Population genomics reveals historical and ongoing recombination in the Fusarium oxysporum species complex

The Fusarium oxysporum species complex (FOSC) is a group of closely related plant pathogens long-considered strictly clonal, as sexual stages have never been recorded. Several studies have questioned whether recombination occurs in FOSC, and if it occurs its nature and frequency are unknown. We analysed 410 assembled genomes to answer whether FOSC diversified by occasional sexual reproduction interspersed with numerous cycles of asexual reproduction akin to a model of predominant clonal evolution (PCE). We tested the hypothesis that sexual reproduction occurred in the evolutionary history of FOSC by examining the distribution of idiomorphs at the mating locus, phylogenetic conflict and independent measures of recombination from genome-wide SNPs and genes. A phylogenomic dataset of 40 single copy orthologs was used to define structure a priori within FOSC based on genealogical concordance. Recombination within FOSC was tested using the pairwise homoplasy index and divergence ages were estimated by molecular dating. We called SNPs from assembled genomes using a k-mer approach and tested for significant linkage disequilibrium as an indication of PCE. We clone-corrected and tested whether SNPs were randomly associated as an indication of recombination. Our analyses provide evidence for sexual or parasexual reproduction within, but not between, clades of FOSC that diversified from a most recent common ancestor about 500 000 years ago. There was no evidence of substructure based on geography or host that might indicate how clades diversified. Competing evolutionary hypotheses for FOSC are discussed in the context of our results.

Comparative transcriptomic analysis of races 1, 2, 5 and 6 of Fusarium oxysporum f.sp. pisi in a susceptible pea host identifies differential pathogenicity profiles

BACKGROUND

The fungal pathogen Fusarium oxysporum f.sp. pisi (Fop) causes Fusarium wilt in peas. There are four races globally: 1, 2, 5 and 6 and all of these races are present in Australia. Molecular infection mechanisms have been studied in a few other F. oxysporum formae speciales; however, there has been no transcriptomic Fop-pea pathosystem study.

RESULTS

A transcriptomic study was carried out to understand the molecular pathogenicity differences between the races. Transcriptome analysis at 20 days post-inoculation revealed differences in the differentially expressed genes (DEGs) in the Fop races potentially involved in fungal pathogenicity variations. Most of the DEGs in all the races were engaged in transportation, metabolism, oxidation-reduction, translation, biosynthetic processes, signal transduction, proteolysis, among others. Race 5 expressed the most virulence-associated genes. Most genes encoding for plant cell wall degrading enzymes, CAZymes and effector-like proteins were expressed in race 2. Race 6 expressed the least number of genes at this time point.

CONCLUSION

Fop races deploy various factors and complex strategies to mitigate host defences to facilitate colonisation. This investigation provides an overview of the putative pathogenicity genes in different Fop races during the necrotrophic stage of infection. These genes need to be functionally characterised to confirm their pathogenicity/virulence roles and the race-specific genes can be further explored for molecular characterisation.

Genetic Diversity of the Fusarium oxysporum Complex Isolated from the Grassland Biome of South Africa

The genetic diversity of pathogenic members of the Fusarium oxysporum species complex (FOSC) has been intensively studied worldwide, yet strains occurring in native soils with low anthropogenic disturbance remain poorly understood. This study focused on 355 F. oxysporum isolates from soils with low anthropogenic activity obtained from the grassland biome of South Africa. Analysis of the translation elongation factor 1-alpha (tef-1α) gene revealed high levels of sequence type diversity within the soil population in comparison with the global dataset. Phylogenetic relationships of the South African isolates revealed that four nested within FOSC clade 1. This is the first report of members of the basal clade recovered from ecosystems with low anthropogenic disturbance from Sub-Saharan Africa. The remaining strains nested within clades 2 to 5. This study contributes significantly to our understanding of the distribution of the FOSC in natural systems as we show that FOSC populations in the South African grassland biome are genetically diverse. This fills in our knowledge gap because previous studies reported only on the occurrence and diversity of the FOSC isolated from plant debris in South Africa. This is the first comprehensive survey of fusaria from grassland soils with low anthropogenic disturbance in South Africa.

