Globisporangium and Pythium Species Associated with Yield Decline of Pyrethrum (Tanacetum cinerariifolium) in Australia

Pyrethrum (Tanacetum cinerariifolium) cultivation in Australia, which accounts for the majority of global production of natural insecticidal pyrethrins, is affected by a persistent yield decline which in part is caused by a complex of pathogens. Globisporangium and Pythium species were isolated from crown and roots of pyrethrum plants showing stunting and brown discoloration of crown tissue, and from soil adjacent to diseased plants from yield-decline-affected sites in Tasmania and Victoria, Australia. Ten known Globisporangium species (Globisporangium attrantheridium, G. erinaceum, G. intermedium, G. irregulare, G. macrosporum, G. recalcitrans, G. rostratifingens, G. sylvaticum, G. terrestris and G. ultimum var. ultimum), two new Globisporangium species (Globisporangium capense sp. nov. and Globisporangium commune sp. nov.) and three Pythium species (Pythium diclinum/lutarium, P. tracheiphilum and P. vanterpoolii) were identified through morphological studies and multigene phylogenetic analyses using ITS and Cox1 sequences. Globisporangium ultimum var. ultimum, G. sylvaticum, G. commune sp. nov. and G. irregulare were most abundant. Globisporangium attrantheridium, G. macrosporum and G. terrestris were reported for the first time in Australia. Seven Globisporangium species were pathogenic on both pyrethrum seeds (in vitro assays) and seedlings (glasshouse bioassays), while two Globisporangium species and three Pythium species only caused significant symptoms on pyrethrum seeds. Globisporangium irregulare and G. ultimum var. ultimum were the most aggressive species, causing pyrethrum seed rot, seedling damping-off and significant plant biomass reduction. This is the first report of Globisporangium and Pythium species causing disease in pyrethrum globally and suggests that oomycete species in the family Pythiaceae may have an important role in the yield decline of pyrethrum in Australia.

Colletotrichum Species Causing Anthracnose of Citrus in Australia

Colletotrichum spp. are important pathogens of citrus that cause dieback of branches and postharvest disease. Globally, several species of Colletotrichum have been identified as causing anthracnose of citrus. One hundred and sixty-eight Colletotrichum isolates were collected from anthracnose symptoms on citrus stems, leaves, and fruit from Victoria, New South Wales, and Queensland, and from State herbaria in Australia. Colletotrichum australianum sp. nov., C. fructicola, C. gloeosporioides, C. karstii, C. siamense, and C. theobromicola were identified using multi-gene phylogenetic analyses based on seven genomic loci (ITS, gapdh, act, tub2, ApMat, gs, and chs-1) in the gloeosporioides complex and five genomic loci (ITS, tub2, act, chs-1, and his3) in the boninense complex, as well as morphological characters. Several isolates pathogenic to chili (Capsicum annuum), previously identified as C. queenslandicum, formed a clade with the citrus isolates described here as C. australianum sp. nov. The spore shape and culture characteristics of the chili and citrus isolates of C. australianum were similar and differed from those of C. queenslandicum. This is the first report of C. theobromicola isolated from citrus and the first detection of C. karstii and C. siamense associated with citrus anthracnose in Australia.

Inheritance of breeding system in Cakile (Brassicaceae) following hybridization: implications for plant invasions

BACKGROUND AND AIMS

Hybridization is commonly assumed to aid invasions through adaptive introgression. In contrast, a recent theoretical model predicted that there can be non-adaptive demographic advantages from hybridization and that the population consequences will depend on the breeding systems of the species and the extent to which subsequent generations are able to interbreed and reproduce. We examined cross-fertilization success and inheritance of breeding systems of two species in order to better assess the plausibility of the theoretical predictions.

