Of 109 soils collected from various location in Kauai, Molokai, Maui and Hawaii Islands, 15 soils and 6 soils were condisered pathogen-and disease-suppressive soils for Rhizoctonia solani respectively, and pH of these soils were acid (3.8-4.7). Suppressiveness was eliminated completely when pH of suppressive soils were adjusted from 3.8 to above 5.6, while suppressiveness was not recovered when the suppressive soils adjusted to alkaline were readjusted to acid equivalent to original soil pH. Elimination of suppressiveness also was shown in adjusted alkaline soil extract from suppressive soils. The concentrations of several elements in soil extracts were determined with atomic absorption spectrophotometer. Water-soluble Al concentration in suppressive soil extracts was higher than conducive soil extract and hyphal growth of R. solani and ascospore germination of Neurospore tetrasperma were inhibited in soil extract. Inhibitions also were observed in equivalent concentration of AlCl_3・6H_2O solution to that of soil extract. When suppressive soil was mixed with conducive soil, the suppressiveness decreased with increasing proportion of conducive soil added. Suppressiveness was partially nullified by amendment with 10,000 ppm streptomycin, 500 ppm rose bengal, 3,000 ppm Botran and 600 ppm Dexon. Suppressiveness was restored in autoclaved suppressive soil by reinfestation with either suppressive soil suspension or conducive one, while reinfestation of autoclaved conducive soil did not convert into suppressive one. Two actinomycetes were screened as antagonists for damping off of green mustard caused by Rhizoctonia solani. These results indicate that soil acidity, antagonists and water-soluble aluminium are responsible for suppressiveness of R. solani-suppressive soil in Hawaii Islands.

View full abstract

In mountainous and cool summer districts in and around Takayama City, Gifu Prefecture, spinach (Spinacia oreacea L.) is successively cultivated, three times, from May to October. During the last few years, some serious root diseases of the plant have occurred in summer and early fall croppings. Among the fungi isolated from the diseased spinach during the growing season, Pythim spp., Rhizoctonia solani and Fusarium oxysporum were obtained most frequently. Other pathogenic fungi, such as Aphanomyces sp., F. solani and binucleat Rhizocotnia sp. were less frequent. Damping-off of the cotyledon and 2-4 leaf-stage of the seedling caused by P. aphanidermatum, P. paroecandrum and R. solani was most serious during the summer cropping, and spinach wilt caused by F. oxysporum f. sp. spinaciae and Rhizoctonia foot rot at more than 5 leafstage from June to September were followed. The isolates of R. solani were assigned to 5 anastomosis groups (AGs). AG-4 was most dominant at all stages of spinach development, and AG-2 Type-2 isolated frequently from the mature plant was followed. Fusarium wilt pathogen has widespreaded throughout the spinach cultivated area, even in the field where the wilt incidence has never been found, and also isolated from the roots of some weed plants grown in the fields. The optimum temperature for hyphal growth of the wilt pathogen was between 25 and 28℃, and the wilt disease was severe above 25℃. The differences of the isolation frequency of the pathogens and of the incidence of the diseases were found among the fields differed in thier cropping history and soil texture.

View full abstract

In attempt to examine the bacterial flora on the roots and in the rhizosphere of cucumber, and soils treated with chemical fertilizer and rice straw, about 200 cultures were isolated. On the basis of morphological and biochemical characteristics, and cellular fatty acid composition, these isolates were classified into ten groups. The strains of P. fluorescens were isolated from the roots, and those of P. maltophilia were isolated from the soils, rhizosphere and roots. The numbers of Alcaligenes sp. 1, Moraxella, and Cytophaga strains were increased on the roots, and in the rhizosphere and the soils by addition of rice straw. The numbers of Gram-postive bacteria and spore formers were proportionally abundant in the soils, and were relatively less abundant in the rhizosphere and on the roots.

View full abstract

Effects of application of compost made from municipal refuse on actinomycete population in soil was investigated. Almost all of the isolates from compost were identified as Nocardiopsis dassonvillei, which were not isolated from soil except just after application of compost. About 90% of the isolates from soil belonged to the genus Streptomyces. Percentage of chromogenic streptomycetes to total streptomycete isolates, which was 75% before application of compost, decreased to 30% after application of compost. Streptomyces phaeochromogenes, the most frequently recorded species in this soil, accounted for about 30% of the total streptomycete population. A possible reason for the predominance of S. phaeochromogenes is considered to be its capability of utilizing a great number of carbohydrates. However, the proportion of S. phaeochromogenes in soil decreased to 7.5% after addition of compost. N. dassonvillei strains isolated from compost were shown to have a higher optimum pH for growth than actinomycete isolates from soil. Relationship between the predominance of certain species in soil and its growth response to the different pH regimes is discussed.