Phylogenomic Analysis of a 55.1-kb 19-Gene Dataset Resolves a Monophyletic Fusarium that Includes the Fusarium solani Species Complex

Scientific communication is facilitated by a data-driven, scientifically sound taxonomy that considers the end-user's needs and established successful practice. In 2013, the Fusarium community voiced near unanimous support for a concept of Fusarium that represented a clade comprising all agriculturally and clinically important Fusarium species, including the F. solani species complex (FSSC). Subsequently, this concept was challenged in 2015 by one research group who proposed dividing the genus Fusarium into seven genera, including the FSSC described as members of the genus Neocosmospora, with subsequent justification in 2018 based on claims that the 2013 concept of Fusarium is polyphyletic. Here, we test this claim and provide a phylogeny based on exonic nucleotide sequences of 19 orthologous protein-coding genes that strongly support the monophyly of Fusarium including the FSSC. We reassert the practical and scientific argument in support of a genus Fusarium that includes the FSSC and several other basal lineages, consistent with the longstanding use of this name among plant pathologists, medical mycologists, quarantine officials, regulatory agencies, students, and researchers with a stake in its taxonomy. In recognition of this monophyly, 40 species described as genus Neocosmospora were recombined in genus Fusarium, and nine others were renamed Fusarium. Here the global Fusarium community voices strong support for the inclusion of the FSSC in Fusarium, as it remains the best scientific, nomenclatural, and practical taxonomic option available.

Cryptic diversity found in Didymellaceae from Australian native legumes

Ascochytakoolunga (Didymellaceae, Pleosporales) was first described in 2009 (as Phomakoolunga) and identified as the causal agent of Ascochyta blight of Pisumsativum (field pea) in South Australia. Since then A.koolunga has not been reported anywhere else in the world, and its origins and occurrence on other legume (Fabaceae) species remains unknown. Blight and leaf spot diseases of Australian native, pasture and naturalised legumes were studied to investigate a possible native origin of A.koolunga.

Ascochytakoolunga was not detected on native, naturalised or pasture legumes that had leaf spot symptoms, in any of the studied regions in southern Australia, and only one isolate was recovered from P.sativum. However, we isolated five novel species in the Didymellaceae from leaf spots of Australian native legumes from commercial field pea regions throughout southern Australia. The novel species were classified on the basis of morphology and phylogenetic analyses of the internal transcribed spacer region and part of the RNA polymerase II subunit B gene region. Three of these species, Nothophomagarlbiwalawardasp. nov., Nothophomanaiawusp. nov. and Nothophomangayawangsp. nov., were isolated from Sennaartemisioides. The other species described here are Epicoccumdjirangnandirisp. nov. from Swainsonagalegifolia and Neodidymelliopsistinkyukukusp. nov. from Hardenbergiaviolacea. In addition, we report three new host-pathogen associations in Australia, namely Didymellapinodes on S.artemisioides and Viciacracca, and D.lethalis on Lathyrustingitanus. This is also the first report of Didymellaprosopidis in Australia.

