Sydowia japonica is a pathogenic fungus of the male strobili of Cryptomeria japonica. The fungus is expected as a biological control agent for pollinosis of Japanese cedar to inhibit the anthesis of male strobili. We investigated the rate of infection of S. japonica in male strobili cluster and the isolation frequency from the infected male strobilus to obtain the fundamental information to evaluate the substantivity of efficacy and the possibility of secondary infection in the natural environment. Although the number of male strobili and the infection rate varied widely at each study site, the number of current-year-infected male strobili positively correlated with the number of current-year-produced male strobili and the number of previous-year-infected male strobili, respectively. It is believed that the infection number in the current year depends on both the amount of the infectious source and the amount of new host tissue. The results showed that S. japonica could survive for more than two years in the natural condition.
We surveyed the occurrence risk of needle rust caused by Coleosporium phellodendri and Asian pine gall rust caused by Cronartium orientale on Japanese black pine (Pinus thunbergii) cultivated at bonsai production areas in Takamatsu City, Kagawa Prefecture, in 2016. Our results suggested that the risk of damage due to needle rust would be very low given that Phellodendron amurense, the alternate host, was not found within 300m from the bonsai garden where bonsai trees for export are cultivated. However, we found alternate hosts of Asian pine gall rust within 300m of the gardens. Nonetheless, based on the fact that a roughly 100-year-old Japanese black pine showed no symptoms of Asian pine gall rust, it is likely that the disease had not occurred for at least 100 years, or if it had, proper management had resulted in complete elimination from the gardens. Considering multiple factors, including the life cycles of the two pathogenic fungi, bonsai cultivation and management methods, disease control, and weather conditions, together with the results of the present survey, it is highly unlikely that damage due to these two diseases would occur in the future as well. With regard to the importation of bonsai of Japanese black pine into Europe, which is wary about the invasion of pathogenic fungi that cause needle rust and Asian pine gall rust of Japanese black pine, the risk of their invasion into European countries would be extremely low by taking the same quarantine measures as those implemented for bonsai of Japanese white pine (Pinus parviflora), which have already been exported.
Considerable diversity was recognized in the herbaceous understories of hundreds of temperate tree species (approximately 70 years old) grown in a uniformly managed arboretum. Of these, species whose understory diversities were categorized as being nonexistent (Chamaecyparis obtusa, Cryptomeria japonica), poor (Calocedrus decurrens, Nageia nagi, Cladrastis platycarpa), and extremely rich (Taxodium distichum, Celtis sinensis, Ulmus parvifolia) were identified, and soils around their trunks were collected for assessment of their biological and chemical properties. The results of soil bioassays examining the growth of three herbaceous plants (lettuce, Italian ryegrass [both grown from seeds], and Solidago altissima [a perennial weed, grown from rhizome segments]) as well as the macroconidial germination of the fungus Fusarium cuneirostrum were fully consistent with the degree of understory diversity at the sites of soil collection. Considerably high growth of test plants was observed in soils from around T. distichum, C. sinensis, and U. parvifolia, indicating bio-promotive modification of soils by these species. Of the other five species, soils from around C. obtusa and C. japonica exhibited acidity as high as pH 3.9 and 4.2, and these soils showed the strongest inhibition of the growth of three test plants, and also completely inhibited the germination of F. cuneirostrum. Soil EC levels did not show any correspondence to bioassay results. The above results indicate that tree species may play a considerable role in understory biological diversity.