Abstract
Longevity and density of saprophytic survival of Xanthomonas citri (Hasse) Dowson was examined with the leaf infiltration technique on various weeds, dead plant leaves or straws, soils and roots of Natsudaidai (Citrus natsudaidai). The infestation of the materials with X. citri was made by dipping the plant materials into bacterial suspensions with the concentration of approximately 108/ml, or by pouring them in soil samples.
1. Infested with bacteria in early spring, the bacterium survived for 1 to 3 months on weeds, dead rice straw and in soils. With the infestation in autum, however, it survived for 7 months on either intact parts or dead leaves of Zoysia japonica and Vetiveria zizanioides, for 5 months in soils which were kept air drying (Fig. 1).2. The patterns of the seasonal population changes differed depending on the kinds of materials. The population of X. citri maintained for several months the level of about 102 to 103 bacterial cells per gram samples after showing quick decrease at the beginning on intact Zoysia japonica and dead rice straw (Fig. 2, 3). In wet soils kept outdoor, the population quickly decreased in almost linear ways, whereas the rather low level of the population was maintained for the period of 8 months in soils which were left under greenhouse and kept drying (Fig. 4). The survival population on the root surface of C. natsudaidai was characterized by no marked decrease at the beginning and by maintaining the high level for 10 months (Fig. 7).
3. Seasonal population changes of X. citri surviving saprophytically on various materials were compared among three phage-types A, B and C. As shown in Fig. 5-8, the population of phage-type C on Zoysia japonica and rice straw became 10 to 100 times higher than those of the other two types in late autumn through winter season, whereas such clear differences were not recognized in soils.
4. The possibility was discussed from the above evidences as to the existence of two different ecotypes, i.e., the parasitic ecotype, phage-type A and B, and the saprophytic one, phage-type C. Although the former can saprophytically survive under natural conditions, its major habitat would be the diseased tissues of the citrus trees and it perpetuates causing repeated infections on the host plants.
On the other hand, the saprophytic ecotype may survive mainly on non-host vegetations without causing severe infections on citrus plants under natural conditions.
