP-3, were analyzed for RG57 fingerprints, mtDNA haplotypes, two allozyme genotypes, and mating types. Genotypes JP-1.1, JP-2.1, JP-2.2, JP-3.1, and JP-4 were newly defined. JP-1.1 and JP-2.1 were isolated discontinuously from potato fields in several years, and JP-1.1 was found in Hokkaido and Kagoshima. These results show that some minor genotypes can overwinter and disperse from their original site.

New race of Phytophthora vignae f. sp. adzukicola, the causal agent of Phytophthora stem rot of the adzuki bean.

Abstract A new race of Phytophthora vignae f. sp. adzukicola, designated race 4, is reported from central and western Hokkaido, Japan. The isolates obtained from diseased plants of a new cultivar, cv. Syumari, which is resistant to races 1, 2, and 3, were determined to be a new race by the pathogenic reaction on a set of differential adzuki bean cultivars (cv. Erimo-shozu, cv. Kotobuki-shozu, cv. Noto-shozu, cv. Urasa-shimane, and cv. Syumari).