Using yield response curves to measure variation in the tolerance and resistance of wheat cultivars to Fusarium crown rot

The disease crown rot, caused predominantly by the fungal pathogen Fusarium pseudograminearum (Fp), is a major disease of winter cereals in many regions of the world, including Australia. A methodology is proposed, using response curves, to robustly estimate the relationship between grain yield and increasing crown rot pathogen burdens. Using data from a field experiment conducted in northern New South Wales, Australia in 2016, response curves were derived for five commercial wheat cultivars exposed to six increasing rates of crown rot inoculum, where the rates served to establish a range of crown rot pathogen burdens. In this way, the response curve methodology is fundamentally different from alternate approaches that rely on genetic or environmental variation to establish a range in pathogen burdens over which yield loss relationships are estimated. By manipulating only the rates of crown rot inoculum and thus pathogen burden directly, the number of additional confounding factors and interactions are minimised, enabling the robust estimation of the rate of change in yield due to increasing crown rot pathogen burdens for each cultivar. The methodology revealed variation in the rate of change in yield between cultivars, along with the extent of crown rot symptoms expressed by the cultivars. Variation in the rate of change in yield between cultivars provides definitive evidence of differences in the tolerance of commercial Australian wheat cultivars to crown rot caused by Fp, while variation in the extent of crown rot symptoms signifies differences in the resistance of the cultivars to this disease. The response curve methodology also revealed variation in how the different mechanisms of tolerance and resistance act to limit yield losses due to crown rot for different cultivars

Stubble Olympics: the cereal pathogen 10cm sprint – growth patterns of fungi causing crown rot, common root rot and yellow leaf spot in post-harvest cereal stubble

Take home messages • Wetter is better (for cereal pathogens): moist conditions promoted growth of pathogenic fungi (Fusarium pseudograminearum, Bipolaris sorokiniana and Pyrenophora tritici-repentis) within post-harvest cereal stubble, meaning inoculum levels of crown rot, common root rot and yellow spot may increase if wet weather is experienced after harvest • Not all cereal stubble is created equally: some pathogens progressed further in oat than bread wheat stubble. Additionally, there are indications that the resistance ratings of varieties and crops do not reflect the extent of saprophytic growth post-harvest • Each cereal pathogen had a unique stubble-colonisation pattern: the crown rot fungus was the quickest to progress within all stubble types and the yellow spot pathogen was the slowest. This is likely to influence which pathogen dominates in following seasons if mixed infections have occurred in the same crop • Reducing cereal stubble biomass may limit the post-harvest progression of pathogenic fungi in stubble, thereby reducing the amount of inoculum carried forward. Options could include selection of low-biomass varieties, low harvest heights or cutting for hay, however field validation is required

Stubble trouble: mapping Fusarium crown rot survival under different cereal stubble management scenarios

The adoption of stripper-front headers, light tillage and rotations with shorter-stature break crops is increasing in northern New South Wales cropping systems. The effect of these practices on survival of Fusarium crown rot inoculum in cereal stubble is unknown but may impact future disease risks. Field experiments at Narrabri and Breeza in NSW examined whether reducing the harvest-height of a cereal crop infected with Fusarium crown rot (Fusarium pseudograminearum, Fp) limits post-harvest pathogen colonisation. Pathogen survival was assessed within standing cereal stubble during a chickpea break crop in 2020. Fp was recovered higher within stems in tall and medium harvest-height treatments in May 2020 compared to harvest levels in 2019. This confirms that wet fallow conditions allow Fp to saprophytically colonise cereal stubble and shortening harvest-heights could limit this inoculum production. Performance and disease levels in a successive wheat crop is required to verify potential benefits of this practise

Stubble trouble! Moisture, pathogen fitness and cereal type drive colonisation of cereal stubble by three fungal pathogens

