Since the first report of Teratosphaeria nubilosa (Cooke) Crous & U.Braun in Uruguay in 2007, young plantations of Eucalyptus globulus Labill. and E. maidenii F.Muell. have been severely damaged by Mycosphaerella leaf disease. The genetic variation in disease resistance and in the timing of heteroblastic phase change was examined in 194 open-pollinated families of E. globulus and 86 families of E. maidenii growing in a field trial in south-eastern Uruguay, naturally infected by T. nubilosa. Disease severity, precocity of vegetative phase change and tree growth were assessed at 14 months. E. globulus was significantly more susceptible to T. nubilosa than was E. maidenii, presenting higher severity of leaf spots (10.6% and 5.6%, respectively), higher defoliation (31.9% and 22.9%, respectively) and higher crown-damage index (39.1% and 27.4%, respectively). However, the heteroblastic transition began significantly earlier in E. globulus than in E. maidenii, with 34.1% and 2.8% of the trees having some proportion of their crown with adult foliage at 14 months, respectively. Significant individual narrow-sense heritabilities were found in E. globulus for severity of leaf spots (0.40), defoliation (0.24), crown-damage index (0.30) and proportion of adult foliage (0.64). Additive genetic variation in E. maidenii was significant only for defoliation and crown-damage index, with a moderate heritability (0.21 and 0.20, respectively). Although E. maidenii was more resistant to T. nubilosa than was E. globulus, the degree of resistance was not enough to consider this species as an alternative to E. globulus for high-risk disease sites. In addition, the small genetic variability for resistance on the juvenile foliage and the late transition to adult foliage suggested that the chances for early selection in E. maidenii are quite limited. By contrast, the genetic variation in E. globulus clearly indicated that through selection for resistance of the juvenile foliage, and especially by selecting for early phase change, it is possible to obtain genetic stock suitable for sites with high risk of T. nubilosa infection.

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