Armillaria root rot of tea in Kenya : characterization of the pathogen and approaches to disease management

Abstract

The rare occurrence of basidiomata and rhizomorphs constrains diagnosis of Armillaria root rot and identification of Armillaria species in Africa. This has had a negative impact on taxonomic research on the genus Armillaria in the continent, where the existence of various genetic groups of the fungus is more easily demonstrated by techniques that do not depend on the presence of basidiomata. DNA analysis by RAPD, ISSR, and RFLP of the IGS and ITS regions showed that Armillaria pathogenic on tea in Kenya consists mainly of two groups. Neither of these is identical to A. mellea , the species to which the disease has been largely attributed in the past. Based on morphological characteristics we conclude that the more predominant of the two groups is probably A. heimii and that the other group may represent a hitherto undescribed species.Infected root residues in the soil comprise the major sources of Armillaria inoculum from which primary infection of tea starts. From these the fungus spreads rapidly within a plantation. Due to the scarcity of rhizomorphs it is assumed that the principal mode of spread of the fungus is through mycelial transfer at points of root-to-root contact. The available control measure against Armillaria root rot is the eradication of the fungus from infested sites through mechanical removal of the inoculum sources and infected plants. However, removal of inoculum is cumbersome and difficult to carry out manually. An isolate of Trichoderma harzianum was selected that readily colonized wood and strongly reduced viability of Armillaria inoculum. Use of this isolate to control Armillaria would work better when applied in combination with soil disinfestation by solarization. By increasing soil temperatures, solarization for 10 wk completely inactivated woody Armillaria inocula, which could be reduced to a solarization period of 5 wk if it was followed with application of T. harzianum . Soil amendment with coffee pulp, earlier reported to enhance development of T. harzianum , did not increase colonisation of Armillaria inoculum sources by the antagonist but did cause a slight reduction in inoculum viability. It is concluded that a combination of solarization with subsequent introduction of T. harzianum has good prospects for managing Armillaria root rot