Stubble-borne cereal diseases are a major constraint to production in Australia, with associated costs rising as a result of increased adoption of conservation agriculture systems. The fungal pathogens that cause these diseases can saprotrophically colonise retained cereal residues, which may further increase inoculum levels post-harvest. Hence, saprotrophic colonisation by the stubble-borne fungal pathogens Fusarium pseudograminearum, Pyrenophora tritici-repentis and Bipolaris sorokiniana were compared under a range of moisture conditions for stubble of six cereal varieties (two bread wheat, two barley, one durum wheat and one oat). Sterile cereal stubble was inoculated separately with two isolates of each pathogen and placed, standing, under constant relative humidity conditions (90, 92.5, 95, 97.5 and 100%) for 7 days at 25 °C. Stubble was then cultured in increments of 1 cm to determine the percentage colonisation height of each tiller. Fusarium pseudograminearum colonised farther within tillers, leaving a greater proportion of the standing stubble colonised compared with B. sorokiniana and P. tritici-repentis, suggesting F. pseudograminearum has higher saprotrophic fitness. Saprotrophic colonisation also increased with increasing relative humidity for all pathogens and varied by cereal type. Disease management strategies, such as reduced cereal harvest height, may limit saprotrophic colonisation and improve stubble-borne disease management in conservation agriculture systems

“Killer Joules”: spores of Bipolaris sorokiniana and fusarium species are susceptible to microwave radiation

Cereal production in Australia is severely impacted by diseases such as Fusarium crown rot (caused predominantly by Fusarium pseudograminearum) and common root rot (caused by Bipolaris sorokiniana). These diseases are particularly difficult to manage because inoculum can survive at least three years within cereal stubble, or four years in soil in the case of B. sorokiniana. Microwave radiation may be able to reduce or eliminate inoculum within stubble and soil. Several cereal pathogens have been previously shown to be susceptible to microwave radiation, but the energy requirements to achieve a significant decrease in pathogen populations were not defined. Laboratory based microwave dose-response experiments on conidia of B. sorokiniana and macroconidia of F. pseudograminearum and F. cerealis revealed that all three pathogens are susceptible to microwave radiation, with lethal dose (LD) thresholds estimated for each pathogen. Bipolaris sorokiniana conidia required 103.8 Jg− 1 and 236.6 Jg− 1 of microwave radiation energy for LD50 and LD99, respectively, whilst F. pseudograminearum required 78.4 Jg− 1 and 300.8 Jg− 1 and F. cerealis required 95.3 Jg− 1 and 152.7 Jg− 1 for LD50 and LD99, respectively. These results were derived from experiments whereby samples were microwaved for up to 10 s using a domestic 1100 W microwave oven. These timing and energy requirements serve as a starting point to define requirements for further development of microwave radiation treatments under field conditions

Is there a disease downside to stripper fronts? Harvest height implications for Fusarium crown rot management

Take home messages • Taller standing stubble allowed vertical progression of the Fusarium crown rot fungus within the stubble after harvest, whilst short stubble prevented further growth (i.e. vertical growth was limited to the height of the cut stubble). • Stripper fronts, which leave higher standing stubble, may increase stubble-borne disease inoculum after harvest of an infected crop, especially if wet fallow conditions are experienced. • In high-risk situations, such as an infected crop with high biomass, cutting the crop shorter at harvest will limit further inoculum development within the stubble after harvest (beyond the levels already present at harvest). • Cutting infected cereal stubble shorter prior to rotation with shorter-stature crops such as chickpea or lentils also prevents the dispersal of infected stubble when harvesting these shorter break crops

Transposon-Mediated Horizontal Transfer of the Host-Specific Virulence Protein ToxA between Three Fungal Wheat Pathogens

Most known examples of horizontal gene transfer (HGT) between eukaryotes are ancient. These events are identified primarily using phylogenetic methods on coding regions alone. Only rarely are there examples of HGT where noncoding DNA is also reported. The gene encoding the wheat virulence protein ToxA and the surrounding 14 kb is one of these rare examples. ToxA has been horizontally transferred between three fungal wheat pathogens (Parastagonospora nodorum, Pyrenophora tritici-repentis, and Bipolaris sorokiniana) as part of a conserved ∼14 kb element which contains coding and noncoding regions. Here we used long-read sequencing to define the extent of HGT between these three fungal species. Construction of near-chromosomal-level assemblies enabled identification of terminal inverted repeats on either end of the 14 kb region, typical of a type II DNA transposon. This is the first description of ToxA with complete transposon features, which we call ToxhAT. In all three species, ToxhAT resides in a large (140-to-250 kb) transposon-rich genomic island which is absent in isolates that do not carry the gene (annotated here as toxa−). We demonstrate that the horizontal transfer of ToxhAT between P. tritici-repentis and P. nodorum occurred as part of a large (∼80 kb) HGT which is now undergoing extensive decay. In B. sorokiniana, in contrast, ToxhAT and its resident genomic island are mobile within the genome. Together, these data provide insight into the noncoding regions that facilitate HGT between eukaryotes and into the genomic processes which mask the extent of HGT between these species.M.C.M. acknowledges The Sun Foundation’s Peer Prize for Women in Science for support to sequence additional ToxA isolates. E.H. acknowledges The Grains and Research Development Corporation (project UHS11002). M.C.M., A.M., S.S., and P.S.S. also acknowledge The Grains and Research Development Corporation for the collection of isolates (projects DAN00203 and DAN00177)

