Seasonal periodicity of coffee leaf rust and factors affecting the severity of outbreaks in Kenya Colony

The seasonal periodicity of Hemileia vastatrix Berk. & Br. in Kenya Colony differs markedly in districts east and west of the Rift Valley, this being attributable to the respective rainfall and climatic conditions of the two areas. The outbreaks in the east districts correspond to the monsoon rain pattern of two wet seasons; two disease peaks are always discernible, but their relationship to one another is often greatly affected by out-of-season rains. West of the Rift, the one extended epiphytotic is a result of the more or less continuous rainy season. For any given disease cycle, three clearly defined phases are recognizable. There is also an inverse relationship between altitude and the severity of outbreaks. Where temperature is not limiting, the course and severity of seasonal outbreaks are determined by the interaction of three factors, any one of which may be limiting. These are: (1) distribution and intensity of rainfall, (2) degree of leanness of the tree, (3) amount of residual inoculum present at the end of the dry season.