The biology, epidemiology and variability of Quambalaria shoot blight of Corymbia species

The recent need to develop a source of high value logs for sawn timber products to replace the declining supplies from native forests has resulted in the rapid expansion of spotted gum plantations in Queensland and northern New South Wales. Quambalaria shoot blight, caused by the fungus Quambalaria pitereka, is a serious disease affecting this expansion (Simpson, 2000; Self et al., 2002; Carnegie, 2007b; Pegg et al., 2008). This study aimed to develop a disease management strategy through investigating aspects of the pathogens biology. Initially we aimed to determine which species of Quambalaria are associated with shoot blight symptoms on Corymbia spp. Surveys identified three Quambalaria spp. — Q. pitereka, Q. eucalypti and Q. cyanescens — from native and plantation eucalypts as well as amenity plantings. Symptom descriptions and morphological studies were coupled with phylogenetic studies using ITS rDNA sequence data. Quambalaria pitereka was the causal agent of blight symptoms on species and hybrids in the Corymbia complex. Quambalaria eucalypti was identified from Eucalyptus species and a single Corymbia hybrid. Quambalaria cyanescens was detected from native and plantation Corymbia spp. Quambalaria spp. are pathogens of growing global importance, yet virtually nothing is known about the manner in which they infect and colonise their hosts. A study of the infection process of Q. pitereka and Q. eucalypti on Corymbia and Eucalyptus species was thus undertaken using light, scanning and transmission electron microscopy after artificial inoculation. For both species, conidial germination was triggered at relative humidity (RH) levels above 90 % and commenced within 2 hours in the presence of free water. Light reduced the percentage germination but did not prevent germination. Conidial germination and hyphal growth occurred on the upper and lower leaf surfaces with penetration occurring via the stomata or wounds on the leaf surface or juvenile stems. There was no evidence of direct penetration of the host epidermis. Following penetration through the stomata, hyphae of Q. pitereka and Q. eucalypti grew only intercellularly without the formation of haustoria or any other interaction apparatus. Instead, the presence of an interaction zone is demonstrated in this thesis. Conidiophores arose through stomatal openings and produced conidia seven days after infection. Incorporation of screening for resistance to Q. pitereka into current breeding programs is essential for the future development of plantations using spotted gum and Corymbia hybrids. An additional part of this study was to determine whether there is variability in susceptibility to infection by Q. pitereka among different species and provenances of spotted gum and to identify variability among and within families of spotted gum. We also considered whether the seed source origin is a significant indicator of susceptibility. Assessments were conducted in trials consisting of spotted gum provenances, families and clones, all at the same site and with high levels of disease pressure and with optimum climatic conditions for disease development. While all species and provenances of spotted gum could be infected by Q. pitereka, results showed that there are high levels of variability in susceptibility among and within species, provenances and families, indicating the potential to select for disease resistance. Provenance was shown to be an unreliable indicator of susceptibility to Q. pitereka. A spotted gum clonal trial provided the opportunity to investigate the impact of the disease on tree growth and factors influencing wood quality. The effect of Q. pitereka on productivity when infection occurs during plantation establishment and the value of deploying resistant germplasm were determined. The results showed that Q. pitereka has a significant impact on plantation establishment and emphasised the negative impact on wood quality and value in sawlog production where unimproved or incorrectly selected germplasm is deployed. Tree growth, based on height and diameter as well as tree form, was significantly improved where germplasm with low susceptibility to Q. pitereka infection was used. Disease development within plantation-grown spotted gum was examined and an understanding of the epidemiology of Q. pitereka and factors influencing disease development established. It was shown that native spotted gum is a primary source of inoculum, followed rapidly by the production of secondary inoculum from infected trees in the plantation. The rate of spread and development of Q. pitereka within plantations increased exponentially over time as additional trees became infected and produced secondary inoculum. Spore concentration was shown to play an important role in disease development with disease severity increasing with increasing disease incidence on individual trees and incidence across the plantation. An understanding of pathogen variability and host-pathogen interactions is needed to develop management strategies for the disease. The aim of the study was to determine if there was any difference in host range of Q. pitereka isolates and any variability in aggressiveness of these isolates. It was evident for the first time that Q. pitereka isolates differ in their level of aggressiveness towards hosts. Of the isolates tested, two were consistently more aggressive, based on incidence and severity of disease and lesion size, on spotted gum species and C. torelliana