Distribution, severity, and spread of Armillaria root disease in Kenya tea plantations

Surveys for Armillaria root disease severity were conducted over a 5-year period in small tea farms (0.5 to 1.0 ha) in the 12 tea-growing districts of Kenya. The disease occurred in all tea districts, but severity was greater in the districts east of the Rift Valley. Disease severity was associated with relative amounts of residual woody debris, especially roots, from trees and shrubs present when the land was converted to tea plantations. Excavation of tea bushes in disease centers showed that infection of tea bushes occurred primarily by mycelial growth from residual tree roots and from infected tea roots rather than from rhizomorphs. Rhizomorphs were scarce, and rarely involved in infection. They were confined mostly to the surface of the residual tree roots 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 tree roots in the deeper, cooler, moister levels of the soil. Inoculum from residual tree debris in the soil was the most important source of infection 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 spread from infected to healthy tea plants was more important resulting in large disease centers or gaps due to plant death and removal. Currently, soil sanitation by thorough removal of roots of forest trees and prompt removal of infected tea bushes is the best available management practice.