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Quambalaria shoot blight in Corymbia calophylla: Detection, pathogenicity, and host resistance

Duong, Hoa ThanhORCID: 0000-0001-6183-6141 (2022) Quambalaria shoot blight in Corymbia calophylla: Detection, pathogenicity, and host resistance. PhD thesis, Murdoch University.

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Quambalaria shoot blight (QSB) caused by the non-native pathogen Quambalaria pitereka has been recently introduced to south-west Western Australia (SWWA). It affects the leaves, buds, flowers, and fruits of the keystone tree species Corymbia calophylla (marri) and can lead to the death of young trees. This project examined the potential to establish a disease resistance program for marri to QSB by (1) developing qPCR assays for the rapid and sensitive detection of Q. pitereka from plant tissues, (2) investigating the pathogenicity of Q. pitereka isolates collected in SWWA; (3) screening for QSB resistance of 3-month-old seedlings under glasshouse conditions; and (4) estimating the genetic control of growth traits and QSB resistance in four-, and six-year old trees in two common garden trials containing 165 - 170 open-pollinated families from 18 provenances of marri from across its natural range. As disease expression in QSB is similar to myrtle rust (Austropuccinia psidii), in a glasshouse experiment, seedlings were also screened for resistance to myrtle rust to test whether resistance to both diseases is linked. A qPCR assay for the rapid and sensitive detection of Q. pitereka from plant tissues was developed. Inoculation of seedlings with several Q. pitereka isolates demonstrated that they varied in their pathogenicity ranging from those that caused no disease symptoms to ones that were highly pathogenic in seedlings. There were significant differences in resistance to both QSB and myrtle rust between seedling provenances but no significant correlation between resistance to QSB and myrtle rust. In the common garden field trials, narrow-sense heritability for all growth traits and QSB damage were low to moderate (h2 = 0.11 - 0.26). A correlation between QSB damage and growth traits indicated that fastgrowing provenances were less damaged by QSB disease. The genetic correlation between the same traits at four and six years was strong, and the Type-B correlations were strongly positive for all traits. Both disease incidence and disease severity were assessed and gave similar results, but disease incidence is the easiest to assess in older trees. There was poor correlation between the provenance resistance assessed at the seedling stage and trees at four and six years of age. Provenances from cooler wetter regions showed higher resistance in the field trials than those from warmer, drier areas. Results suggest that selection for QSB resistance cannot be assessed in seedlings but may be reliably undertaken by assessing disease incidence on four-year old trees at the earliest. The results of this study indicate a strong potential for a resistance breeding program to develop populations of genetically diverse and resistant trees.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Environmental and Conservation Sciences
Supervisor(s): Hardy, Giles and Burgess, Treena
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