Out of 1,067 clinical sheets recorded by about 270 “tree doctors” in an nationwide investigation on large-sized old trees in the years from 1996 to '98, 1,041 sheets bearing exact description of trunk girth were chosen for the present study. After the variations of decline recorded on the sheets were sorted into three major categories, i.e., decay of wood, mechanical damage and debility (or weakening), and a few sub-categories, the relationships were examined between the frequency and/or extent of respective categories or subcategories and any of the following three items of the trees, i.e., the trunk size (girth at the breast height), the habitats or the agents causing such decline. Two features of tree decline became clear by this examination. (1) When trees are comparatively young and small, the decline is mostly represented by those caused by parasitic attack to the living parts of the tree bodies. And as the trees become older and larger, the decline caused by saprophytic attack to the wood increases its rate, (2) The decay of wood is found mainly in the trunks of more than 80% of trees in every girth class. The mechanical damage is caused by too heavy branch-cutting on trees of smaller girth, by weather damage on those of larger girth. The mechanical injuries to root system by repeated human treading gives damage to the trees in every girth class. The deterioration of chemical and physical properties of soil causes much more weakening of trees than do the injuries by pests and diseases. On the other hand, the individual examination of two conifers and three broad- leaved species, selected by the plenty of record sheets, expresses some similarity to the whole as well as speciality in the trend and features of their decline.

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Since 1997, a hitherto unrecorded disease has been observed on Stewertia pseudocamellia trees planted in Ibaraki, Saitama and Chiba Prefs. It occurred seriously in 2000 not only on S. pseudocamellia but also on S. monadelpha. Many small circular spots occur on leaves and they soon enlarge to 10-20 mm in sizes widely surrounded by scarlet or yellowish discoloration. Diseased leaves are successively defoliated. Lower surface of the spotted areas is covered with white powdery masses of conidiophores and conidia. Disease could develop on sound leaves by artificial inoculations spraying each of mycelial or conidial suspensions. For this disease on Stewartia, “scarlet leaf spot” is proposed as a new disease name. Causal fungus was tentatively identified as a species of the genus Ramularia Unger with the confused concept.

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