Take home messages
- Yield losses due to crown rot of 40 per cent (%) and 60% were measured in susceptible bread wheats in the Wimmera and Mallee respectively, with losses up to 90% occurring in durum wheat.
- Crown rot yield losses were related to the inoculum level present at planting and were exacerbated by the dry finish to the season.
- Growers should avoid planting wheat, especially durum, in paddocks with high crown rot inoculum levels. Test paddocks prior to sowing using PREDICTA B if unsure of inoculum levels.
Background
Crown rot (caused by the fungus Fusarium pseudograminearum) has become a common disease in Victoria due to the intensification of cereals and stubble retention practices. A recent study in Victoria and South Australia of 892 PREDICTA B test results (2015-2017) showed that 54% and 31% of paddocks in the Wimmera and Mallee respectively, had medium to high levels of crown rot inoculum detected. The losses caused by crown rot are much greater in seasons with a dry finish and are related to the inoculum levels in the paddock at planting.
The distinctive symptom of crown rot is the presence of white heads in the crop at early grain fill. These heads mature early and contain shrivelled or no grain. White head expression is more common in seasons with a dry spring. The other, and more reliable, symptom of crown rot infection is browning of stem bases. To check for stem browning pull back the leaf sheath and inspect the base of the stem for a brown honeycomb colour. A PREDICTA B test provides a reliable option to identify crown rot risk in paddocks before planting.
Aim
To investigate the effects of increasing levels of crown rot inoculum on yield loss in bread and durum wheat cultivars in both the Wimmera and Mallee regions of Victoria.
Paddock details
| Location: | Horsham (Wimmera) and Birchip (Mallee) |
| Crop year rainfall (Nov-Oct): | Horsham – 284mm Birchip – 189mm |
| GSR (Apr-Oct): | Horsham – 189mm Birchip – 129mm |
| Soil type: | Horsham – Clay vertisol Birchip – Clay loam |
| Paddock history: | Horsham – Peas (brown manure) 2017 Birchip – Lentils 2017 |
Trial details
Crop type:
Table 1. Bread and durum wheat cultivars with associated crown rot resistance reaction.
| Treatments: | Six rates of crown rot inoculum were applied at planting (0, 0.25, 0.5, 1.0, 2.0 and 4.0g/m row) using sterile durum wheat seed colonised with Fusarium pseudograminearum. |
| Target plant density: | Horsham – 150 plants/m² Birchip – 140 plants/m² |
| Seeding equipment: | Knife points, press wheels, 30 cm row spacing |
| Sowing date: | Horsham – 26 June 2018 Birchip – 23 May 2018 |
| Replicates: | Three |
| Harvest date: | Horsham – 11 December 2018 Birchip – 3 December 2018 |
| Trial average yield: | Horsham – 2.47t/ha (disease free) Birchip – 1.06t/ha (disease free) |
Trial inputs
| Fertiliser: | Managed to maximise grain yield and quality according to soil test results. |
| Herbicide: | Weeds were controlled according to best management practice. |
| Insecticide: | Pests were controlled according to best management practice. |
| Fungicide: | Foliar fungicides were applied to control rust as required. |
Method
The same field experiment was undertaken at both Horsham and Birchip during 2018. Within each experiment the same four bread and one durum wheat cultivars (Table 1) were planted with increasing rates of crown rot inoculum (applied as sterilised wheat seed colonised with Fusarium pseudograminearum). The field trials were sown as split plot designs with three replicates. Both trials were assessed for white head severity (% of white heads/plot) and grain yield.
Results and interpretation
Increasing rates of crown rot inoculum present at planting resulted in more white heads and reduced grain yield in field trials conducted in both the Wimmera (Horsham) and Mallee (Birchip) (Tables 2 to 5). At the 2.0g inoculation rate (a high crown rot level) mean yield losses of 37 and 56% were measured in the Wimmera and Mallee, respectively but importantly the actual yield losses were greater in the Wimmera at 0.9t/ha than in the Mallee at 0.6t/ha. Yield losses were greatest in the very susceptible durum wheat (WID802) and least in the moderately susceptible bread wheat (Emu Rock).
These results highlight how damaging crown rot can be in seasons with a dry finish. The results also demonstrate that in paddocks with crown rot inoculum present, if wheat is to be grown, selecting cultivars with partial resistance (such as Emu Rock) can reduce the extent of yield loss.
Table 2. Effect of increasing rates of crown rot inoculum present at planting on white head severity (%) in bread and durum wheat at Horsham during 2018.
At Horsham sowing the MS rated bread wheat variety Emu Rock provided a 0.52 to 0.85t/ha yield benefit (30-60%) over the S rated bread wheat varieties Shield, Suntop and Cobra under high inoculum levels (2.0g/m row). The yield benefit was even higher at 1.35t/ha (153%) compared with the VS rate durum variety WID802 (Table 3).
Table 3. Effect of increasing rates of crown rot inoculum present at planting on grain yield (t/ha; percentage yield loss relative to the untreated control in parenthesis) of bread and durum wheat at Horsham during 2018.
Table 4. Effect of increasing rates of crown rot inoculum present at planting on white head severity (%) in bread and durum wheat at Birchip during 2018.
Under lower yield potentials, given the season at Birchip there were still significant yield benefits associated with cereal crop and variety choice in the presence of high crown rot inoculum levels. At Birchip during 2018 sowing the MS rated bread wheat variety Emu Rock provided a 0.41 to 0.56t/ha yield benefit (80-156%) over the S rated bread wheat varieties Shield, Suntop and Cobra under high inoculum levels (2.0g/m row). The yield benefit was even higher at 0.82t/ha (820%) compared with the VS rated durum variety WID802 (Table 5).
Table 5. Effect of increasing rates of crown rot inoculum present at planting on grain yield (t/ha; percentage yield loss relative to the untreated control in parenthesis) of bread and durum wheat at Birchip during 2018.
Commercial practice
Crown rot must be managed prior to sowing as there are no in-crop control options available. A PREDICTA B soil test can be used to identify crown rot risk in paddocks before sowing, thus enabling high losses during seasons conducive to crown rot, as demonstrated in these studies, to be avoided (Hollaway et al. 2013). Even if crown rot is detected in a paddock the risk associated with crown rot can be reduced by avoiding sowing more susceptible cereals (eg. durum wheat which is rated VS) or S rated bread wheat varieties.
In paddocks with medium to low crown rot levels, yield losses can be minimised by avoiding durum wheat and growing barley in preference to susceptible bread wheat varieties. Even though barley is a good option for reducing yield loss from crown rot, it will still increase inoculum levels for the next crop (Evans et al. 2010). Barley is a very good host of the crown rot fungus. It tends to suffer less yield loss through its earlier maturity relative to bread wheat. This generally provides a level of escape from hotter and drier conditions during grain filling which exacerbate the expression of crown rot as white heads and hence the extent of yield loss. However, growers should be aware that if moisture or heat stress occurs during the vegetative stage crown rot can kill barley plants.
In paddocks with high levels of crown rot it is best to avoid growing cereals. Previous research showed that cereals increased inoculum levels while broadleaf crops and fallow decreased crown rot inoculum levels (Evans et al. 2010). In general, a two-year grass-free break from cereals is required to reduce medium to high inoculum levels to low levels. A three-year break may be required following the dry season of 2018.
References
Acknowledgements
This research was funded by the GRDC as part of the ‘Yield Loss Response Curve Project’ (DAW00245) with national project collaborators NSWDPI, DPIRD, QDAF, SARDI and AgVic. Clayton Forknall (SAGI North) produced the experimental designs.
