First Report of Canker and Dieback Disease of Grevillea robusta in East Africa Caused by Botryosphaeria spp.

Abstract

An outbreak of a canker and dieback disease was observed for the first time on Grevillea robusta in Uganda during October of 2001. Disease symptoms included dieback of shoots and branches, lesions and canker formation on the stems, clear or yellow-to-red exudates on stems and branches, formation of clusters of small and deformed leaves, and sapling and tree mortality. Isolations were made from the inner bark and wood of stems and branches in the Masaka and Rakai districts of Uganda in March and August to October of 2003. Samples from the border between healthy and necrotic tissues were plated on potato dextrose agar, vegetable juice agar, and water agar as well as on selective Phytophthora medium. Fungal isolates from several of the symptomatic trees were identified as belonging to the genus Botryosphaeria on the basis of morphological characteristics. Botryosphaeria-like fungi were not isolated from healthy trees. The internal transcribed spacer regions of the ribosomal DNA, as well as parts of the beta-tubulin and elongation factor 1-alpha genes, were sequenced from eight of these isolates. Five isolates matched Botryosphaeria parva in GenBank and one isolate matched B. rhodina. Two isolates matched an unidentified Botryosphaeria sp. previously isolated from Eucalyptus grandis in Uganda (GenBank Accession Nos. AY228102 and AY226852). Pathogenicity tests were performed by inoculating 10 18-month-old G. robusta seedlings with each isolate. A 10-mm3 malt agar plug with mycelium was inserted into a cut in the bark and the wound was sealed using grafting tape. One month after inoculation, the number of seedlings displaying death from the point of inoculation to the top (TM) and whole seedling mortality (SM) were recorded. Six of the isolates produced pathogenic responses in the seedlings: Botryosphaeria sp. isolate 1 (TM 10, SM 2), Botryosphaeria sp. isolate 2 (TM 10, SM 1), B. parva isolate 1 (TM 6, SM 1), B. parva isolate 2 (TM 3, SM 1), B. parva isolate 3 (TM 3, SM 0), and B. parva isolate 4 (TM1, SM 0). Inoculation with B. parva isolate 5 and B. rhodina, as well as sterile agar plugs, resulted in no TM or SM. The canker and dieback disease seems to be widespread in the East African Region since typical symptoms have been observed on G. robusta in Kenya (February 2004) and Ethiopia (February 2006). Botryosphaeria spp. are known to have multiple hosts and have been isolated from several other species in this region, including Eucalyptus spp. (1,2). From these findings, it appears that the same species of Botryosphaeria, which are previously known to be pathogenic on Eucalyptus, also cause serious disease on G. robusta.