Mycosphaerella species of Tasmania and their interactions with eucalyptus plantations

The genus Mycosphaerella includes pathogens capable of severely reducing the growth of eucalypts. The threat of these fungal species to the eucalypt plantation industry prompted this investigation into the Mycosphaerella species occurring on Eucalyptus globulus and E. nitens plantations in Tasmania. This research focused on establishing which Mycosphaerella species are present on eucalypts in Tasmania, their biology and host interactions. A survey of 36 plantations across the island of Tasmania was conducted. Five Mycosphaerella species and three species from associated anamorph genera were isolated and identified. The most frequently isolated species with the highest incidence and severity of infection were Mycosphaerella cryptica and M. nubilosa. These two species appear to have the greatest potential to damage juvenile leaves on E. globulus and E. nitens plantations in Tasmania. A link between Mycosphaerella vespa and Coniothyrium ovatum was described for the first time. A severe epidemic of M. nubilosa in a field trial in north-west Tasmania provided an opportunity to examine quantitative genetic variation in susceptibility in a genetically diverse population of E. globulus families. Significant genetic variation for susceptibility was detected with a narrow-sense heritability of disease severity (measured as percent of whole tree leaf necrosis) being the highest yet reported (h2 = 0.60) for a Mycosphaerella disease of eucalypts. A possible cause for this high heritability is that the level of disease severity encountered in the trial was very high (40% of trees had 32% or greater of leaf area damaged) and this may have helped the detection of genetic differences in susceptibility. This damage had a significant deleterious impact on tree growth (for example at year 7 a 1% reduction in leaf area damaged resulted in a 0.84 mm increase in diameter growth r2 = 33%; P < 0.001). There is ample opportunity to select genotypes of E. globulus that are relatively resistant to damage and if these were deployed in areas of high disease risk, significant benefits in plantation productivity could be obtained. Genetic structure of a M. cryptica population was studied following natural infection of an E. globulus genetic trial and an adjacent E. nitens plantation in Tasmania. Significant genetic variation in resistance was found between and within E. globulus families in the trial (three cloned and control-crossed F2 and four open-pollinated families). Single ascospore isolates were taken from 72 E. globulus trees which differed in their level of resistance, 21 adjacent E. nitens and five trees in distant plantations. DNA from these isolates was scored for the presence/absence of 75 random amplified polymorphic DNA (RAPD) loci. Only 18 RAPD genotypes, which appc:ared to recombine rarely, were present among the 98 isolates which indicates that M. cryptica is not strictly heterothallic. Cluster analysis using genetic distance revealed that the RAPD genotypes grouped into two clusters that matched differences in isolate culture morphology, indicating that the fungal population comprised at least two distinctive biotypes. The two biotypes differed markedly in their host interactions. Biotype 2 was only found on E. globulus while biotype 1 infected both eucalypt species. Within E. globulus, biotype 2 was almost exclusively collected on resistant trees while biotype 1 was found on both resistant and susceptible trees. These data may provide evidence for specialisation ofbiotypes within the eucalypt pathogen M. cryptica. These studies have helped to identify the current suite of Mycosphaerella species parasitising young eucalypt plantations in Tasmania. Further evidence of wide genetic variation in susceptibility to Mycosphaerella in E. globulus with high heritability is provided. The first study on the population structure of M. cryptica has revealed complex interactions with its hosts.