When mulberry trees, which had been severely infected with the dwarf virus in the previous year, were cut from the basal part of trunk in winter or were allowed to remain unpruned throughout the growing stages, most of these trees developed normal shoots and leaves, although some trees showed only slight symptoms in summer. When the cuttings obtained from these apparently normal trees were grown in field or in greenhouse, most of them remained to be free from the disease, not only at the time of sprouting, but also at their growing stages. When the affected trees with apparently normal shoots or those showing slight symptoms were cut from the base of trunk in summer season, the typical or sometimes severe symptoms appeared. It is considered that the recovery from the disease by the cutting back of affected mulberry trees in winter or no cutting back throughout the year is not attributable to masking of the symptoms, but to the retardation of movement of the virus from the roots, which is accelerated by the cutting back in summer.
Mycelia of Penicillium expansum Link and Botrytis cinerea Pers., and also conidia of the former were irradiated with Cobalt-60 gamma radiation. The irradiation took place at Tokaimura Atomic Center (Cobalt source: 1.6×104C), Ibaragi prefecture and Tokyo Metropolitan Isotope Center (Cobalt source: 0.3×104C), Tokyo. The original cultures of P. expansum and B. cinerea used were isolated from affected apples and strawberries, respectively. The mycelium samples to be irradiated were prepared by removing the mycelia shake-cultured in potato decoction media (with 2% of saccharose) for 3 days at 25°C, washing them with sterile distilled water, and then suspending them in sterile distilled water in Pyrex tubes. The conidium samples to be irradiated were prepared by obtaining conidia with a sterile needle from a well-sporulating 10-day old culture and putting them in small Pyrex tubes. After the irradiation, the samples were transferred to potato saccharose agars, being incubated for 7 days at 25°C. The survival of the irradiated samples transferred was confirmed by their resumpted growth on the medium. The temperatures during the irradiation were 19°-24°C. The results obtained are summarized below: 1. The mycelia of P. expansum and B. cinerea irradiated with the dose of 80×104 rad at a high dose rate (approximately 100×104rad/hr.) were almost completely killed, while the samples irradiated with the same dose at a low dose rate (approximately 2×104rad/hr.) showed resumption of mycelial growth. 2. The minimum doses that lost the conidia of P. expansum the ability to form colony were 40×104 rad at a low dose rate (approximately 1×104rad/hr.), and 20×104rad at a high one (approximatel 40×104rad/hr.), respectively. 3. The irradiation with the doses of 5-40×104rads on the mycelia of P. expansum and B. cinerea delayed their growth and the conidial formation. The sclerotial formation of B. cinerea was also suppressed by the above doses. 4. It should be concluded that the dose rate as well as the dose in the irradiation is one of the factors in determining the sterilizing or fungistatic effect of gamma radiation.
Orymycin, a new antifungal antibiotic, has been obtained from Streptomyces albochromogenes isolated from the soil collected in Tokyo. The antifungal spectrum of Orymycin against several plant pathogenic fungi and bacteria was determined by the agar streak dilution method as well as the cup method. Potato-sucrose agar was used in these tests with the exception of its test with shoot blight fungus of larch which was grown on yeast-starch agar. 1. The antifungal spectrum of Orymycin against plant pathogenic fungi was considerably wide, being especially inhibitory to rice blast fungus (Piricularia oryzae), anthracnose fungi of vegetables and fruits (Colletotrichum atramentarium, C. glycines, C. lagenarium, Gloeosporium kaki, G. laeticolor and Glomerella cingulata) and shoot blight fungus of larch (Guignardia laricina). The minimum inhibitory concentrations of Orymycin against rice blast fungus, anthracnose fungi and shoot blight fungus of larch were detected less than 0.1 or 0.5ppm, 0.5 or 1.0ppm, and 0.5ppm, respectively. 2. It was observed that the resistance to Orymycin varied with different species or isolates of anthracnose fungi and black spot fungi, and with different isolates of rice blast fungus. 3. In rice blast fungus, the isolates which have been cultured for a long time after the isolation from the host plants seemed to be sensitive to Orymycin, while the isolates cultured for a short period appeared to be resistant. On the other hand, no correlation was found between the pathogenicity or appressorial formation type of the isolate and the resistance to Orymycin. 4. It appeared that rice blast fungus was suitable as the test fungus for the cup method, but many isolates of this fungus produced relatively small number of conidia, and accordingly it needed considerably a long incubation period to observe the results. Therefore, it is necessary to find out a better test fungus for the cup method against Orymycin.
This paper reports the results of experiments on the influence of Forsyth's fractions A, B, C, and D of Tiurin's fulvic acid on the growth of Helicobasidium mompa Tanaka and Rosellinia necatrix (Hartig) Berlese, the causal agent of violet root rot and white root rot, respectively. The results obtained are summarized as follows: 1. The Forsyth's fractions A and B showed marked growth-promoting effects on H. mompa and R. necatrix. The same effects were not evident in the case of the fractions C and D. 2. The fraction A was well utilized by both H. mompa and R. necatrix as a carbon source, and the latter also slightly utilized the fraction as a nitrogen source. 3. The fraction B had no nutritive effect on the growth of H. mompa and R. necatrix. 4. The fraction C was utilized by both H. mompa and R. necatrix as carbon and nitrogen sources. 5. The fraction D had no nutritive effect on the growth of H. mompa, but exhibited a marked growth-inhibiting effect on it. The fraction, however, was slightly utilized by R. necatrix as a carbon source, showing no evident inhibiting effect on the growth of the fungus. 6. It seems that the predominance of H. mompa in uncultivated soil and its extinction in old cultivated one are connected with the variation of the organic constituents of fulvic acid in soil, i.e., the increasing fraction D of fulvic acid in cultivated soil retards the growth of H. mompa, and therefore accelerates the decrease of the fungus.