THE ROLE OF SCLEROTIA AS AN INFECTION STRUCTURE OF SCLEROTIUM SP., THE CAUSAL PATHOGEN OF SCLEROTIAL DIEBACK DISEASE OF CRYPTOMERIA JAPONICA
Y MIYASAWA, N SAHASHI, T KUBONO and S ITO
Tohoku Research Center, Forestry and Forest Products Research Institute, 72 Nabeyashiki, Shimo-kuriyagawa, Morioka 020-0123, JAPAN
Background and objectives
Sclerotial dieback disease causes serious damage of Cryptomeria japonica (Japanese cedar) in Northern Japan. The causal pathogen is characterized by the production of mycelial strands and sclerotia, but no sexual and conidial state have been found. Therefore it is placed in the genus Sclerotium (Sclerotium sp.). The process of forming the necrotic lesions of this disease is as follows: mycelial strands of the pathogen elongate from dead buds and spread on the surface of healthy twigs of the host plants, but never induce any necrotic lesion. After they stop their growth, sclerotia production is initiated by sporadic swellings on them. Then the new necrotic lesions are observed around the newly formed sclerotia. Fruiting bodies of this pathogen are not produced from these scierotia. From these facts, it appears that the sclerotia of this fungus are closely related to the formation of necrotic lesions. However the detailed mechanism of the necrotic lesion formation has not been elucidated. The purpose of this study is to observe the architecture of the sclerotia of Sclerotium sp. and to clarify their role in forming new necrotic lesions.
Materials and methods
Various developmental stages of the sclerotia of Sclerotium sp. formed on the host twigs were collected in early October from Akita Prefecture, Japan. Degree of maturation of those sclerotia was determined by their size and colour. The immature sclerotium was grayish black and slightly swollen on mycelial strands. While the mature one was black and hemispheroidal, and had the small necrotic lesion around itself. The collected sclerotia with the host tissue were fixed, sectioned to approximately 10 Ám and stained with safranine and fastgreen for light microscopy.
Result and conclusion
The mature sclerotia of Sclerotium sp. were comprised of three distinct cell layers: (a) an inner medulla made of filamentous hyphae; It was contact with the host tissue directly and arranged at right angle to the surface of the host tissue, (b) a middle cortex made of globose pigmented cells closely packed, which surrounded the inner medulla, and (c) an outer rind made of tangentially flattened cells, which surrounded the middle cortex. It was found that these hyphae composing the inner medulla penetrated the host cuticule directly and spreaded into the host tissue. On the other hand, the immature sclerotia had more thick and loosely arranged outer rind, a middle cortex similar to the mature one, and imperfect inner medulla smaller than the mature one. This imperfect inner medulla has already penetrated into the host cuticule. Therefore it was suggested that the sclerotia of Sclerotium sp. was the infection structure for the direct penetration into the host. A number of studies, on architecture and function of the sclerotia of many sclerotium-forming fungi were reported by many researchers. It was suggested that most sclerotia of the filamentous fungi were produced on the infected host tissues. Before penetrating into the host tissues again, they usually produced the fruiting body and/or germinated after a resting stage . However the sclerotia of this pathogen were produced on the healthy host tissues, and played an important role in forming the new necrotic lesions. Emmett & Parbery  suggested that the appressoria were the structures adhering to host surface and their basic function was entry into a host. But they have not been defined morphologically. We suggest that the sclerotia of this fugus as described in the present study have a function of appressoria.
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2. Emmett RW, Parbery DG, 1975. Ann. Rev. Phytopathol. 13,147-167.