DISEASE CYCLE OF JAPANESE CEDAR TWIG BLIGHT
T KUBONO and N SAHASHI
Tohoku Research Center, Forestry and Forest Products
Research Institute, Shimo-kuriyagawa, Morioka, lwate 020-01, Japan
Background and objectives
A twig blight of Japanese cedar (Sugi; Cryptomeria japonica D. Don) was first recognized in 1950 at Amagi, Shizuoka Prefecture, in central Honshu . It is now widespread throughout Japan and is one of the most serious diseases of cedar plantations. Although many forest pathologists have studied the disease after 1950, the infection source had not been discovered . Therefore, it had been very difficult to develop appropriate countermeasures. In more recent years, the disease has again become particularly severe across the range of Japanese cedar plantings. Accordingly, a project to determine the causal pathogen and elucidate the disease cycle was commenced in earnest in 1988. This paper describes these studies and the disease cycle of Japanese cedar twig blight as we now understand it.
Materials and methods
Across all seasons, the search for a causal fungus and investigation of symptom development were carried out in 25-year-old plantations located in the second nursery at Tohoku Research Center in Morioka, lwate Prefecture, in northern Japan.
Results and conclusions
Fungal sampling and inoculation tests resulted in recognition of the causal pathogen in 1993. This fungus was described as a new species Stromatinia cryptomeriae Kubono and Hosoya. The disease cycle was defined in 1994. In nature, apothecia of Stromatinia cryptomeriae are found on fallen dead branches during March to April, just after the continuous snow cover has melted. Field study has shown that primary infection occurs most frequently at the male strobili and occasionally at immature female strobili and twig buds damaged by late frost or attacked by insects (Contarinia inouyei Mani) by ascospores from March to mid-May. The period of cedar pollen dispersal begins in mid-March and approximately corresponds with the period of primary infection. Infections of male strobili by ascospores may be stimulated by nutrients associated with the pollen grains. Incipient mycelial mats appear from the bases of infected male strobili in early June. Therefore, the incubation period before the first evident symptoms is about three months. The mycelial mats continued to grow on the surface of the twigs from June to July. In early August they began to disappear macroscopically, and they could not be seen during the summer season (August). Then purplish discolorations occurred on all twigs where the mycelial mats had been seen to occur. Secondary infections of the lateral branches, caused by infected male strobili failing during late April to early May, form necrotic lesions on the branches from June to early July. Observations by a scanning electron microscope (SEM) show that hyphae from the mycelial mats enter the twig tissue through the stoma; the hyphae directly enter the stoma or it produces infection cushions-like bodies around the stoma, which produce new hyphae to enter the twig tissue. Just after the mycelial mats disappeared in summer season, the color of green twigs and branches which have been supporting the mycelial mats quickly changes purplish red. The discolored portion then shows the necrosis of the twig tissue and finally the death of the twigs. In several years, the infected trees develop into severe "twig blight" and "dieback". The eight stage seasonal infection cycle of Japanese cedar twig blight is presented diagrammatically.
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