Characterization of Fusarium graminearum from Acacia and Eucalyptus using beta-tubulin and histone gene sequences

During routine surveys of diseased Acacia mearnsii and Eucalyptus grandis i n South Africa, isolates of an unknown, nonsporulating fungus with a red my celium were collected. Symptoms associated with the disease included branch dieback and stem cankers on both hosts. None of the isolates of the fungus produced spores, making identification using morphological characteristics impossible. An attempt was thus made to identify the isolates using DNA se quence data for the beta -tubulin and histone genes. Using this approach, t he fungus was tentatively identified as Fusarium graminearum. Sequences wer e then compared to those for isolates of F. graminearum from cereal hosts. The relative pathogenicity of F. graminearum to A. mearnsii and an E. grand is clone was determined in pathogenicity trials. Pathogenicity tests were c onducted by inoculating 18-mo-old A. mearnsii trees and 12-mo-old E. grandi s clone trees under field conditions. All the isolates tested produced sign ificant lesions on both the A. mearnsii and E. grandis clones. The isolates collected from A. mearnsii and E. grandis were further compared to other F . graminearum isolates using beta -tubulin and histone gene sequence. In th ese comparisons the iso-lates collected from A. mearnsii and E. grandis con sistently grouped with F. graminearum isolates. The occurrence of F. gramin earum on A. mearnsii and E. grandis is intriguing, and as far as we are awa re, this is the first report of the fungus associated with a disease of a w oody host.