THE INFLUENCE OF ENVIRONMENTAL CONDITIONS ON THE DEVELOPMENT OF THE ANGULAR LEAF‐SPOT DISEASE OF COTTON

Abstract

SUMMARY.1. Experiments on the influence of variations in the environmental conditions on the bacterial disease of cotton plants caused by Bacterium malvacearum are described.2. A regular diurnal variation in soil temperature is shown to have the same effect on primary infection of seedlings as a constant temperature near the mean of the fluctuations.3. The mean soil temperature at the time of sowing and for the first few days of germination is the chief controlling factor in primary infection, other factors being equal. Subsequent variations in the soil temperature have little effect on the incidence of the disease.4. The amount of disease resulting from infection of the seed, i.e. primary infection, is higher at soil moisture contents approaching saturation than at normal moisture contents in a given type of soil.5. The amount of primary infection at a given soil temperature and moisture content varies with the type of soil.6. A regular diurnal variation of air temperature has the same effect on secondary infection resulting from spray‐inoculation of young plants as a constant temperature near to, or slightly above, the mean of the variations.7. Other things being equal, the amount of infection resulting from spray‐inoculation depends upon the mean temperature prevailing during the incubation period of the disease, the actual temperature at the time of inoculation being unimportant.8. Atmospheric humidity is a conditioning factor in secondary infection only during a short period (less than 48 hours) following inoculation. Its importance lies in its control of the time during which the infection droplets persist. Once penetration of the tissues has been effected variations in the external humidity have little direct effect.9. Plants kept in total darkness are relatively resistant to infection.10. Plants grown in continuous light are no more susceptible than those grown under a daily period of 17 hours' illumination of approximately 1200 foot‐candles.11. The time of inoculation in relation to the period of illumination does not appear to have any marked effect on the amount of infection.12. The relations of the whole series of experiments on the influence of environmental conditions are discussed.