New and Interesting Fungi. 3

Seven new genera, 26 new species, 10 new combinations, two epitypes, one new name, and 20 interesting new host and / or geographical records are introduced in this study. New genera are: Italiofungus (based on Italiofungus phillyreae) on leaves of Phillyrea latifolia (Italy); Neolamproconium (based on Neolamproconium silvestre) on branch of Tilia sp. (Ukraine); Neosorocybe (based on Neosorocybe pini) on trunk of Pinus sylvestris (Ukraine); Nothoseptoria (based on Nothoseptoria caraganae) on leaves of Caragana arborescens (Russia); Pruniphilomyces (based on Pruniphilomyces circumscissus) on Prunus cerasus (Russia); Vesiculozygosporium (based on Vesiculozygosporium echinosporum) on leaves of Muntingia calabura (Malaysia); Longiseptatispora (based on Longiseptatispora curvata) on leaves of Lonicera tatarica (Russia). New species are: Barrmaelia serenoae on leaf of Serenoa repens (USA); Chaetopsina gautengina on leaves of unidentified grass (South Africa); Chloridium pini on fallen trunk of Pinus sylvestris (Ukraine); Cadophora fallopiae on stems of Reynoutria sachalinensis (Poland); Coleophoma eucalyptigena on leaf litter of Eucalyptus sp. (Spain); Cylindrium corymbiae on leaves of Corymbia maculata (Australia); Diaporthe tarchonanthi on leaves of Tarchonanthus littoralis (South Africa); Elsinoe eucalyptorum on leaves of Eucalyptus propinqua (Australia); Exophiala quercina on dead wood of Quercus sp., (Germany); Fusarium californicum on cambium of budwood of Prunus dulcis (USA); Hypomyces gamsii on wood of Alnus glutinosa (Ukraine); Kalmusia araucariae on leaves of Araucaria bidwillii (USA); Lectera sambuci on leaves of Sambucus nigra (Russia); Melanomma populicola on fallen twig of Populus canadensis (Netherlands), Neocladosporium syringae on branches of Syringa vulgarishorus (Ukraine); Paraconiothyrium iridis on leaves of Iris pseudacorus (Ukraine); Pararoussoella quercina on branch of Quercus robur (Ukraine); Phialemonium pulveris from bore dust of deathwatch beetle (France); Polyscytalum pinicola on needles of Pinus tecunumanii (Malaysia); Acervuloseptoria fraxini on Fraxinus pennsylvanica (Russia); Roussoella arundinacea on culms of Arundo donax (Spain); Sphaerulina neoaceris on leaves of Acer negundo (Russia); Sphaerulina salicicola on leaves of Salix fragilis (Russia); Trichomerium syzygii on leaves of Syzygium cordatum (South Africa); Uzbekistanica vitis-viniferae on dead stem of Vitis vinifera (Ukraine); Vermiculariopsiella eucalyptigena on leaves of Eucalyptus sp. (Australia).

Pathogenic, Morphological, and Phylogenetic Characterization of Fusarium solani f. sp. cucurbitae Isolates From Cucurbits in Almería Province, Spain

Fusarium solani f. sp. cucurbitae (syn. Neocosmosporum cucurbitae) is one of the most devastating soilborne pathogens affecting the production of cucurbits worldwide. Since its first detection in Almería Province in Spain in the spring of 2007, it has become one of the main soilborne pathogens affecting zucchini production. It has also been reported on melon, watermelon, and squash rootstocks in Spain, representing a high risk of dissemination in the area. The objectives of this study were to investigate the incidence and distribution of this disease in southeastern Spain and characterize isolates collected over 5 years. These strains were characterized on the basis of greenhouse aggressiveness assays on a range of cucurbit hosts, morphological characteristics, and elongation factor 1-α and RNA polymerase II second largest subunit phylogenies. All pathogenic isolates were highly aggressive on zucchini plants, causing a high mortality rate a few weeks after inoculation. The rest of the cucurbit hosts showed differential susceptibility to the pathogen, with cucumber being the least susceptible. Plants belonging to other families remained asymptomatic. Morphological characterization revealed the formation of verticilate monophialides and chlamydospores forming long chains, characteristics not described for this forma specialis. Phylogenetic studies of both the individual loci and combined datasets revealed that all pathogenic isolates clustered together with strong monophyletic support, nested within clade 3 in the F. solani species complex.

