A simple method to detect the tolerant strain of Venturia nashicola (causal fungus of Japanese pear scab) to benomyl was developed. Conidiospores on the diseased leaves were seeded directly on potato sucrose agar media containing given concentrations of benomyl. After incubation for 48 or 72hr at 15C, minimum inhibitory concentration (MIC) of benomyl was determined on the basis of septum formation in germ tubes. By this method, the tolerance and its degree could be determined much easier and faster than the conventional hyphal growth method with the same accuracy, This method was applicable to ascospores as well as conidiospores. The method was named “Germ-tube Septum Method”.

View full abstract

Rice ragged stunt virus (RRSV), which has been prevalent in South and Southeast Asia since 1977, caused typical symptoms on japonica rice, such as Kinmaze and Mihonishiki. However, barley, oat, wheat and maize, which had been known as hosts of rice black-streaked dwarf virus (RBSDV), were not susceptible to RRSV. The infected japonica rice developed symptoms of stunting, leaf twisting and vein swelling, that were identical to those caused by RBSDV. Other typical symptoms, such as nodal branching and ragged leaves, differed from those caused by RBSDV. A colony of Nilaparvata lugens collected in the fields of Japan and confined in the greenhouse for more than five years in Sapporo, Japan, was capable of transmitting RRSV in a persistent manner. Laodelphax striatellus, a vector of RBSDV, failed to transmit RRSV, and N. lugens, the vector of RRSV, did not carry RBSDV. RRSV was also carried by N. lugens when the insects were inoculated with the extracts from diseased rice plants and viruliferous insects by artificial injection. Purified preparations associated with the low infectivity contained numerous isometric particles of 36-40nm in diameter. The diameter of isometric particles in dip preparations varied from 44 to 63nm according to compounds used for fixing and staining. Electron micrographs of ultrathin sections showed abundant RRSV particles in phloem cells of infected rice leaves. The results suggest that RRSV and RBSDV are not identical, and although both may belong to the plant reovirus group, RRSV is probably a new member of planthopper-borne subgroup.

View full abstract

The whole cycle of fungal growth of Diaporthe citri, i.e., pycnospore germination, appressorial formation, hyphal penetration, penetrated mycelia, pycnidium formation and pycnospore formation was observed on alcohol-treated epidermal strips of onion bulb which might be free from some inhibitors against hyphal penetration. The results obtained on epidermal strips of onion bulb were nearly same as those obtained on citrus leaves or twigs. Therefore, the method using epidermal strips of onion bulb would be very useful for assaying the effects of fungicides on the different growth stages of D. citri. The maximum rate of pycnospore germination was approximately 80 to 100% and that of hyphal penetration was approximately 50 to 70%. The shapes of appressoria were mostly fist-like or globular form and they did not necessarily have relation to hyphal penetration. Therefore, the function of appressorium of D. citri seems to adhere on host plants, but the role does not seem to be so important, judging from the fact that the appressorial formation rate is only 20 to 30%. When suspension of pycnospores in pycnidia formed on epidermal strips of onion bulb was inoculated on young citrus leaves, typical symptoms of melanose were observed, indicating that the pycnospores were virulent.

View full abstract

The fine structure of the infection process of Rhizoctonia solani Kühn was studied on the cucumber seedlings inoculated with an isolate of type III-A at 25C under continuous light. The attachment of the hyphae on the epidermal surface of hypocotyls was clearly observed 12hr after inoculation. Plasmolysis was revealed in the epidermal cells of hypocotyls on which the infection cushions had been formed 18hr after inoculation. The tip of the primary penetration pegs from the hyphae at the basal part of the infection cushion did not have the wall, and the fungal lomasome-like structures were in direct contact with the cuticle layer of the host epidermal cells. At 24hr after inoculation, the cuticle and cell wall of the hypocotyl epidermal cells were penetrated simultaneously by infection pegs from each hyphal tip. Initial penetration by the infection pegs occurred mostly through the suture of epidermal cells, and the penetrating hyphae had invaded the cortical parenchyma of hypocotyls. As soon as the infection pegs penetrated the cuticle, the cuticular layer was detached from subcuticular walls of epidermal cells, the epidermal cell walls swelled, and cellulose fibers were laminated, dispersed and decomposed. The cytoplasm of the invaded epidermal and cortical parenchymatous cells and the adjacent cells was extremely disorganized and plasmolysis was observed.

View full abstract

Phleichrome was isolated from the culture of Cladosporium phlei and its actions were examined on the invertases from the leaves of timothy, Phleum pratense. Phototransformed phleichrome was almost identical with the pigment from the diseased spots in some properties, suggesting that phleichrome may be a pathotoxin. Susceptibility and resistibility of SH requiring and non-requiring invertases, respectively, to phleichrome were demonstrated as was the case for p-chloromercuribenzoate. Its inhibitory action on this enzyme was partially neutralized by the addition of dithiothreitol or L-cysteine. Exposed to visible light (474nm), phleichrome showed less inhibitory action on this enzyme. The inhibition of SH enzymes by this toxin is assumed to be a factor to incite the timothy leaf spot disease.

