Relationship Between Defoliation and Disease Severity in Citrus Canker

Abstract

The relationship between disease severity and leaf abscission was analysed with leaves of Natsudaidai (Citrus Natsudaidai) infected by Xanthomonas citri. When young shoots were inoculated by spray method under field conditions, defoliation occurred on more than 80% of leaves when lesions covered as much as 10% of leaf surface area. In leaves with the lesions more than 20% of the leaf surface area, 100% defoliation occurred. These figures corresponded approximately to one large lesion (4.5mm in average diameter) per cm² leaf or 15 to 20 lesions per mature leaf. Most of the leaves with the lesions covering less than 5% of the total leaf surface area did not fall throughout the season. When the leaf infiltration method was applied to inocu-late leaves of young seedlings in greenhouse, leaf abscission was directly related to the leaf surface area covered with lesions. The most of defoliated leaves belonged to those with about 20% leaf surface area covered with lesions. This was significantly larger than that in spray inoculation in field. The rate of absission increased when the site of inouclation was set in lower half of the leaf, particularly close to the joint of leaf blade to leaf stipule.