METHODS

Reciprocal artificial crosses were made to produce F1, F2 and backcrosses between Cakile maritima (self-incompatible, SI) and Cakile edentula (self-compatible, SC) (Brassicaceae). Flowers were emasculated prior to anther dehiscence and pollen was introduced from donor plants to the recipient's stigma. Breeding system, pollen viability, pollen germination, pollen tube growth and reproductive output were then determined. The results were used to replace the assumptions made in the original population model and new simulations were made.

KEY RESULTS

The success rate with the SI species as the pollen recipient was lower than when it was the pollen donor, in quantitative agreement with the 'SI × SC rule' of unilateral incompatibility. Similar outcomes were found in subsequent generations where fertile hybrids were produced but lower success rates were observed in crosses of SI pollen donors with SC pollen recipients. Much lower proportions of SC hybrids were produced than expected from a single Mendelian allele. When incorporated into a population model, these results predicted an even faster rate of replacement of the SC species by the SI species than previously reported.

CONCLUSIONS

Our study of these two species provides even clearer support for the feasibility of the non-adaptive hybridization hypothesis, whereby the colonization of an SI species can be assisted by transient hybridization with a congener. It also provides novel insight into reproductive biology beyond the F1 generation.

Population Structure of Colletotrichum tanaceti in Australian Pyrethrum Reveals High Evolutionary Potential

Colletotrichum tanaceti, the causal agent of anthracnose, is an emerging pathogen of commercially grown pyrethrum (Tanacetum cinerariifolium) in Australia. A microsatellite marker library was developed to understand the spatio-genetic structure over three sampled years and across two regions where pyrethrum is cultivated in Australia. Results indicated that C. tanaceti was highly diverse with a mixed reproductive mode; comprising both sexual and clonal reproduction. Sexual reproduction of C. tanaceti was more prevalent in Tasmania than in Victoria. Little differentiation was observed among field populations likely due to isolation by colonization but most of the genetic variation was occurring within populations. C. tanaceti was likely to have had a long-distance gene and genotype flow among distant populations within a state and between states. Anthropogenic transmission of propagules and wind dispersal of ascospores are the most probable mechanisms of long-distance dispersal of C. tanaceti. Evaluation of putative population histories suggested that C. tanaceti most likely originated in Tasmania and expanded from an unidentified host onto pyrethrum. Victoria was later invaded by the Tasmanian population. With the mixed mode of reproduction and possible long-distance gene flow, C. tanaceti is likely to have a high evolutionary potential and thereby has ability to adapt to management practices in the future.

Identification, prevalence and pathogenicity of Colletotrichum species causing anthracnose of Capsicum annuum in Asia

Anthracnose of chili (Capsicum spp.) causes major production losses throughout Asia where chili plants are grown. A total of 260 Colletotrichum isolates, associated with necrotic lesions of chili leaves and fruit were collected from chili producing areas of Indonesia, Malaysia, Sri Lanka, Thailand and Taiwan. Colletotrichum truncatum was the most commonly isolated species from infected chili fruit and was readily identified by its falcate spores and abundant setae in the necrotic lesions. The other isolates consisted of straight conidia (cylindrical and fusiform) which were difficult to differentiate to species based on morphological characters. Taxonomic analysis of these straight conidia isolates based on multi-gene phylogenetic analyses (ITS, gapdh, chs-1, act, tub2, his3, ApMat, gs) revealed a further seven known Colletotrichum species, C. endophyticum, C. fructicola, C. karsti, C. plurivorum, C. scovillei, C. siamense and C. tropicale. In addition, three novel species are also described as C. javanense, C. makassarense and C. tainanense, associated with anthracnose of chili fruit in West Java (Indonesia); Makassar, South Sulawesi (Indonesia); and Tainan (Taiwan), respectively. Colletotrichum siamense is reported for the first time causing anthracnose of Capsicum annuum in Indonesia and Sri Lanka. This is also the first report of C. fructicola causing anthracnose of chili in Taiwan and Thailand and C. plurivorum in Malaysia and Thailand. Of the species with straight conidia, C. scovillei (acutatum complex), was the most prevalent throughout the surveyed countries, except for Sri Lanka from where this species was not isolated. Colletotrichum siamense (gloeosporioides complex) was also common in Indonesia, Sri Lanka and Thailand. Pathogenicity tests on chili fruit showed that C. javanense and C. scovillei were highly aggressive, especially when inoculated on non-wounded fruit, compared to all other species. The existence of new, highly aggressive exotic species, such as C. javanense, poses a biosecurity risk to production in countries which do not have adequate quarantine regulations to restrict the entry of exotic pathogens.

