Studies on Saprophytic Survival of Xanthomonas citri (Hasse) Dowson

1. Detection of the bacterium from a grass (Zoysia japonica)

Abstract

Xanthomonas citri (Hasse) Dowson could be detected from a grass, Zoysia japonica, growing along the paths in citrus groves all the year round. The isolates obtained from the grass were phage-typed. Frequency of each phage-type was as follow: phage-type A…11 isolates (38%), B…13 isolates (45%) and C…5 isolates (17%). Population density surviving on the grass was generally less than 103 cells per gram samples. It was interested that the isolation frequency of phage-type C from the grass was higher than that from diseased citrus plants.
Detection ratio of X. citri from Zoysia japonica was generally higher in late autumn through winter than in summer. Phage-type A and C were frequently detected in winter, whereas phage-type B in summer.
To investigate whether X. citri detected from the grass had originated in direct dispersion from the canker lesions by rain splash, the phage-types isolated from Zoysia japonica were compared with those from diseased citrus trees grown nearby. Only the isolates belonged to phage-type B were isolated from canker lesions on Unshu trees which were grown most close to the grass. On the other hand, phage-type A was found on the other citrus trees such as navel orange, lemons, Natsudaidai etc., which were grown 80 to 100m away from the colonies of the grass, and interrupted by the three lines of tall trees of Pondocarpus chinensis planted as wind breaks. No isolate of phage-type C was detected from any citrus plants. From these facts, it was considered that the phage-type C had been surviving on Zoysia japonica independently from the diseased citrus trees. The phage-type A was also assumed to survive fairly long period of time on the grass because it was repeatedly detected from the grass even several months after typhoon which might be responsible for dispersion of the bacterium from the lesions. The population density of phage-type B in canker lesions of Unshu trees quickly declined by autumn so that the isolation of the bacterium after late autumn became very difficult by the conventional methods. Nevertheless, the phage-type B was isolated from the grass in winter through the next spring. Therefore, the phage-type B was also considered to survive on the grass for at least several months under natural conditions.
X. citri could be isolated from the colonies of Zoysia japonica which were collected at the distance of 600m from citrus trees. However, detection of the bacterium was difficult from the grass in area where no citrus tree was grown.