Phylogenetic relationship between Australian Fusarium oxysporum isolates and resolving the species complex using the multispecies coalescent model

BACKGROUND

The Fusarium oxysporum species complex (FOSC) is a ubiquitous group of fungal species readily isolated from agroecosystem and natural ecosystem soils which includes important plant and human pathogens. Genetic relatedness within the complex has been studied by sequencing either the genes or the barcoding gene regions within those genes. Phylogenetic analyses have demonstrated a great deal of diversity which is reflected in the differing number of clades identified: three, five and eight. Genetic limitation within the species in the complex has been studied through Genealogical Concordance Phylogenetic Species Recognition (GCPSR) analyses with varying number of phylogenetic 'species' identified ranging from two to 21. Such differing views have continued to confuse users of these taxonomies.

RESULTS

The phylogenetic relationships between Australian F. oxysporum isolates from both natural and agricultural ecosystems were determined using three datasets: whole genome, nuclear genes, and mitochondrial genome sequences. The phylogenies were concordant except for three isolates. There were three concordant clades from all the phylogenies suggesting similar evolutionary history for mitochondrial genome and nuclear genes for the isolates in these three clades. Applying a multispecies coalescent (MSC) model on the eight single copy nuclear protein coding genes from the nuclear gene dataset concluded that the three concordant clades correspond to three phylogenetic species within the FOSC. There was 100% posterior probability support for the formation of three species within the FOSC. This is the first report of using the MSC model to estimate species within the F. oxysporum species complex. The findings from this study were compared with previously published phylogenetics and species delimitation studies.

CONCLUSION

Phylogenetic analyses using three different gene datasets from Australian F. oxysporum isolates have all supported the formation of three major clades which delineated into three species. Species 2 (Clade 3) may be called F. oxysporum as it contains the neotype for F. oxysporum.

Resolving Fusarium: Current Status of the Genus

The fungal genus Fusarium is one of the most important groups of plant-pathogenic fungi and affects a huge diversity of crops in all climatic zones across the globe. In addition, it is also a human pathogen and produces several extremely important mycotoxins in food products that have deleterious effects on livestock and humans. These fungi have been plagued over the past century by different perspectives of what constitutes the genus Fusarium and how many species occur within the genus. Currently, there are conflicting views on the generic boundaries and what defines a species that impact disease diagnosis, management, and biosecurity legislation. An approach to defining and identifying Fusarium that places the needs of the community of users (especially, in this case, phytopathologists) to the forefront is presented in this review.

Genealogical concordance phylogenetic species recognition in the Fusarium oxysporum species complex

Fusarium oxysporum is an important plant and human pathogenic ascomycetous group, with near ubiquity in agricultural and non-cultivated ecosystems. Phylogenetic evidence suggests that F. oxysporum is a complex of multiple morphologically cryptic species. Species boundaries and limits of genetic exchange within this complex are poorly defined, largely due to the absence of a sexual state and the paucity of morphological characters. This study determined species boundaries within the F. oxysporum species complex using Genealogical Concordance Phylogenetic Species Recognition (GCPSR) with eight protein coding loci. GCPSR criteria were used firstly to identify independent evolutionary lineages (IEL), which were subsequently collapsed into phylogenetic species. Seventeen IELs were initially identified resulting in the recognition of two phylogenetic species. Further evidence supporting this delineation is discussed.

One fungus, one name: defining the genus Fusarium in a scientifically robust way that preserves longstanding use

In this letter, we advocate recognizing the genus Fusarium as the sole name for a group that includes virtually all Fusarium species of importance in plant pathology, mycotoxicology, medicine, and basic research. This phylogenetically guided circumscription will free scientists from any obligation to use other genus names, including teleomorphs, for species nested within this clade, and preserve the application of the name Fusarium in the way it has been used for almost a century. Due to recent changes in the International Code of Nomenclature for algae, fungi, and plants, this is an urgent matter that requires community attention. The alternative is to break the longstanding concept of Fusarium into nine or more genera, and remove important taxa such as those in the F. solani species complex from the genus, a move we believe is unnecessary. Here we present taxonomic and nomenclatural proposals that will preserve established research connections and facilitate communication within and between research communities, and at the same time support strong scientific principles and good taxonomic practice.

