Japanese Journal of Phytopathology

The weak photon emission from Japanese black pine inoculated with pine wood nematodes were characterized. The first peak of photon emission appeared within 30 min after inoculation with nematodes, and the second peak was observed about 70 hr after inoculation. The first photon emission lasted for 1 hr, but the second emission lasted over 100 hr longer. Although treatment with dead nematodes and with supernatant of nematode-suspended water both induced the first peak, the second peak was not detected after either of these treatments. Loblolly pine (resistant host plant) inoculated with nematodes emitted the same pattern of photons as that of Japanese black pine (susceptible host plant), although the intensity of emission from Loblolly pine was weaker than that of Japanese black pine. However, photon emission from the inoculated Japanese cedar (nonhost plant) did not increase. From these results, the first and second peaks seem to be related to substance(s) originating from the nematodes and from cell damage caused by nematodes, respectively.

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A new disease of European pear (Pyrus communis L. var. sativa de Candolle), causing black leaf spot and bud rot, was found on cv. Le Lectier in orchards in Niigata Prefecture, Japan, in 1993. Black spot lesions on cv. Le Lectier leaves began to appear in late May and the disease developed rapidly in summer. Severe symptoms are characterized by defoliation of young leaves. The same disease was also observed on cv. General Leclerc. Alternaria sp. was consistently isolated from the diseased leaves and buds. Based on morphological characteristics, the fungus was identified as A. alternata (Fries:Fries) Keissler. In inoculation tests with conidia, the fungus was pathogenic on the original hosts, cvs. Le Lectier and General Leclerc, and re-isolated from lesions of the inoculated leaves. European pear cvs. Ballade, Koshisayaka, La France, Marguerite Marillat and Silver Bell, and Japanese pear cv. Nijisseiki were not affected by the fungus. This is the first report on a disease of European pear caused by A. alternata. Therefore, black spot (kokuhanbyo in Japanese) of European pear was proposed for the name of the new disease.

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In observations of tumor of melon (Cucumis melo) caused by Streptomyces sp., most tumor tissue formed at the branching of roots. Cross sections of tissue from spontaneous tumors were stained with thionin-orange G and observed by optical microscopy, showing that microorganisms including the pathogenic Streptomyces sp. were located on the tumor surface and in the epidermal intercellular space of tumor tissue. The boundary between the root and tumor was also stained, as were the tissues surrounding the vascular bundles and the exterior-layer of tissue of branched roots containing tumors were also stained in the same manner. Scanning electron microscopic observation of the diseased roots after artificial inoculation with the pathogen without contamination revealed large numbers of the pathogen that had proliferated on the surface of root tumors. The pathogen was present on the surface of the epidermis and of the endodermal layers exposed by eruption of the branching roots through the cortex. However, few cells of the pathogen were found on the surface of hypertrophied parenchyma cells that were exposed by the broken endodermis. The pathogen was observed only in the intercellular spaces of the first and second outer layers of tumor tissue. Tumor tissue was mostly found on branching roots or vascular bundle structures. Root tumors were composed of hyperplastic tissue, comprised of parenchyma cells in the endodermis, pericycle, and inner cortex or around the vascular bundle structures. In addition, the outer layer of cells of tumor tissue were hypertrophied. The surface of the tumor had a rough appearance from the warty hypertrophied cells that were exposed on the surface during tumor development.

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In late August 1998, leaf blight symptoms of anthracnose disease and preharvest defoliation were found on ‘Kousui’, the most popular cultivar (cv.) of Japanese pear, in several orchards in Akita Prefecture, Japan. The fungus isolated from diseased leaves was determined to be Colletotrichum acutatum Simmonds ex Simmonds, based on temperature relationship of the colony growth, colony characteristics and conidial morphology, as well as its lesser sensitivity to the fungicides benomyl and diethofencarb added in potato dextrose agar. This is a new anthracnose pathogen on Japanese pear, in addition to Glomerella cingulata (Anam.: C. gloeosporioides) hitherto known to cause anthracnose disease on Japanese pear. The isolates of C. acutatum were pathogenic to the leaves of Japanese pear ‘Kousui’, ‘Housui’, ‘Choujuurou’, ‘Nijisseiki’, ‘Yasato’ and others in artificial inoculation tests. The strobilurin fungicides, azoxystrobin (100 μg/ml) and kresoxim-methyl (235 μg/ml) effectively controlled the disease in field experiments.

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