Late infection of citrus melanose upon Satsuma mandarin fruit

Abstract

For the purpose of surveying damage by late infection of citrus melanose, the author carried out inoculation experiments upon Satsuma mandarin (Citrus unshu) fruits with the causal fungus, Diaporthe citri (Faw.) Wolf, during the period from September to November. The method of experiment was as follows: two kinds of infected twigs, as inoculum, were hung near above fruits on trees for several days. The one was the twigs artificially inoculated with the causal fungus in test tubes, while the other was the dead twigs gathered from an old tree of Natsudaidai (Citrus natsudaidai) in the field. The spore horns of the fungus had been abundantly produced on those twigs. Fruits were covered with paraffin paper bags in order to protect them from natural infection.
Surveys on late infection of the disease both in the fields and in packing houses were carried out at three places in Hiroshima prefecture.
Results of these experiments and surveys are summarized as follows:
1. By means of inoculation using infected twigs, the symptoms of melanose made their appearance on Satsuma mandarin fruits even in early November. The occurrence of the disease seemed to depend primarily on quantity of rain fall, but the ages of fruit were proved to be of a rather minor factor for the infection.
2. Two types of symptoms appeared on Satsuma mandarin fruits. The one, caused by inoculation before the end of August, was characterized by blackish brown protrusive pimples by slight whitish border. The other, caused by inoculation after September, showed blackish surrounded brown pimples surrounded by greenish halo.
3. The results of survey in the field demonstrated that damage caused by late infection are as much as those by earlier infection in summer.