SPOTTED GUMS – Dealing with the Shoot Blight problem
The Corymbia “Variegata” being grown by Trees Impact provide an exciting Spotted Gum alternative to the much desired but problematic, Corymbia maculata.
What we know as “Spotted Gum” extends over a range from the South Coast of NSW to Southeast Queensland. It has been a long-favoured “Gum” for use in East Australia, both as a landscape tree and as a plantation species, because of the strength and durability of its timber.
Historically, “Spotted Gum” was Eucalyptus maculata, regardless of where in its range it was found.However,today, following some taxonomical decisions around the turn of the century, the Spotted Gum from the northern half of their range have largely been renamed Corymbia citriodora subsp variegata (Variegata), and the Spotted Gum from the southern sector arenow Corymbia maculata (Maculata).
In all respects, Variegata continues to present as Maculata, including the absence of lemon scent in the leaves. Furthermore, discussions with arborists and academics (SE Qld.), botanists (Royal Botanic Gardens) and foresters (DPI NSW) support our conclusions that, from a practical and structural point of view, Variegata and Maculata are effectively the same thing. Timber composition and strength, branch attachment, foliage, form and ‘spottiness’ all appear to be consistent. The differences are botanical rather than aesthetic or practical.
However, Maculata is very susceptible to attack by a native fungus called Shoot Blight (Quambalaria pitereka). Shoot Blight is such a significant problem that growing healthy specimens in Eastern Qld and NSW plantations and nurseries has become almost impossible. And, given that the issues with Shoot Blight have been escalating over the past 20 – 30 years, in future it may well prove to be a problem in our landscapes. Because of Maculata’s susceptibility to Shoot Blight, both Qld and NSW forestry have turned to Variegata as the favoured Spotted Gum for use in plantations.
Some variation in Shoot Blight resistance has been seen between different provenances of Variegata. For five years, Trees Impact has grown trees from seed sourced from provenances that have shown a high degree of resistance in studies and in the nursery, and we have now settled on a provenance originating in Northern NSW. Variegata are, climatically, well adapted to our target market; and in more southern regions, Variegata grown from seed collected from northern NSW may indeed be better suited than Maculata to use in a warming world.
Trees Impact production of Spotted Gums for landscape use
Maculata can no longer be grown successfully in Eastern NSW production nurseries without the seemingly unavoidable risk of significant damage from Shoot Blight. Selected provenances of Variegata are proving that they can be.
Our trials over the past five seasons are notable for the minimal impact of any Shoot Blight on the large number of Variegata in production at our nursery. That is a striking difference when compared with our experiences growing Maculata over the previous 10-15 years.
Conclusion
While we cannot claim that the Variegata now being grown by Trees Impact will be entirely Shoot Blight resistant, Variegata does provide an exciting and more viable Spotted Gum alternative to the much desired, but problematic, Maculata.
References:
Early plantation growth and tolerance to ramularia shoot blight of provenances of three spotted gum taxa on a range of sites in Queensland.
G R Dickinson, D J Lee and J R Huth: Queensland Department of Primary Industries, AFFS – Forestry Research; October 2003
Growth, form and Quambalaria Shoot Blight Tolerance of Spotted Gums in north-eastern New South Wales Australia.
I G Johnson, A J Carnegie and M Hensen: Silvae Genetica (2009) 58-4, 180-191
Variable resistance to Quambalaria pitereka in spotted gum reveal opportunities for disease screening
G.S Pegg, A.J. Carnegie, M.J. Wingfield, A. Drenth: Australian Plant Pathology Society Inc. 2010
Preliminary results of Corymbia maculata (Syn. Eucalyptus maculata) (SPOTTED GUM) Provenance Trial using Street Tree Selection Criteria.
Sarah Bone, The University of Melbourne, Burnley College. Treenet Proceedings of the 2nd national Street Tree Symposium September 2001.
Forests NSW SPOTTED GUM (Corymbia spp.) Tree Improvement and Deployment Strategy.
H.J. Smith, M. Hanson and S. Boyton. Forests NSW
Wood Properties and uses of Australian Timbers; SPOTTED GUM.
Farms, fishing and forestry. Business Qld.
