Studies on the causal fungus of citrus melanose in Japan

Abstract

The authors carried out studies on the morpho-logical and physiological characters of the causal fungus of citrus melanose in Japan, and particu-larly its pathogenicity during the year 1948 and 1949. Results of the experiments are summarized as follows. 1. Several strains of the causal fungus were collected from the various growing centers of the Satsuma orange such as in Shizuoka, Kanagawa, and Wakayama prefectures. After cultural and inoculation experiments they were all identified to be the same species. Strain No. 1 was isolated from a dead twig of Satsuma orange in the citrus grove of the Tokai Horticultural Experiment Station, Okitsu, and used as the type specimen in the present experiments. 2. By the natural and artificial infection, the causal organism caused symptoms of relanose on fruits, leaves and twigs, and also stem end rot on fruits and die back of twigs. 3. The fungus showed vigorous mycelial growth and abundant sporulation on potato decoction agar, vigorous mycelial growth but poor sporulation on such media as soy agar, corn meal agar, CZAPECK's synthetic ager and HOPKINS' synthetic agar. While both mycelial growth and sporulation were scanty on apricot decoction agar. 4. Optimum temperature for mycelial growth of this fungus is approximately 27-28°C, maximum about 34°C and minimum 7-8°C. 5. Optimum hydrogenion concentration for mycelial growth is about 6 in pH value. 6. According to the inoculation experiments and field observations, , the fruits of Natsudaidai, grapefruit, navel orange and lemon are susceptible to the present fungus. 7. The infection occurred on citrus fruits con-tinuously from June to October when fruits were almost matured, although BURGER and others (5, 21, 27) did not recognize the infection on rather young fruits 4 to 6 weeks after the petal fall. While citrus leaves acquired immunity one month after sprouting. 8. From the anatomical study and bagging experiment in the grove, it was recognized that the causal fungus invades citrus fruits through stomata and cuticle, increasing the cuticular infection in later season. 9. Considering the results of the present investigation, the authors recognize the causal fungus of citrus melanose in Japan is identical to Diaporthe citri (FAWCETT) WOLF (Phomopsis citri FAW) 10. Seasonal difference of infection between Japanese and American citrus seems to depend mainly on the difference of climatic conditions.