Comparative genome analysis indicates high evolutionary potential of pathogenicity genes in Colletotrichum tanaceti

Colletotrichum tanaceti is an emerging foliar fungal pathogen of commercially grown pyrethrum (Tanacetum cinerariifolium). Despite being reported consistently from field surveys in Australia, the molecular basis of pathogenicity of C. tanaceti on pyrethrum is unknown. Herein, the genome of C. tanaceti (isolate BRIP57314) was assembled de novo and annotated using transcriptomic evidence. The inferred putative pathogenicity gene suite of C. tanaceti comprised a large array of genes encoding secreted effectors, proteases, CAZymes and secondary metabolites. Comparative analysis of its putative pathogenicity gene profiles with those of closely related species suggested that C. tanaceti likely has additional hosts to pyrethrum. The genome of C. tanaceti had a high repeat content and repetitive elements were located significantly closer to genes inferred to influence pathogenicity than other genes. These repeats are likely to have accelerated mutational and transposition rates in the genome, resulting in a rapid evolution of certain CAZyme families in this species. The C. tanaceti genome showed strong signals of Repeat Induced Point (RIP) mutation which likely caused its bipartite nature consisting of distinct gene-sparse, repeat and A-T rich regions. Pathogenicity genes within these RIP affected regions were likely to have a higher evolutionary rate than the rest of the genome. This "two-speed" genome phenomenon in certain Colletotrichum spp. was hypothesized to have caused the clustering of species based on the pathogenicity genes, to deviate from taxonomic relationships. The large repertoire of pathogenicity factors that potentially evolve rapidly due to the plasticity of the genome, indicated that C. tanaceti has a high evolutionary potential. Therefore, C. tanaceti poses a high-risk to the pyrethrum industry. Knowledge of the evolution and diversity of the putative pathogenicity genes will facilitate future research in disease management of C. tanaceti and other Colletotrichum spp.

Chloroplast variation is incongruent with classification of the Australian bloodwood eucalypts (genus Corymbia, family Myrtaceae)

Previous molecular phylogenetic analyses have resolved the Australian bloodwood eucalypt genus Corymbia (~100 species) as either monophyletic or paraphyletic with respect to Angophora (9-10 species). Here we assess relationships of Corymbia and Angophora using a large dataset of chloroplast DNA sequences (121,016 base pairs; from 90 accessions representing 55 Corymbia and 8 Angophora species, plus 33 accessions of related genera), skimmed from high throughput sequencing of genomic DNA, and compare results with new analyses of nuclear ITS sequences (119 accessions) from previous studies. Maximum likelihood and maximum parsimony analyses of cpDNA resolve well supported trees with most nodes having >95% bootstrap support. These trees strongly reject monophyly of Corymbia, its two subgenera (Corymbia and Blakella), most taxonomic sections (Abbreviatae, Maculatae, Naviculares, Septentrionales), and several species. ITS trees weakly indicate paraphyly of Corymbia (bootstrap support <50% for maximum likelihood, and 71% for parsimony), but are highly incongruent with the cpDNA analyses, in that they support monophyly of both subgenera and some taxonomic sections of Corymbia. The striking incongruence between cpDNA trees and both morphological taxonomy and ITS trees is attributed largely to chloroplast introgression between taxa, because of geographic sharing of chloroplast clades across taxonomic groups. Such introgression has been widely inferred in studies of the related genus Eucalyptus. This is the first report of its likely prevalence in Corymbia and Angophora, but this is consistent with previous morphological inferences of hybridisation between species. Our findings (based on continent-wide sampling) highlight a need for more focussed studies to assess the extent of hybridisation and introgression in the evolutionary history of these genera, and that critical testing of the classification of Corymbia and Angophora requires additional sequence data from nuclear genomes.

