Quantitative genetic control of Mycosphaerella resistance in Eucalyptus globulus and impact on growth

Fungi of the genus Mycosphaerella is one of the major leaf disease of Eucalyptus globulus worldwide. The main species that infect E. globulus in southern Australia are M. cryptica and M. nubilosa. M. nubilosa is mainly confined to the juvenile foliage whereas M. cryptica may occur on both foliage types. Mycosphaerella damage to E. globulus plantations can be severe and the risk of disease damage is one of the main reasons for the shift towards planting the more resistant E. nitens in northern Tasmania. We examined the quantitative genetic variation in susceptibility to infection by M. nubilosa in a genetically diverse population of E. globulus families growing in a field trial in north-west Tasmania. The trees were two years old and still entirely in the juvenile foliage stage when a heavy epidemic of M. nubilosa occurred. Disease incidence was uniform across the trial and the mean leaf area damage (severity assessed as % of necrotic of lost leaves on whole tree basis) was very high at 34%. Significant genetic variation for susceptibility was detected with a narrow-sense heritability of disease severity of 0.6 being the highest yet reported for a Mycosphaerella disease of eucalypts. We followed the effects of this disease outbreak on growth up to age 7 years and found that M. nubilosa damage had a significantly deleterious impact on tree growth at both the phenotypic and genetic level. At age 7, the top 10% of families had a mean DBH 20.8% greater than the trial mean. Approximately half of this gain would have been achieved by early selection for disease resistance (9.1%) or height (11.0%) at age 2, with a time advantage of 5 years. This is similarly the case for selection of above average families. It is likely such gains would be reduced in homogenous plantings of resistant genotypes, or if genotype x environment interactions are significant. Nevertheless, a large component of this gain is likely to be due to disease resistance per se, and collection of seed from