View full abstract

Oogonia and oospores of Peronospora manshurica were proved to be formed in lesions on Glycine soja (G. ussuriensis) by immersion treatment in water at 10-20C. A few oogonia were formed by 3-day immersion at 15 and 20C, and their number increased by 4-day immersion at 10, 15 and 20C. The oogonium formation was most abundant at 15C. After the formation of oogonia, oospores were more rapidly formed when temperature was higher in the range of 10-20C. The size and shape of oospore formed by immersion treatment were identical to those hitherto reported on natural host, average diameter being 27.1μm.

View full abstract

In June, 1977, bacterial stem rot of potato caused by Erwinia chrysanthemni Bur-kholder et al. occurred in Niigata, Japan, for the first time. It was the second record in the world that the disease was found on potatoes. Streaks developed first on petiole of lower leaves of the affected plants, and then on the stems in which vessels later discolored. These leaves became pale and then yellow before the plants finally died. The stems near the soil level turned dark, appearing similar symptoms to those of potato black leg disease caused by Erwinia carotovora var. atroseptica. When severely attacked, pith was rotted with a hollow and plants collapsed. The bacteriological characteristics of the pathogen were determined. The bacterium was gram-negative, non-spore-forming rods, motile by several peritrichous flagella, measuring 0.3∼0.8×1.0∼4.0μm; facultative anaerobe. Colonies on nutrient agar were greyish white, round or amoeboid, smooth, shining, convex to slightly raised. The blue pigment was not produced on GYCA. Gelatin was hydrolysed, but not starch. Potato slices were strongly rotted. Nitrate was reduced. Glucose was fermentatively metabolized. Acid was produced within 7 days from the following compounds: arabinose, xylose, galactose, mannose, fructose, glucose, sucrose, raffinose, inositol and salicin; but not from lactose, maltose, α-methyl glucoside, trehalose, adonitol, dulcitol and melezitose. Acid was produced from lactose after 14 days. Tartrate, citrate and malonate were used as carbon source; but not propionate or benzoate. Catalase, lecithinase and phosphatase reactions were positive. Kovacs' oxidase, urease, lipase, tyrosinase, phenylalanine deaminase and amino acid decarboxylases were all negative. Production of reducing substances from sucrose was negative. Maximum NaCl tolerance was 5%. Maximum growth temperature was 37C.

View full abstract

Soil microorganisms, i.e. fungi, actinomycetes and bacteria, were isolated from soils collected in both reclaimed and matured fields. Some of them were randomly selected, and their antagonistic actions were tested against three plant pathogens, Fusarium solani f. sp. radicicola, Rhizoctonia solani and Corticium rolfsii on potato sucrose agar plates. The percentage of antagonistic microorganisms in soils from reclaimed fields increased year by year, but it was still lower than that from matured fields even four years after reclamation. This was attributed partly to few chances to have been invaded by plant pathogens, and partly to species and nature of soil microorganisms in reclaimed fields. Results obtained suggest that careful attention must be paid to the control of soil-borne diseases in reclaimed fields for at least more than four years after reclamation. The results also indicate that crop rotation or growing of grasses may be effective for increasing the population of antagonistic microorganisms in soil.

View full abstract

A previously undescribed disease of the yellowing type was found to occur on cucumber and muskmelon in commercial greenhouses in Kanto area. Long, flexuous, rod-shaped virus particles, 12×1, 000nm approximately, were detected in dip preparations from diseased plants of cucumber and muskmelon. The detection was not easy, because the particles were small in number and labile in nature. The particles were also observed in thin sections prepared from both naturally and artificially infected plants of cucumber and muskmelon. Their presence was always restricted to phloem cells, but sometimes they were found in xylem parenchyma cells of vascular bundles. Virus-infected cells were also characterized by the development of amorphous granular structures and small vesicles containing fibrous materials in the cytoplasm. Both of them seemed to be closely related to virus multiplicaiton, and was considered to be viroplasms. Phloem necrosis was usually observed in diseased plants. It caused a remarkable accumulation of starch grains in chloroplasts in mesophyll cells. The virus was transmitted to healthy seedlings of cucumber and muskmelon by viruliferous greenhouse whiteflies, but not by inoculation of sap. From these results, the yellowing disease of cucumber and muskmelon was concluded to be caused by the long, flexuous, rod-shaped virus, which was named cucumber yellows virus.

View full abstract

In 1978, a new disease of rice broke out in Hokkaido, the northern part of Japan, and the causal fungus was identified as Helminthosporium sativum P.K. & B. (=Bipolaris sorokiniana). Most of the diseased plants were observed in the paddy fields adjacent to spring wheat fields. Brown blotch appeared on leaves and glumes. Temperature at 28C was optimum for infection and disease development. The fungus was pathogenic to wheat and barley in addition to rice.

View full abstract