Paraphoma Crown Rot of Pyrethrum (Tanacetum cinerariifolium)

Pyrethrum (Tanacetum cinerariifolium) is commercially cultivated for the extraction of natural pyrethrin insecticides from the oil glands inside seed. Yield decline has caused significant yield losses in Tasmania during the last decade. A new pathogen of pyrethrum causing crown rot and reduced growth of the plants in yield decline affected fields of northern Tasmania was isolated from necrotic crown tissue and described as Paraphoma vinacea. Multigene phylogenetic identification of the pathogen also revealed that P. vinacea was a new species different from other Paraphoma type strains. Glasshouse pathogenicity experiments showed that P. vinacea significantly reduced belowground and total biomass of pyrethrum plants 2 months after inoculation. Dull-tan to reddish-brown discoloration of the cortical and subcortical crown tissue was observed in 100% of the infected plants. P. vinacea infected 75% of the plants inoculated with root dip and soil drench inoculation techniques in an inoculation optimization experiment. P. vinacea, the causal agent of Paraphoma crown rot disease, represents an important pathogen that will negatively impact the commercial cultivation of pyrethrum in Tasmania.

IMA Genome-F 4: Draft genome sequences of Chrysoporthe austroafricana, Diplodia scrobiculata, Fusarium nygamai, Leptographium lundbergii, Limonomyces culmigenus, Stagonosporopsis tanaceti, and Thielaviopsis punctulata

The genomes of Chrysoporthe austroafricana, Diplodia scrobiculata, Fusarium nygami, Leptographium lundbergii, Limonomyces culmigenus, Stagonosporopsis tanaceti, and Thielaviopsis punctulata are presented in this genome announcement. These seven genomes are from endophytes, plant pathogens and economically important fungal species. The genome sizes range from 26.6 Mb in the case of Leptographium lundbergii to 44 Mb for Chrysoporthe austroafricana. The availability of these genome data will provide opportunities to resolve longstanding questions regarding the taxonomy of species in these genera, and may contribute to our understanding of the lifestyles through comparative studies with closely related organisms.

Rapid Changes in the Genetic Composition of Stagonosporopsis tanaceti Population in Australian Pyrethrum Fields

A novel set of microsatellite markers were developed and employed for geographical and temporal population analyses of Stagonosporopsis tanaceti, the cause of ray blight of pyrethrum in Australia. Genotyping of 407 isolates, using 13 markers, suggested an asexual mode of reproduction with significant linkage disequilibrium and high levels of clonality. Low geographical differentiation and widespread distribution of a few multilocus genotypes (MLGs), in the absence of airborne ascospores, suggested the role of human-mediated movement of seed as a major means of long-distance pathogen dispersal. The genetic composition of S. tanaceti was stable for a decade then changed rapidly in only 2 years. Bayesian clustering analyses and minimum spanning networks determined only two major clonal lineages in and prior to 2010. However, in 2012, a previously unobserved cluster of MLGs was detected, which significantly increased in frequency and displaced the historically dominant MLGs by 2013. This rapid change in the genetic composition of S. tanaceti could indicate a second introduction then a selective sweep, or strong selection pressures from recently introduced fungicides or pyrethrum varieties. These results may have serious implications for durability of management strategies for this disease.

