Stem canker and dieback disease on Grevillea robusta Cunn ex R. Br.

Abstract

A widespread stem canker and dieback disease threatened the cultivation of Grevillea robusta on farms in Kenya. Disease index increased from 18% in the humid zone to 67% in the semi-arid zones and strongly correlated with altitude and length of drought period in the zones. Analysis of morphological characteristics and molecular data of the internal transcribed spacer (ITS) rDNA revealed that forty fungal species were associated with G. robusta. Seven species of the Botryosphaeriaceae comprised 42% of the total isolations. Five fungal species known to cause canker and dieback symptoms on woody tree species, Neofusicoccum parvum, Lasiodiplodia theobromae, Diplodia seriata, Botryosphaeria sp. and a Phomopsis sp. were selected for pathogenicity tests on four agroforestry tree species: Grevillea robusta, Senna siamea, Azadirachta indica and Melia volkensii. Neofusicoccum parvum was the most virulent pathogen, L. theobromae, Botryosphaeria sp, and D. seriata were moderate and Phomopsis sp. was the least virulent. Under hot conditions, G. robusta, S. siamea and A. indica were most susceptible to the Botryosphaeriaceae pathogens and the only native species M. volkensii was least susceptible. The symptoms caused by the Botryosphaeriaceae species in the field and laboratory conditions were indistinguishable leading to the suggestion that they formed a disease complex. Phylogenetic analyses showed that the fungal species associated with G. robusta also occurred in other plant species. The plurivorous nature of these pathogens threatens trees as well as crops in agroforestry systems. The study recommended that species site matching should be emphasized in agroforestry systems with a focus on native tree species such as M. volkensii.