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Summary
Early height growth and field tolerance to Quambalaria pitereka, the casual agent of ramularia shoot blight, were assessed for eleven provenances of Corymbia citriodora ssp. variegata, nine provenances of C. citriodora ssp. citriodora and eight provenances of C. henryi, within existing trials involving 22 diverse sites in Queensland. Not all taxa and provenances were present at all sites. The age at assessment varied, but was primarily between 10 and 21 mo. Assessments indicated that ramularia shoot blight at a specific site was closely associated with local climatic conditions, with mean annual rainfall being a useful indicator of the potential disease risk. It also appears that the mean annual rainfall at the origin of the provenance is a good indicator of the potential tolerance of the provenance to ramularia shoot blight, with the most tolerant provenances of all three spotted gum taxa originating from sites with more than 1000 mm.
Of the taxa and provenances tested, the Woondum provenance of C. citriodora ssp. variegata has consistently demonstrated good growth rates and high tolerance to ramularia shoot blight across a wide range of sites. It is recommended that this provenance or other similarly tolerant provenances be selected for use in all future C. citriodora ssp. variegata plantations in southern Queensland where annual rainfall exceeds 800 mm. For C. citriodora ssp. citriodora, the Herberton and Kirrama provenances demonstrated good growth rates and high tolerance to ramularia shoot blight across a range of sites. For C. henryi there was little difference between provenances in growth or tolerance to ramularia shoot blight. While general tolerance was low, some good individuals were observed in the Gold Coast and Myrtle Creek provenances.
The results presented in this paper provide guidance towards provenance selection for plantation establishment and on-going tree improvement for the three spotted gum taxa. It is anticipated that within a few years increasing quantities of seed from commercial seed orchards will become available, giving plantation growers a seed supply with considerably greater growth potential and tolerance of ramularia shoot blight than any source currently available from natural provenances.
