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Severe Mycosphaerella outbreak hits genetics trial

Matthew Hamilton, Brad Potts, Paul Tilyard, Tim Wardlaw and Dean Williams


Severe MLD outbreak at Gould's Country

A Eucalyptus globulus progeny trial at Goulds Country in north-east Tasmania was affected by a severe summer/autumn outbreak of the Mycosphaerella (Teratosphaera) leaf disease (MLD) in 2010/11.  The trial is part of a series of five progeny trials established on the Forestry Tasmania estate to study the extent of genetic variation in MLD susceptibility –Togari (trial established 2005), Salmon River (2005), Temma (2006), Salmon River (2006) and Gould’s Country (2008).  The recent MLD outbreak at Gould’s Country, unlike previous mild outbreaks at the five trials, was severe enough to cause the partial defoliation of many trees.  It is anticipated that data collected from the severely affected trial will provide valuable information on the impact of such outbreaks on growth and form, and our ability select genotypes more suited to MLD susceptible sites.

Prior to the severe outbreak at the Goulds Country trial, MLD was visually-assessed after trials were exposed to mild outbreaks of the disease.  Disease severity was assessed as the percentage of juvenile leaf area covered with necrotic lesions (mean = 13% at all trials; see article).  Despite the low severity of these MLD outbreaks, significant additive genetic variation was revealed at all trials, suggesting that genotypes could be selected which exhibit minimal necrosis when exposed to mild MLD outbreaks.  Narrow-sense heritabilities (h2) ranged from 0.14 (SE=0.03) to 0.34 (SE=0.06; data not adjusted for crown size or the extent phase change). 


Mild MLD outbreak at Gould's Country

After the severe outbreak at Goulds Country, visual assessment of MLD severity was repeated and expressed as the percentage of leaf area of the entire crown (juvenile and adult) covered with necrotic lesions (mean=32%).  Furthermore, an attempt was made to assess the percentage of the pre-outbreak crown that had been defoliated due to disease (mean=31%), enabling the estimation of the total area of the pre-outbreak crown lost from MLD. 

Significant additive genetic variation was evident for leaf area lost after the severe outbreak when expressed as either a percentage of the retained crown (h2 = 0.30, SE=0.06) or a percentage of the pre-outbreak crown (h2 = 0.41, SE = 0.06).  Encouragingly, genetic correlations between leaf area necrosis after the mild outbreaks at the five trial sites and leaf area lost after the severe outbreak at Goulds Country were, in general, moderately to strongly positive and significantly different from zero - ranging from 0.40 (SE=0.17) to 0.73 (SE=0.08) for leaf area lost as a percentage of the retained crown and from 0.02 (SE=0.18) to 0.66 (SE=0.08) for leaf area lost as a percentage of the pre-outbreak crown.

These results indicate that it is possible to assess leaf area necrosis after mild MLD outbreaks and select genotypes in which less leaf area would be lost in the event of a severe outbreak.  However, leaf area lost is not of direct interest to forest growers and the conclusion to this story will only be known once it has been established that leaf area necrosis after minor MLD outbreaks can be used to identify genotypes that exhibit good growth and form on MLD susceptible sites.  It is proposed that the Goulds Country trial be assessed for growth and form in coming years to address this question.