View full abstract

Morphological, physiological and biochemical characteristics of 65 oligotrophic isolates from terrestrial environments and the distribution of oligotrophic bacteria on organic debris and rice roots in a paddy field soil were investigated. All of the isolates did not grow on full-strength nutrient broth (NB) but grew on a 100-fold diluted NB, and were called "DNB organisms". They were divided into 4 groups on their cellshape: (1) regular rods (Group I, 23 isolates), (2) filament-forming rods (Group II, 13 isolates), (3) irregular rods (Group III, 27 isolates), (4) appendaged organisms (Group IV, 2 isolates). All the isolates were aerobic, nonspore-formers. Almost all the isolates were gram-negative. Twenty-two isolates were motile and possessed polar flagella. Twenty-one isolates utilized fenolic acids such as ferulic acid or P-coumaric acid. Ten isolates showed acetylene-reducing (nitrogenase) activity under the atomosphere of 89% Ar, 10% C_2H_2 and 1% O_2. These nitrogen-fixing isolates were characterized chemotaxonomically and their taxnomic status wasdiscussed; five isolates (Group I) corresponded to Psedomonas and for the other 5 isolates (Group III), Agromonas oligotrophica gen. nov., sp. nov. was proposed. In both manured and unmanumed soils, DNB organisms were predominant in the bacterial communities on organic debris and rice roots throughout most of the rice cultivation, although a transient decresease in the portion of DNB organisms was observed immediately after an application of manure. Many DNB orgaisms from the rice roots showed nitrogenase activity and were identical with A. oligotrophica in the morphological and biochemical characteristics.

View full abstract

Most of the studies on rhizosphere have been concerned with the plants grown in the upland soils. Even a few studies which dealt with rhizosphere of rice plants disregarded the specific physiological properties of the rice plants and reduced condition of the submerged paddy soils. In order to know the general characteristics of rice rhizosphere, R/S ratios were first measured with rice plant grown in flooded soils and abnormally low R/S ratios were obtained for aerobic heterotrophs. As the causes of abnormally low R/S ratios, (i) genetical characteristics of rice plant, (ii) low concentration of microbial substrates exuded from rice roots, and (iii) reduced condition of flooded soils were considered. But the results that R/S ratios of rice plant during upland nursery stage were as high as those of upland plants, that the R/S ratios of anaerobic microorganisms were very high during flooded stage and that the concentration of exuded sugars, amino acids and organic acids were higher during flooded stage than during upland nursery stage denied the above (i) and (ii) considerations. The numbers of aerobic microorganisms decreased as the soil Eh decreased in both rhizosphere and non-rhizosphere soils. More reduced condition at each stage in the rhizosphere caused the low R/S ratios, though the numbers of aerobic microorganisms were higher in the rhizosphere at the same Eh level. The degree of Eh decrease and the physiological properties of rice plant also influenced the succession of microorganisms and precipitation-redissolution of iron oxides on root surfaces and microbial activities in the rhizosphere. The disappearances of Polymyxa graminis like organisms (P. graminis is a strict aerobe) and precipitation-redissolution of iron oxides occurred around maximum tillering stage and this phenomena did not change with different soil treatments. Several gases were produced successively from the rhizosphere (CO_2→N_2O→N_2→H_2→CH_4). Carbon dioxide, N_2O and N_2 were produced during tillering stage. The time of H_2 production coincided with the maximum tillering stage (the time of sharp Eh fall). Fe^<2+> production was also active at this stage. After Eh reached the very low level, CH_4 production was observed. From the authors' researhes, the general (specific) characteristics of rice rhizosphere were known to originate from the reduction of the rice rhizosphere and physiological properties of rice plant.

View full abstract

The inputs of nitrogen from atmosphere and medium into the symbiotic system of Azolla were evaluated with the aid of ^<15>N using ammonium, urea and nitrate as combined-nitrogens in the medium. The uptake rates, which showed concentration-dependent curve, were the fastest in ammonium and the slowest in nitrate when compared at the same concentration. The acetylene reduction activites were reduced to about one half of control by the additions of ammonium and urea at 10 mM, and nitrate at 25 mM to the medium. The contributions of medium nitrogens to nitrogen budget in Azolla were about 50, 40 and 20% for 5 mM of ammonium, urea and nitrate, respectively. As a consequence of the balance between nitrogen inputs from atrosphere and meidum, the positive nitrogen budget was observed in the case of ammonium and urea. Nitrate brought about no net increase in total nitrogen. Anabaena azollae isolated from the fern which were incubated in the medium containing ^<15>N-labelled ammonium and nitrate was enriched with ^<15>N in total nitrogen and free ammonium. The result suggests that there is a bidirectional movement of nitrogen between the host plant and the symbiont or the there are two types of the symbiont which depends on either atmosphere or medium for the nitrogen requirement.

View full abstract