Lack of genetic variation in tree ring delta13C suggests a uniform, stomatally-driven response to drought stress across Pinus radiata genotypes

We assessed the variation in delta(13)C signatures of Pinus radiata D. Don stemwood taken from three genetic trials in southern Australia. We sought to determine the potential of using delta(13)C signatures as selection criteria for drought tolerance. Increment cores were taken from P. radiata and were used to determine the basal area increment and the delta(13)C signature of extracted cellulose. Both growth increment and cellulose delta(13)C were affected by water availability. Growth increment and delta(13)C were negatively correlated suggesting that growth was water-limited. While there was significant genetic variation in growth, there was no significant genetic variation in cellulose delta(13)C of tree rings. This suggests that different genotypes of P. radiata display significant differences in growth and yet respond similarly to drought stress. The delta(13)C response to drought stress was more due to changes in stomatal conductance than to the variation in photosynthetic capacity, and this may explain the lack of genetic variation in delta(13)C. The lack of genetic variation in cellulose delta(13)C of tree rings precludes its use as a selection criterion for drought tolerance among P. radiata genotypes.

Leaf osmotic potential of Eucalyptus hybrids responds differently to freezing and drought, with little clonal variation

Concentrations of solutes, and thus leaf osmotic potential (Psi pi), often increase when plants are subject to drought or sub-zero (frost) temperatures. We measured Psi pi and concentrations of individual solutes in leaves of 3-year-old Eucalyptus camaldulensis Dehn., E. globulus Labill., E. grandis W. Hill ex Maid. and 29 hybrid clones on a site subjected to both summer drought and winter frost. We sought to characterize seasonal and genetic variations in Psi pi and to determine whether Psi pi or leaf turgor is related to bole volume increment. Leaf osmotic potential at full turgor (Psi pi(100)) was 0.7 MPa more negative in winter than in late summer, and this trend was uniform across genotypes. Soluble carbohydrates were confirmed as key contributors to Psi pi, accounting for 40-44% of total osmolality. The seasonal trend in Psi pi(100) was facilitated by changes in leaf morphology, such as reduced turgid mass:dry mass ratio and increased apoplastic water fraction in winter. Cell wall elasticity increased significantly from winter to summer. Our results suggest that elastic adjustment may be more important than osmotic adjustment in leaves exposed to drought. Although Psi pi(100) was a reasonable predictor of in situ osmotic potential and turgor, we found no relationship between any physiological trait and bole volume increment. Clone-within-family variation in Psi pi(100) was small in both summer and winter and was unrelated to bole volume increment. We conclude that, for the study species, tree improvement under water-limited conditions should concentrate on direct selection for growth rather than on indirect selection based on osmotic potential.

Mechanical stability of trees under dynamic loads

Tree stability in windstorms and tree failure are important issues in urban areas where there can be risks of damage to people and property and in forests where wind damage causes economic loss. Current methods of managing trees, including pruning and assessment of mechanical strength, are mainly based on visual assessment or the experience of people such as trained arborists. Only limited data are available to assess tree strength and stability in winds, and most recent methods have used a static approach to estimate loads. Recent research on the measurement of dynamic wind loads and the effect on tree stability is giving a better understanding of how different trees cope with winds. Dynamic loads have been measured on trees with different canopy shapes and branch structures including a palm (Washingtonia robusta), a slender Italian cypress (Cupressus sempervirens) and trees with many branches and broad canopies including hoop pine (Araucaria cunninghamii) and two species of eucalypt (Eucalyptus grandis, E. teretecornus). Results indicate that sway is not a harmonic, but is very complex due to the dynamic interaction of branches. A new dynamic model of a tree is described, incorporating the dynamic structural properties of the trunk and branches. The branch mass contributes a dynamic damping, termed mass damping, which acts to reduce dangerous harmonic sway motion of the trunk and so minimizes loads and increases the mechanical stability of the tree. The results from 12 months of monitoring sway motion and wind loading forces are presented and discussed.

