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Latitudinal differences in Mycosphaerella susceptibility

Matt Hamilton
University of Tasmania

Figure 1: Most of the trees in the trial at Gould's Country have not yet undergone phase change. The Teratosphaera damage can be seen as brown patches on the leaves [image: Brad Potts]

A Eucalyptus globulus open-pollinated base-population progeny trial, established by Forestry Tasmania in 2008 at Gould’s Country in north-east Tasmania was subject to a Teratosphaera (formerly known as Mycosphaerella) leaf disease outbreak sometime between April and August 2010.  Damage to the juvenile foliage was assessed visually in September 2010, at a stage when most trees (85%) had not undergone change to the adult foliage type (Figures 1 and 2).  Damage was assessed as the percentage of the juvenile foliage that was necrotic. The exact Teratosphaera species responsible for the observed damage is yet to be determined.


Figure 2:  Matt Hamilton visits the Gould's Country E. globulus provenance trial in north-eastern Tasmania.  [image: Brad Potts]

The damage levels averaged 6.2%, which was in the range of the levels previously observed in four sister trials, established in 2005 and 2006 in north-west Tasmania (2.5 - 13.1%).  There were significant differences amongst the E. globulus subraces and their ranking was similar to that observed in the trials in north-west Tasmania.  There was a strong latitudinal cline in the disease susceptibility of the subraces (Figure 1).  The juvenile foliage of populations from Victoria was generally less susceptible to Teratosphaera damage than that from populations in the southern half of Tasmania.

Significant genetic variation within subraces was also observed and the heritability was relatively high (h2 = 0.31), indicating that a substantial proportion of the variation in damage observed among trees was explained by genetic differences.  Furthermore, genetic correlations between Gould’s Country and the far north-west Tasmanian trials were positive, albeit generally weaker than those observed among the trials in north-west Tasmania, and ranged from 0.42 to 0.62.  


Figure 3:  The mean percentage of the juvenile leaf area of each E. globulus subrace that was damaged by Teratosphaera leaf disease following the 2010 infection of the Goulds Country trial.  Subraces are Gippsland Coastal Plain (GCP), Eastern Otways (EOt), Flinders Island (FI), King Island (KI), North-eastern Tasmania (NET), Recherche Bay (RB), Southern Furneaux (SF), Southern Tasmania (ST), South-eastern Tasmania (SET), St Helens (StH), Strzelecki Ranges (StR), Western Otways (WOt) and Western Tasmania (WT).  

The significant genetic variation and the positive genetic correlations at the subrace level and within subraces among sites are encouraging for tree breeders aiming to decrease susceptibility of E. globulus to Teratosphaera leaf disease.  Selection for reduced susceptibility in the trials in far north-west Tasmania or Gould’s country would result in genetic gains across the environments represented by all these trials.  

Biobuzz issue thirteen, December 2011