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How disease susceptible are Eucalyptus globulus x nitens hybrids?

Brad Potts and Paul Tilyard
University of Tasmania

Figure 1. Mycosphaerella damage (lesions) on juvenile foliage expressed as a percentage of the total juvenile leaf area across three cross types:  E. nitens, E. globulus, and their F1 hybrid.  Significance values are based on log 10 transformed data. Damage values have been back transformed for graphical presentation.  The dashed line shows the (back transformed) mid-parent value and the significance of the corresponding level from the hybrid value (*, P < 0.05; **, P < 0.01). Different letters indicate significance between cross types at the P < 0.05 level.

Eucalyptus globulus is generally considered more susceptible to Mycosphaerella leaf disease than E. nitens.  This has lead to E. nitens being planted in preference to E. globulus in many lower altitude areas of high disease risk in Tasmania.  However, there is only one published study where the two species have been compared directly and the inheritance of the difference in disease susceptibility between the species examined (Dungey et al. 1997).  In their study the samples of the two species did not differ in susceptibility of their juvenile foliage to Mycosphaerella leaf disease but did in their adult foliage. The F1 hybrids tended to be either more susceptible than either species (mainly for juvenile foliage) or at least above the mid-parent.  We had the opportunity to re-test this pattern of susceptibility in a small hybrid trial planted in 2006 in north western Tasmanian (Salmon River).  We assessed Mycosphaerella damage to juvenile foliage on a percentage scale across two years (2007 and 2008; Figure 1). While the trial only contained germplasm from eight mother trees (four from E. nitens and four from E. globulus) the discrepancy in the results from the previous study is noteworthy.  In each year the E. globulus was significantly more damaged than the E. nitens and the hybrids were intermediate but slightly below the mid-parent value. This difference in response to the disease could arise due to numerous factors including differences in the composition of Mycosphaerella species or genotypes causing the disease, differences in eucalypt germplasm, and/or a genotype by environment interaction.   

Biobuzz issue ten, December 2009