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Early phase change may protect against Mycosphaerella leaf disease

Matthew Hamilton1, Dean Williams2, Tim Wardlaw2 and Brad Potts1
1School of Plant Science
University of Tasmania
2Forestry Tasmania

Figure 1.    Contrast in the timing of the transition from juvenile (right) to adult (left) foliage between same-age trees of E. globulus in a plantation in Western Australia

The process and timing of phase change (Figure 1) in Tasmanian blue gum, Eucalyptus globulus, fundamentally affects its interaction with pathogens and pest species in natural and plantation environments (de Little et al. 2008).  Some pathogens and pests favour juvenile foliage, while others favour adult foliage.  For example, juvenile E. globulus foliage is more susceptible to infection by Teratosphaeria nubilosa (formerly Mycosphaerella nubilosa) than adult foliage (Carnegie and Ades 2005; de Little et al. 2008; Dungey et al. 1997; Milgate et al. 2005); Mnesampela privata (autumn gum moth) preferentially oviposits on soft, glaucous, juvenile foliage (Rapley et al. 2004); Paropsisterna agricola (a chrysomelid leaf beetle formerly known as Crysophtharta agricola) larvae most commonly feed on juvenile foliage; and larvae of P. bimaculata typically feed on adult foliage (de Little et al. 2008).  Early phase change may represent a means of ‘escape’ from the most severe impacts of these pests and pathogens (Carnegie et al. 1994) but in some environments trees that change early may be jumping out of the frying pan and into the fire.  For example, a survey of Tasmanian plantations indicated that the prevalence of Uraba lugens (gum-leaf skeletoniser) and Gonipterus scutellatus (eucalypt snout weevil) tends to increase as the amount of adult foliage in a plantation increases (de Little et al. 2008).  In the case of Mycosphearella leaf disease, the juvenile foliage is susceptible to both Teratosphaeria nubilosa and T. cryptica, whereas adult foliage tends to be mainly impacted by T. cryptica (Carnegie and Ades 2002).  As early transition to adult foliage may allow a tree to reduce foliar damage from T. nubilosa, we examined the patterns of genetic variation in this trait in E. globulus and how this relates to variation in juvenile foliage susceptibility to Mycosphearella leaf disease. 

We assessed the presence of adult foliage on two-year old trees growing in the four progeny trials in which we have been studying the genetic control of juvenile leaf susceptibility to Mycosphearella leaf disease.  These trials were established in 2005 and 2006 by Forestry Tasmania in the north-west of Tasmania, a region known to experience severe Mycosphaerella outbreaks (Mohammed et al. 2003).  They contain genetic material from native-forests (open-pollinated seed from 14 sub-races), as well as breeding (control pollinated) and deployment (open-pollinated and mass-supplementary pollinated) populations.  The breeding and deployment populations in these trials are representative of the genetic makeup of recently-planted and future E. globulus plantations.  The trials provide an opportunity to compare the characteristics, including phase change, of elite populations with unimproved native-forest populations.

Significant differences in the proportion of trees that had adult foliage at age two years were observed among native sub-races (see results for the Togari trial in Figure 2; n=3681) and native sub-race rankings were broadly similar across trials in this (data not shown) and other studies (Jordan et al. 2000; Jordan et al. 1999).  Less variation was observed among 'selected population' means than among the 'native-forest population' means and the overall mean of the 'selected population' was similar to the overall 'native forest population' mean.  This most likely reflects the mix of native-forest populations that make up the E. globulus breeding and deployment populations, of which King Island (later than average phase change, Figure 2), Strzelecki Ranges (average), Eastern Otways (average), Western Otways (earlier), North Cape Barren Island (near average) and Flinders Island (earlier) are most prevalent.


Figure 2.     Proportion of E. globulus trees in native (red) and selected (yellow) populations that had adult foliage at age two years in the Togari Mycosphaerella trial planted in the north-west of Tasmania. The sub-races from which native forest seedlots were collected follow Dutkowski and Potts (1999) (6-12 families per sub-race).  The selected populations are: commercially available seedlots derived from mass supplementary pollination (MSP, 36 families); progeny from selections derived from the Portugese landrace (Portugese, 5 families); seedlots amongst the top-20 for diameter growth at 8-years in a base-population trial established at Woolnorth, Tasmania, (Dutkowski and Potts 1999) which was impacted by Mycosphaerella leaf disease early in its development (Base population top DBH myco site, 7 families); second generation full-sib families from the STBA National Breeding Program (2nd Gen CP, 18 families); open-pollinated seedlots derived from selected trees in a seed orchard at Meunna in NW Tasmania that originated from open-pollinated seed collected from King Island (NW SO OP (King Island), 5 families) and Flinders Island (NW SO OP (Flinders Is), 3 families); and elite seedlots identified in an early analysis of growth in base population trials across Australia (Base pop selected multisite, 17 families).

The 'Base pop top DBH myco site’ population (Figure 2) consisted of native-forest families that had previously exhibited superior year-eight growth in a base population trial that was impacted by Mycosphaerella leaf disease early in its development.  If early phase change (i.e. disease escape) was the principal driver of growth on sites affected by Mycosphaerella leaf disease, native-forest families selected for growth on such sites would be expected to originate from sub-races exhibiting early phase change.  However, a number of the selected families were from King Island, a race that tends to exhibit later than average phase change.  It is also noteworthy that in native stands the genetically-based differences between sub-races in their timing of the transition to adult foliage is not associated with genetic variation in the susceptibility of juvenile leaves to Mycosphearella leaf disease (rsub-race = -0.17 ±0.30; n.s.).  Such independence suggests that there are sub-races of E. globulus which have the potential to both avoid and resist damage by Mycosphearella leaf disease.

Further examination of correlations among phase change, foliar resistance and growth, on disease-prone, pest-prone and ‘normal’ sites, is required to determine the true influence of the timing of phase change on plantation growth across different environments.


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Biobuzz issue twelve, August 2010