Latent Infections by Bacteria

Abstract

Bacteria may be present in plant tissues without effecting any gross change in form or function. According to Gaumann (25) an infection of this type is termed inappar­ ent or symptomless, and the individuals affected are known as carriers. However, whether this condition of the plant is recognized as latent or diseased depends on the sensitivity of the physiological or microscopic techniques by which the state of the host plant is assessed (86). In any disease there is a latent period during which symptoms are not evident; in some latent infections there may be a greatly prolonged period of incubation due to resistance in the host or presence of some environmental factor, such as temperature, which is unfavorable for the pathogen. When conditions change, the disease is expressed. A change in the virulence of the pathogen while resident in healthy tissues may also lead to an expression of disease symptoms in a previously latent condition. Latent infection may imply low numbers of the pathogen. Internal tissues of some plants usually considered to be sterile may contain relatively large numbers of bacteria that may become parasitic under certain conditions. In this review the concept of latency is restricted neither to temporarily symptomless infections that subsequently break out nor to latent pathogenic bac­ teria. Latent bacteria occur within, rather than upon the surface of the plant, but this distinction is not clear; there are, for example, bacteria in the depressions at the juncture of anticlinal walls of epidermal cells, sometimes in clusters and often in lines (45, 47). Pathogenic and saprophytic b� ;teria may occur within the buds of healthy plants (17, 48-51). Survival of pathogenic bacteria in buds is also a factor in the overwintering of certain diseases of perennial hosts, such as blight and canker of stone and pome fruits caused by Pseudomonas syringae (20, 23, 48). Distribution of bacteria on root surfaces is three-dimensional rather than planar. Examination of sections of epidermal cells of the root system of plants has shown the presence of bacteria embedded in a discontinuous matrix up to 5 /Lm thick and