High levels of diversity in Fusarium oxysporum from non-cultivated ecosystems in Australia

The Fusarium oxysporum species complex (FOSC) is a ubiquitous ascomycetous group that includes both pathogenic and non-pathogenic strains, the former being responsible for disease in over 100 cultivated plant species. Previous phylogenetic studies have uncovered at least four major clades within the FOSC, with Clade 1 hypothesised as being ancestral. However, the origin of these clades and pathogenic strains is poorly understood. Due to an emphasis on agricultural isolates in previous studies, the underlying diversity of this species complex in non-cultivated soils is largely unknown. To address this imbalance an extensive survey of isolates associated with native vegetation geographically isolated from cultivation throughout the Australian continent was conducted. A multi-gene phylogenetic analysis of the translation elongation factor (EF-1α) and the mitochondrial small subunit (mtSSU) rDNA loci did not recover any novel clades. However, the Australian isolates had high levels of intra-Clade diversity based on EF-1α sequence type (ST) comparison with a global dataset. The ST diversity was not equally distributed across the four clades, with the majority of novel STs recovered from Clade 1. Implications on the origin of the FOSC are discussed.

Additions to the Mycosphaerella complex

Species in the present study were compared based on their morphology, growth characteristics in culture, and DNA sequences of the nuclear ribosomal RNA gene operon (including ITS1, ITS2, 5.8S nrDNA and the first 900 bp of the 28S nrDNA) for all species and partial actin and translation elongation factor 1-alpha gene sequences for Cladosporium species. New species of Mycosphaerella (Mycosphaerellaceae) introduced in this study include M. cerastiicola (on Cerastium semidecandrum, The Netherlands), and M. etlingerae (on Etlingera elatior, Hawaii). Mycosphaerella holualoana is newly reported on Hedychium coronarium (Hawaii). Epitypes are also designated for Hendersonia persooniae, the basionym of Camarosporula persooniae, and for Sphaerella agapanthi, the basionym of Teratosphaeria agapanthi comb. nov. (Teratosphaeriaceae) on Agapathus umbellatus from South Africa. The latter pathogen is also newly recorded from A. umbellatus in Europe (Portugal). Furthermore, two sexual species of Cladosporium (Davidiellaceae) are described, namely C. grevilleae (on Grevillea sp., Australia), and C. silenes (on Silene maritima, UK). Finally, the phylogenetic position of two genera are newly confirmed, namely Camarosporula (based on C. persooniae, teleomorph Anthracostroma persooniae), which is a leaf pathogen of Persoonia spp. in Australia, belongs to the Teratosphaeriaceae, and Sphaerulina (based on S. myriadea), which occurs on leaves of Fagaceae (Carpinus, Castanopsis, Fagus, Quercus), and belongs to the Mycosphaerellaceae.

Local origin of two vegetative compatibility groups of Fusarium oxysporum f. sp. vasinfectum in Australia

Pathogenicity and genetic diversity of Fusarium oxysporum from geographically widespread native Gossypium populations, including a cotton growing area believed to be the center of origin of VCG 01111 and VCG 01112 of F. oxysporum f. sp. vasinfectum (Fov) in Australia, was determined using glasshouse bioassays and AFLPs. Five lineages (A-E) were identified among 856 isolates. Of these, 12% were strongly pathogenic on cotton, 10% were weakly pathogenic and designated wild Fov, while 78% were nonpathogenic. In contrast to the occurrence of pathogenic isolates in all five lineages in soils associated with wild Gossypium, in cotton growing areas only three lineages (A, B, E) occurred and all pathogenic isolates belonged to two subgroups in lineage A. One of these contained VCG 01111 isolates while the other contained VCG 01112 isolates. Sequence analyses of translation elongation factor-1α, mitochondrial small subunit rDNA, nitrate reductase and phosphate permease confirmed that Australian Fov isolates were more closely related to lineage A isolates of native F. oxysporum than to Fov races 1-8 found overseas. These results strongly support a local evolutionary origin for Fov in Australian cotton growing regions.

