Abstract
This paper describes the times of apothecium formation, primary infection, secondary infection and the disease cycle of Japanese cedar twig blight in northern Japan. In nature, apothecia of the causal fungus Stromatinia cryptomeriae KUBONO and HOSOYA were found on fallen dead branches during March to April, just after the continuous snow cover had melted. Apothecia with mature ascospores were observed from early March to late April. From field study, it was found that primary infection of the male strobili and twig buds occurred from March to the middle of May. The pollen in male strobili begins to disperse in mid-March. This coincides with the period of abundant production of apothecia. Infections of male strobili by ascospores may be stimulated by the pollen grains as nutrients. Incipient mycelial mats appearing from the bases of infected male strobili were observed in early June. Therefore, the incubation period for the appearance of the first symptom was about three months. Secondary infections of the lateral branches were caused by falling, infected male strobili from late April to early May. The symptom of secondary infections appeared as necrotic lesions on the lateral branches from June to early July. In view of the seasonal development, the infection cycle of the Japanese cedar twig blight is divided into eight stages as follows: 1) Formation of apothecia of S. cryptomeriae on fallen dead branches; 2) Primary infection of male strobili and twig buds by ascospores; 3) Development of mycelial mats on twigs ; 4) Secondary infection of lateral branches due to fallen, infected male strobili; 5) Development of mycelial mats in relationship to the necrotic lesions on the branches; 6) Formation of sclerotioid bodies on necrotic lesions on branches; 7) Formation of acervuli of the Gloeosporidina cryptomeriae KUBONO on necrotic lesions; 8) Occurrence of the most typical symptoms, the discoloration of primary branch. The infection cycle of Japanese cedar twig blight is shown diagrammatically.
