Among different mechanisms of action, it remains unclear which individual or combined traits could be used as predictors in the selection of the best strains for crop productivity improvement.
identified as potential biocontrol agents and plant growth promoters. Numerous studies of a wide range of plant species revealed a steady increase in the number of Bacillus spp. might be useful in formulating new preparations.
Antagonistic and plant growth-promoting strains of Bacillus spp. promote plant growth via nitrogen fixation, phosphate solubilization, and phytohormone production. Besides being the most promising biocontrol agents, Bacillus spp. improve plant response to pathogen attack by triggering induced systemic resistance (ISR). Bacilli exhibit antagonistic activity by excreting extracellular metabolites such as antibiotics, cell wall hydrolases, and siderophores. produce a variety of compounds involved in the biocontrol of plant pathogens and promotion of plant growth, which makes them potential candidates for most agricultural and biotechnological applications. In the present communication discussion will be limited to the case in which all members of the community are initially equally susceptible to the disease, and it will be further assumed that complete immunity is conferred by a single infection.Bacillus spp. It is difficult to treat this problem in its most general aspect. One of the most important probems in epidemiology is to ascertain whether this termination occurs only when no susceptible individuals are left, or whether the interplay of the various factors of infectivity, recovery and mortality, may result in termination, whilst many susceptible individuals are still present in the unaffected population. In the course of time the epidemic may come to an end. Since the course of an epidemic is short compared with the life of an individual, the population may be considered as remaining constant, except in as far as it is modified by deaths due to the epidemic disease itself. As the epidemic spreads, the number of unaffected members of the community becomes reduced. The chances that the affected may convey infection to the unaffected are likewise dependent upon the stage of the sickness. The chances of recovery or death vary from day to day during the course of his illness. Each infected person runs through the course of his sickness, and finally is removed from the number of those who are sick, by recovery or by death. The disease spreads from the affected to the unaffected by contact infection. The problem may be summarised as follows: One (or more) infected person is introduced into a community of individuals, more or less susceptible to the disease in question. The problem is here carried to a further stage, and it is considered from a point of view which is in one sense more general. Reference may here be made to the work of Ross and Hudson (1915-17) in which the same problem is attacked.
It was with a view to obtaining more insight regarding the effects of the various factors which govern the spread of contagious epidemics that the present investigation was undertaken. (1) One of the most striking features in the study of epidemics is the difficulty of finding a causal factor which appears to be adequate to account for the magnitude of the frequent epidemics of disease which visit almost every population.
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