In Kenya macadamia (Macadamia integrifolia Maiden and Betche and Macadamia tetraphylla L.A.S. Johnson) is grown by over 100,000 small scale rural farmers. However root rots and trunk cankers caused by Phytophthora cinnamomi Rands are major production constraints. Macadamia tree death due to the two diseases is currently estimated at 60%. No single effective method exists for management of the Phytophthora induced root rots. Knowledge on variability within species is a pre-requisite to development of strategies for effective disease management. In this study morphological and physiological characteristics of 76 P. cinnamomi isolates recovered from rhizospheres, stems and roots of symptomatic macadamia trees in different regions of Kenya were investigated. Phenotypic variations were demonstrated in radial growth rate, colony morphology and sporangial dimension.  To determine pathogenicity and virulence, green apples were inoculated with each of the isolates.  The isolates differed significantly (P=0.001) in growth rate on apples. There was a significant relationship (X24 =94.1, P<0.001) between colony type and isolate sub-population. Colony morphology was influenced by temperature. Colonies were predominantly petaloid at 24 °C. Thirty five out of the 76 isolates were pathogenic. The homothallic isolates were the most virulent and killed macadamia seedlings 29 days after inoculation.  Phytophthora cinnamomi was isolated 10 cm above the inoculation point from asymptomatic seedlings. These findings of large phenotypic variation among isolates have important taxonomic and disease management implications. This is the first such study undertaken in Kenya. The knowledge generated will be crucial in development of integrated management strategies for macadamia root rots and trunk canker in Kenya.

Keywords: Macadamia, management, Phytophthora, variation, root rots