Studies on Citrus Melanose Disease of Satsuma Orange

Abstract

In October 1969, the perithecia of citrus melanose fungus, Diaporthe citri (Fawc.) Wolf, were found in the orchard of Satsuma orange in Kanagawa Prefecture. The author conducted experiments to know the time and environmental conditions on perithecia formation, and also to ascertain the virulence of ascospores against citrus fruits comparing with that of pycnospores. Results obtained are summarized as follows:Perithecia of the fungus were found mainly on dead twigs left on the ground and also in some cases on the dead branches or twigs remained on trees. Its morphological characters were accordant with those of Diaporthe citri (Fawc.) Wolf and other workers. The fungus isolated from ascospores produced Phomopsis-type pycnospores on potato dextrose agar and citrus twigs. The perithecia were formed on the twigs of Satsuma orange inoculated with pycnospores. The melanose symptoms of the fruits were induced by the inoculation with ascospores of the fungus. On dead and pruned branches which had been left on the ground of citrus orchard in February to March, immature perithecia were formed in August and September. They became matured in September and October of the same year. In laboratory tests, perithecia of the fungus were formed on the inoculated dead twigs only under high relative humidity over 95%.Symptoms on fruits appeared 6∼11 days after the inoculation with ascospores. In the early stage of fruit development, they were quite similar to those caused by pycnospores, however in September to October, some differences were recognized: that is melanose spots caused by the former were more circular and slightly larger. When the similar amount of ascospores and pycnospores were inoculated on citrus fruits, the symptoms were severe in the former. From these results, it may be concluded that the virulence of ascospores on citrus fruits is higher than that of pycnospores.