Species and ecological diversity within the Cladosporium cladosporioides complex (Davidiellaceae, Capnodiales)

The genus Cladosporium is one of the largest genera of dematiaceous hyphomycetes, and is characterised by a coronate scar structure, conidia in acropetal chains and Davidiella teleomorphs. Based on morphology and DNA phylogeny, the species complexes of C. herbarum and C. sphaerospermum have been resolved, resulting in the elucidation of numerous new taxa. In the present study, more than 200 isolates belonging to the C. cladosporioides complex were examined and phylogenetically analysed on the basis of DNA sequences of the nuclear ribosomal RNA gene operon, including the internal transcribed spacer regions ITS1 and ITS2, the 5.8S nrDNA, as well as partial actin and translation elongation factor 1-α gene sequences. For the saprobic, widely distributed species Cladosporium cladosporioides, both a neotype and epitype are designated in order to specify a well established circumscription and concept of this species. Cladosporium tenuissimum and C. oxysporum, two saprobes abundant in the tropics, are epitypified and shown to be allied to, but distinct from C. cladosporioides. Twenty-two species are newly described on the basis of phylogenetic characters and cryptic morphological differences. The most important phenotypic characters for distinguishing species within the C. cladosporioides complex, which represents a monophyletic subclade within the genus, are shape, width, length, septation and surface ornamentation of conidia and conidiophores; length and branching patterns of conidial chains and hyphal shape, width and arrangement. Many of the treated species, e.g., C. acalyphae, C. angustisporum, C. australiense, C. basiinflatum, C. chalastosporoides, C. colocasiae, C. cucumerinum, C. exasperatum, C. exile, C. flabelliforme, C. gamsianum, and C. globisporum are currently known only from specific hosts, or have a restricted geographical distribution. A key to all species recognised within the C. cladosporioides complex is provided.

Genetic structure of Fusarium pseudograminearum populations from the Australian grain belt

Crown rot, caused by the fungus Fusarium pseudograminearum (teleomorph Gibberella coronicola) is a major disease of wheat in the Australian grain belt. However, there is little information available on the population structure of this pathogen. We measured genetic diversity as assessed with amplified fragment length polymorphism (AFLP) analysis within and between populations of F. pseudograminearum from northeastern, south central, and southwestern regions of the Australian grain belt. Amongst the 217 isolates, 176 haplotypes were identified and grouped into two main clusters. One cluster contained isolates from populations in northeastern Australia, and the other cluster contained isolates from populations in south central and southwestern Australia. The southern populations were distinguished from the northeastern populations by higher levels of population differentiation (Gst) between them and genetic identity amongst the regional populations. We hypothesize that the F. pseudograminearum populations from northeastern and southern Australia are independent, which could result from different founding events or from geographic isolation and the accumulation of genetic differences due to genetic drift and/or selection.

Description of Gibberella sacchari and neotypification of its anamorph Fusarium sacchari

We described the teleomorph of Fusarium sacchari as Gibberella sacchari, sp. nov. This species can be separated from other species of Gibberella on the basis of the longer, narrower ascospores found in G. sacchari and by sexual cross fertility. Female-fertile mating type tester strains were developed that can be used for making sexual crosses with this heterothallic fungus under laboratory conditions. The anamorph, Fusarium sacchari, was neotypified.

Gibberella konza (Fusarium konzum) sp. nov. from prairie grasses, a new species in the Gibberella fujikuroi species complex

The Gibberella fujikuroi species complex (Fusarium section Liseola and allied taxa) is composed of an increasingly large number of morphological, biological and phylogenetic species. Most of the known species in this group have been isolated from agricultural ecosystems or have been described from a small number of isolates. We sampled Fusarium communities from native prairie grasses in Kansas and recovered a large number of isolates that superficially resemble F. anthophilum. We used a combination of morphological, biological and molecular characters to describe a new species, Gibberella konza (Gibberella fujikuroi mating population I [MP-I]), from native prairie grasses in Kansas. Although female fertility for field isolates of this species appears to be low, G. konza is heterothallic, and we developed reliably female fertile mating population tester strains for this species. The F. konzum anamorph is differentiated from F. anthophilum and from other Fusarium species in section Liseola by mating compatibility, morphology, AFLP fingerprint profile and differences in β-tubulin DNA sequence.