Impact of Fusarium Crown Rot on Root System Area and Links to Genetic Variation within Commercial Wheat Varieties

Fusarium crown rot (FCR), caused by the fungal pathogen Fusarium pseudograminearum (Fp), is a major constraint to cereal production worldwide. The pathogen restricts the movement of solutes within the plant due to mycelial colonisation of vascular tissue. Yield loss and quality downgrades are exacerbated by this disease under water stress conditions. Plant root systems are adaptive and can alter their architecture to optimise production in response to changes in environment and plant health. This plasticity of root systems typically favours resource acquisition of primarily water and nutrients. This study examined the impact of FCR on the root system architecture of multiple commercial bread and durum wheat varieties. Root system growth was recorded in-crop in large transparent rhizoboxes allowing visualization of root architecture over time. Furthermore, electrical resistivity tomography was used to quantify spatial root activity vertically down the soil profile. Results demonstrated a significant reduction in the total root length and network area with the inoculation of FCR. Electrical resistivity measurements indicated that the spatial pattern of water use for each cultivar was influenced differently from infection with FCR over the growing season. Specifically temporal water use can be correlated with FCR tolerance of the varieties marking this investigation the first to link root architecture and water use as tolerance mechanisms to FCR infection. This research has implications for more targeted selection of FCR tolerance characteristics in breeding programs along with improved specific varietal management in-crop

Fusarium Crown Rot Reduces Water Use and Causes Yield Penalties in Wheat under Adequate and above Average Water Availability

The cereal disease Fusarium crown rot (FCR), caused by the fungal pathogen Fusarium pseudograminearum, is a worldwide major constraint to winter cereal production but especially in Australia's northern grain's region (NGR) of NSW and Queensland. Conventionally, FCR induced yield penalties are associated with semi-arid water-limited conditions during flowering and grain-filling. In this study, yield penalties associated with FCR infection were found to be significant under both adequate and above average water conditions which has implication for global wheat production in more favorable environments. This research was conducted to understand the impact of FCR on water availability, yield and grain quality in high protein bread and durum wheat varieties in controlled environment and replicated field experiments across three locations in the NGR over a two-year period. Under controlled conditions, FCR infection significantly decreased water use by 7.5% with an associated yield reduction of 9.5% irrespective of water treatment. Above average rainfall was experienced across all field experimental sites in both 2020 and 2021 growing seasons. The field studies demonstrated a decrease in water use of upwards of 23% at some sites and significant yield penalties across all cultivars of up to 18.4% in natural rainfed scenarios to still 13.2% with further supplementary irrigation

PREDICTA®B update and new tests for 2018

- Soil-borne pathogens most likely to pose the greatest risk to cereal crops in the northern region during 2018 include crown rot, common root rot and root lesion nematodes. - PREDICTA®B has added new tests for ascochyta blight and phytophthora root rot of chickpeas, yellow leaf spot and white grain disorder of wheat, fusarium stalk rot of sorghum, charcoal rot of summer crops and arbuscular mycorrhizal fungi (AMF). - Follow sampling recommendations in the manual V10 (Broadacre Soilborne disease manual), including the additional of pieces of stubble to improve the detection of stubble-borne pathogens. - Frequently more than one soil-borne disease exists within a paddock with the interaction between pathogens (e.g. Pratylenchus thornei and crown rot) exacerbating losses. PREDICTA B is assisting pathologists to understand these interactions to advise growers on management options to limit the impact of these disease complexes