Abstract

A canker and dieback disease was recently reported on Grevillea robusta in east Africa but little was known about its magnitude, distribution and associated pathogens. In our survey of the disease approximately 36% of 17,994 G. robusta trees assessed showed canker and dieback symptoms. Disease index increased from the humid zone (18%) and to the semi-arid zones (67%). Tree mortality increased from 0.3 to 20% respectively on the same scale and was highest in trees less than 7 years old in all zones. Disease index and tree mortality showed positive correlations with drought period which increased from an average of 0.7 months (humid) to 7 months (semi-arid zones). Both variables showed negative correlations with farm altitude. Disease index was higher in the woodlots compared to other types of tree planting in all zones. Two Botryosphaeriaceae species, Neofusicoccumparvum and Lasiodiplodia theobromae known to be pathogenic on G. robusta were highly abundant in severely diseased stems and branches and their occurrence also showed positive correlations with disease severity. From the results of this study, it was recommended that G. robusta should not be planted in the semi arid zones of Kenya, due to its susceptibility to the canker and dieback disease.

Key words: Agro-ecological zones, agroforestry, Botryosphaeriaceae, canker and dieback disease, disease index, Grevillea robusta.