Armillaria root rot

Armillaria luteobubalina is a fungal phytopathogen endemic to Australia. First described by Podger et al, this species affects a wide range of plants in horticultural and native environments of temperate regions within Australia, colonising root and trunk tissue. This colonisation causes tissue necrosis and ultimately death of the host, giving it the disease name of Armillaria root rot. This disease has brought about considerable economic loss to horticultural, forestry and amenity plantings. To date, control options are limited, with removal of the infected material as the only proven successful management procedure.

Phosphate-solubilizing bacteria associated with proteoid roots of seedlings of waratah [Telopea speciosissima (Sm.) R.Br.]

Bacteria isolated from proteoid roots and non-proteoid lateral roots of Telopea speciosissima (Sm.) R.Br. seedlings were able to acidify the medium and solubilize calcium phosphates when grown in culture in the presence of ammonium salts and an appropriate carbon source. In general this activity was not detected when NO₃ ^- was substituted for NH₄ ⁺ , and it is proposed that protons were secreted in exchange for ammonium ions. Cation exchange between these protons and calcium in the medium is a possible cause of the calcium phosphate solubilizing activity. The relevance of these data to phosphate-solubilizing activity in natural environments is discussed.

Long-term effects of stubble management on the incidence of infection of wheat by Fusarium graminearum Schw. Group 1

The effect of 3 stubble management regimes (burning after harvest, incorporation with a disc plough, retention on the surface) on the incidence of infection of wheat with Fusarium graminearum Schw. Group 1 was studied for 5 seasons at 2 sites at Moree, New South Wales. One site had high initial incidence (site A) and the other low initial incidence (site B). There were no differences in incidence of infection between retained and incorporated treatments. Stubble burning reduced the increase in incidence of infection in 2 of 5 years at site A and 3 of 4 years at site B. Failure of control in other years was attributed to susceptible weed hosts and poor burns. When stubble was retained on the plots at site B that had been burnt, incidence of infection in the next season increased to a level not significantly different from the retained or incorporated treatments. Incidence of infection at the fourth consecutive wheat crop at both sites was close to the maximum recorded, which was 92% at site A and 65% at site B. There was no evidence of a decline in incidence by the time of the most recent season assessed (eighth year of continuous wheat cultivation at site A, and sixth year at site B). In most years, the differences in yield between treatments were not significant.

The impact of stubble management on the incidence of crown rot of wheat

The influence of 3 stubble management practices, stubble retention, stubble incorporation and stubble burning, on the incidence of crown rot of wheat (Triticum aestivum L.) caused by Fusarium graminearum Schwabe Group 1, and on plant development and grain yield was examined. The incidence of disease (percentage plants affected) was assessed in a susceptible (cv. Sunstar) and moderately resistant cultivar (cv. Suneca) in 1986. In 1987 Sunstar was planted into stubble of the 2 cultivars to assess the influence of host resistance on disease carryover. Crown rot was highest in the stubble retention plots (81% incidence in 1986 and 59% in 1987), whereas stubble burning decreased disease incidence in both years, with the reduction being greater in the second year (47% and 16%). Stubble incorporation was ineffective in reducing disease levels (76% and 53% in years 1 and 2). The incidence of crown rot did not differ in the 2 cultivars. At the harvest sampling in 1987 there were no differences in crown rot incidence in plants sown into stubble of the 2 cultivars. Grain yield did not differ significantly between treatments, but early season plant dry weight was reduced in the retained plots. Grain protein levels were reduced (P< 0.05) in the stubble burnt plots.