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Taking stock of what we know about Crown rot

Crown rot symptoms at the base of tillers. Source: lee Hickey.Crown rot has long been present in Australian cropping systems, but the introduction of minimum tillage systems, changing rainfall patterns and increased heat events has meant an increase in the severity and prevalence of this disease.

This article looks at the key findings from the recent review Crown rot of wheat in Australia: Fusarium pseudograminearum taxonomy, population biology and disease management which examined what is already known about crown rot and what new management strategies could be on the horizon.

Crown rot can survive in most grass species

The fungal pathogen which causes crown rot, Fusarium pseudograminearum (Fp), has a broad host range including all winter cereals (bread wheat, barley, durum wheat, oats and triticale) and numerous native and introduced grasses, some of which may be infected by the disease but do not express symptoms (e.g. oats and grass weeds). The fungus is present in cereal and grass residues and can infect plants at any stage of growth if adequate moisture is present. This wide host range means that crop rotation to non-host species such as pulse and oilseeds, along with effective grass weed control, are key management strategies.

The impacts of stubble retention

The crown rot fungus, Fp, is a stubble-borne pathogen which survives as hyphae inside winter cereal and grass weed residues. Hence, the increased adoption of stubble retention systems has seen a rise in inoculum pressure. Moisture retention also improves with minimum tillage and creates ideal conditions for crown rot infection in emerging cereal crops post rainfall events. Infection primarily occurs below ground (coleoptile, sub-crown internode and crown) but in stubble retention systems can also occur through the outer leaf sheaths right at the base of tillers. Rain splash of spores produced around lower nodes on stems infected with Fp can also cause head infections around flowering, resulting in Fusarium head blight (for more information on Fusarium head blight watch the video ‘What caused low levels of Fusarium head blight in 2016?’). Management strategies for reducing inoculum of Fp in paddocks will reduce the incidence of both types of infection. Stubble burning has historically provided good control of the disease through significant inoculum reduction, however, it only destroys the above-ground portion of the inoculum and does not reduce inoculum levels surviving in the crowns below ground. In addition, stubble burning is no longer standard practice in minimum tillage systems and incomplete burning can compromise efficacy.

Impacts of environmental conditions on crown rot expression

Environmental stress is key to the expression of crown rot infection. Drier conditions result in higher susceptibility to infection, while water stress at flowering causes dead or partially filled whiteheads. The impact of minimum tillage on water retention in the soil profile has generally led to lower plant stress at flowering, and less expression of whiteheads is seen; however, inoculum levels are still building up. Temperature is also an environmental determinant of the severity of the crown rot infection. At 25°C, the disease becomes more aggressive when comparing moderately resistant and susceptible varieties, while at 13°C there is no difference in the aggressiveness of the disease on varieties with varying levels of resistance.

Fungicides only provide limited control

Research into the use of fungicides to control this disease has covered both seed treatments and post-emergent applications. Treatment of seed can reduce seedling death but not provide longer-term protection, yield advantage in the presence of inoculum, or limit inoculum build up which will impact future crops. Ipoconazole + metalaxyl is the only registered product for crown rot suppression and has some efficacy against seedling death but limited efficacy in reducing yield loss as a standalone management practice. Targeted in-crop application of tebuconazole + prothioconazole at the base of tillers was found in one study to provide some yield benefit (19%), but the average yield was still 0.71 t/ha lower than yield in the absence of crown rot – demonstrating that in-crop fungicide application, even when targeted at the base of tillers where crown infection in concentrated, does not provide complete control.

Rotation is the number one option

Due to the nature of the disease, and the current stubble management practices, rotation with non-host crops is considered the most effective control strategy. Rotation with crops such as chickpea, faba beans, field peas, canola, lupins and summer crops such as sorghum (which is one of the best crops to be included in rotation) needs to be implemented over at least two seasons. Not only does rotation to non-host crops reduce the incidence of crown rot infection, but the aggressiveness of Fp has been shown to increase in wheat-on-wheat rotations.

Row placement, variety resistance and PREDICTA® B testing can be used to reduce losses

The ability to precision plant a new cereal crop between the previous cereal rows with inter-row sowing has provided an option to reduce crown rot infection levels. Contact between the lower plant parts of the new crop and Fp infected stubble from the previous cereal crop is reduced with inter-row sowing which subsequently reduces the incidence of plants which become infected. Research has shown yield can increase by 9% with inter-row sowing compared to planting on the previous cereal rows due to an average 50% reduction in the incidence and severity of crown rot. It is important to remember that despite the reduced incidence of infection with inter-row sowing, crown rot inoculum levels will still build-up at a reduced rate in the system which does still not allow for continuous wheat production.

Our limited knowledge of the genetic basis and mechanisms of resistance to crown rot has so far limited progress with resistance breeding, with a range of partial resistance genes currently identified. However, the levels of moderate resistance incorporated into some varieties including Sunguard and Emu Rock and improved yield performance in the presence of crown rot infection in varieties such as Suntop, still provide moderate and useful reductions in the extent of yield loss from this disease. PREDICTA® B is a critical genetic molecular tool for understanding the potential risk from crown rot infection along with a range of other soil and stubble-borne pathogens which can be used to inform management practices and rotation decisions. More information on PREDICTA® B testing is available on the SARDI website.

How else might we look to control crown rot?

Due to the effect which environmental stress has on the impact of crown rot on yield, this may be leveraged to manage the disease. Management of water stress could offer options to decrease the effect of crown rot on yield by enhancing the plant’s ability to access deeper stored moisture through desirable root architecture. Desirable root architecture could mean plants with narrow root angle and high root number in deeper soil types, or conversely wider growth habit and shallow root systems for shallow soil types with sporadic rainfall patterns such as in Western Australia. Early maturing varieties also limit terminal drought stress and heat stress during grain filling which may reduce the expression of crown rot. Heat tolerance traits (waxy leaves, leaf rolling) may also offer benefits to reduce crown rot expression and reduce impacts on yield and grain quality.

Read the full review in the Australasian Plant Pathology Journal here: Crown rot of wheat in Australia: Fusarium pseudograminearum taxonomy, population biology and disease management.

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