Cyanogenesis in Eucalyptus polyanthemos seedlings: heritability, ontogeny and effect of soil nitrogen

Cyanogenic plants release cyanide from endogenous cyanide-containing compounds (generally cyanogenic glycosides) and thus have an effective means of chemical defense. The capacity for cyanogenesis can be highly variable, even among individuals within a population. The genetic, environmental and developmental factors determining this variability are poorly understood, particularly in tree species. We used Eucalyptus polyanthemos Schauer subsp. vestita L. Johnson & K. Hill to quantify aspects of the regulation of cyanogenic capacity, which in this species is determined by foliar cyanogenic glycoside concentration. A half-sibling progeny trial, based on seed collected from open-pollinated trees covering a range of cyanogenic capacities, was used to assess the heritability of cyanogenesis in E. polyanthemos. Narrow sense heritability (h(2) +/- 1 SE) was estimated to be 0.82 +/- 0.20 from an intra-class correlation and 0.78 +/- 0.11 from a standardized progeny-parent regression. Foliar cyanogenic glycoside concentrations were on average about 70% lower in seedlings than in maternal trees, suggesting that there is a developmental delay in the accumulation of cyanogenic capacity in this species. The high h(2) values indicate that cyanogenic capacity is largely genetically determined and that environmental factors have little effect. To test this supposition, we grew seedlings at two soil nitrogen (N) concentrations (N influences cyanogenic capacity in some species) and found no appreciable effect on cyanogenic glycoside concentration, biomass partitioning or relative growth rate. Highly cyanogenic seedlings grew more slowly than seedlings with lower cyanogenic capacities, and relative growth rate was positively associated with net assimilation rate in seedlings in both N treatments.

QTL analysis for ascochyta blight resistance in an intraspecific population of chickpea (Cicer arietinum L.)

In both controlled environment and the field, six QTLs for ascochyta blight resistance were identified in three regions of the genome of an intraspecific population of chickpea using the IDS and AUDPC disease scoring systems. One QTL-region was detected from both environments, whereas the other two regions were detected from each environment. All the QTL-regions were significantly associated with ascochyta blight resistance using either of the disease scoring systems. The QTLs were verified by multiple interval mapping, and a two-QTL genetic model with considerable epistasis was established for both environments. The major QTLs generally showed additive gene action, as well as dominance inter-locus interaction in the multiple genetic model. All the QTLs were mapped near a RGA marker. The major QTLs were located on LG III, which was mapped with five different types of RGA markers. A CLRR-RGA marker and a STMS marker flanked QTL 6 for controlled environment resistance at 0.06 and 0.04 cM, respectively. Other STMS markers flanked QTL 1 for field resistance at a 5.6 cM interval. After validation, these flanking markers may be used in marker-assisted selection to breed for elite chickpea cultivars with durable resistance to ascochyta blight. The tight linkage of RGA markers to the major QTL on LG III will allow map-based cloning of the underlying resistance genes.

Preliminary investigation of QTLs associated with seedling resistance to ascochyta blight from Cicer echinospermum, a wild relative of chickpea

Accessions from Cicer echinospermum, a wild relative of chickpea (Cicer arietinum L.), contain resistance to the fungal disease ascochyta blight, a devastating disease of chickpea. A linkage map was constructed based on an interspecific F(2) population, derived from a cross between a susceptible chickpea cultivar (Lasseter) and a resistant C. echinospermum accession (PI 527930). The linkage map incorporated 83 molecular markers, that included RAPD, ISSR, STMS and RGA markers; eight markers remained unlinked. The map comprised eight linkage groups and covered a map distance of 570 cM. Six out of the eight linkage groups were correlated to linkage groups from the integrated Cicer map using STMS markers. Quantitative trait loci (QTLs) associated with ascochyta blight resistance were detected using interval mapping and single-point analysis. The F(2) population was evaluated for seedling and stem resistance in glasshouse trials. At least two QTLs were identified for seedling resistance, both of which were located within linkage group 4. Five markers were associated with stem resistance, four of which were also associated with seedling resistance. QTLs from previous studies also mapped to LG 4, suggesting that this linkage group is an important region of the Cicer genome for resistance to ascochyta blight.