DISTRIBUTION, SEVERITY, AND SPREAD OF ARMILLARIA ROOT DISEASE IN KENYA TEA PLANTATIONS

Surveys for Armillaria root disease severity were conducted over a 5-y ear period in small tea farms (0.5 to 1.0 ha) in the 12 tea-growing di stricts of Kenya. The disease occurred in all tea districts, but sever ity was greater in the districts east of the Rift Valley. Disease seve rity was associated with relative amounts of residual woody debris, es pecially roots, from trees and shrubs present when the land was conver ted to tea plantations. Excavation of tea bushes in disease centers sh owed that infection of tea bushes occurred primarily by mycelial growt h from residual tree roots and from infected tea roots rather than fro m rhizomorphs. Rhizomorphs were scarce, and rarely involved in infecti on. They were confined mostly to the surface of the residual tree root s and were found growing freely in the soil in only one tea district. Rhizomorphs were more abundant in higher elevation plantations than in lower elevation plantations, where they occurred only on residual tre e roots in the deeper, cooler, moister levels of the soil. Inoculum fr om residual tree debris in the soil was the most important source of i nfection in plantations of seed origin. Secondary spread from infected tea plants to healthy ones was limited and disease centers were small . In tea plantations derived from clonal cuttings, secondary disease s pread from infected to healthy tea plants was more important resulting in large disease centers or gaps due to plant death and removal. Curr ently, soil sanitation by thorough removal of roots of forest trees an d prompt removal of infected tea bushes is the best available manageme nt practice.