Crown rot: be aware of the balancing act or the fall may be harder

Steven Simpfendorfer and Matthew Gardner, NSW DPI Tamworth

Introduction

Crown rot, caused predominantly by the fungus Fusarium pseudograminearum is a significant disease of winter cereals in the northern region. Infection is characterised by a light honey-brown to dark brown discolouration of the base of infected tillers, while major yield loss from the production of whiteheads is related to moisture stress post-flowering. It is critical that growers understand that there are three distinct and separate phases of crown rot, namely survival, infection and expression and management strategies can differentially effect these phases.

Survival: the crown rot fungus survives as mycelium (cottony growth) inside winter cereal (wheat, barley, triticale and oats) and grass weed residues, which it has infected. The crown rot fungus will survive as inoculum inside the stubble for as long as it remains intact, which varies greatly with soil and weather conditions as decomposition is a very slow process.

Infection: given some level of soil moisture the crown rot fungus grows out of stubble residues and infects new winter cereal plants through the coleoptile, sub-crown internode or crown tissue which are all below the soil surface. The fungus can also infect plants above ground right at the soil surface through the outer leaf sheathes. However, with all points of infection, direct contact with the previously infected residues is required and infections can occur throughout the whole season given moisture. Hence, wet seasons (2010, 2011 and start of 2012) favour increased infection events by the crown rot fungus when combined with the production of greater stubble loads in wet seasons significantly builds-up inoculum levels.   

Expression: Yield loss is related to moisture/temperature stress around flowering and through grain-fill. Moisture stress is believed to trigger the crown rot fungus to proliferate in the base of infected tillers, restricting water movement from the roots through the stems, and producing whiteheads that contain either no grain or lightweight shrivelled grain. The expression of whiteheads in plants infected with crown rot (i.e. still have basal browning) is restricted in wet seasons and increases greatly with increasing moisture stress during grain-fill.

Crown rot was widespread in the northern region in 2012 but was not responsible for low grain protein levels (see Gardner, Fettell and Brill 2012 nitrogen paper). This was not surprising as the 2010 and 2011 seasons largely masked the expression of crown rot but were very conducive to infection and inoculum build-up within paddocks. However, 2012 was generally not a ‘big year’ for yield loss from crown rot, with the possible exception of durum crops, which are very susceptible to this disease. Most paddocks had a full profile of soil moisture at sowing in 2012 and good early rain (even some flooding) that provided soil moisture at depth and limited the expression of crown rot (yield loss) during grain-fill. These reserves of soil moisture buffered the impact of crown rot even though the second half of the 2012 season was very dry (see Gardner and Simpfendorfer 2012 crown rot paper). That is, the balancing act between crown rot inoculum and soil water was critical to individual paddock outcomes in 2012. How much soil moisture are you likely to have down the profile in 2013? How will this tip the balance?

What is the balancing act?

The two graphs below summarise some collaborative work conducted by the Northern Grower Alliance (NGA) and NSW DPI across 11 sites in 2007 as reported at pervious GRDC updates. However, they are worth revisiting here as they brilliantly illustrate the relative impact of both crown rot inoculum level and moisture stress on the extent of yield loss.

Figure 1a represents the average yield loss in cv. Lang across the 11 sites in 2007 compared to plots where no crown rot inoculum (0 g/m) was added. Even though artificially introduced it looks at the impact of starting inoculum levels on yield loss with the levels roughly representing a high risk (2 g/m), medium risk (1 g/m) or low risk (0.5 g/m). Extended lines represent the maximum yield loss at each inoculum level at an individual site in 2007, which was either Millie or Cryon where very hot/dry conditions during grain-fill were experienced which exacerbated the expression of crown rot. With a high level of crown rot inoculum (2 g/m), cv. Lang averaged 25% yield loss with up to 55% yield loss occurring under hot/dry conditions during grain-fill. Halving the level of crown rot inoculum (1 g/m) resulted in an average yield loss of 14% with losses topping out at 35% under moisture stressed conditions during grain-fill. Even the ‘low’ inoculum risk level (¼ of the top inoculum level, 0.5 g/m) resulted in an average yield loss of 9% with maximum yield loss of 25% still occurring under a ‘tough’ seasonal finish. Reducing crown rot inoculum levels within a paddock decreases the potential for yield loss but the actual extent of yield loss is very sensitive to the level of moisture stress during grain-fill. Up to 25% yield loss can still occur with a ‘low’ starting inoculum level if the crop becomes severely stressed during grain-fill.

Figure 1: Impact of crown rot inoculum load (a) and moisture stress during grain-fill (b) on yield loss to crown rot in 2007

Figure 1b further re-enforces the balance between soil moisture and inoculum by looking at the yield loss from crown rot averaged across 5 bread wheat varieties at 11 sites in 2007. Moisture stress during flowering/grain-fill clearly dictates the extent of yield loss from crown rot infection. Sites ranged from minimal moisture stress at Breeza, which was irrigated, through to Millie and Cryon where there were very dry and hot finishes in 2007. All inoculated plots at each site had the same source and ‘high’ rate of crown rot added at sowing. Hence, the seasonal finish, even under high crown rot infection, can mean 3% (no moisture stress) to 55% yield loss (hot/dry finish) in bread wheat (Figure 1b) or 13% to 90% yield loss in the durum variety EGA Bellaroi (data not shown).

Inoculum level is important in limiting the potential for yield loss from crown rot but the overriding factor dictating the extent of loss is moisture/temperature stress during grain-fill. Thus, the balance is heavily tipped towards soil water as evidenced by the 2012 season. Who had 55% yield loss in their bread wheat crop in 2012? Some paddocks probably had sufficient starting inoculum levels but full soil moisture profiles at sowing and early rainfall buffered losses in the majority of situations.

Some management strategies however, focus heavily on trying to combat crown rot inoculum, sometimes without full consideration of their impact on soil water. Any management strategy that limits storage of soil water or creates constraints (e.g. nematodes see Daniel et al. 2012 paper) which reduce the ability of roots to access this water increases the probability and/or severity of moisture stress during grain-fill and therefore exacerbates the impact of crown rot.

What is the effect of cultivation?

Growers may cultivate their stubble for a range of reasons e.g. after sowing chickpeas to improve herbicide safety. However, the effect of cultivation on crown rot is complex as it potentially impacts on all three phases of the disease cycle.

Survival: stubble decomposition is a microbial process driven by temperature and moisture. Cultivating stubble in theory increases the rate of decomposition as it reduces particle size of stubble, buries these particles in the soil where microbial activity is greater and the soil environment maintains more optimal moisture and temperature conditions compared to the soil surface or above ground. However, cultivation also dries out the soil in the cultivation layer, which immediately limits the potential for decomposition of the incorporated stubble. Decomposition of cereal stubbles is a very slow process that requires adequate moisture for an extended period of time to occur completely. A summer fallow (even if extremely wet and stubble has been cultivated) is not long enough!

Infection: as covered earlier, the majority of infection sites with crown rot are below ground and physical contact between an infected piece of residue and these plant parts is required to initiate infection. Cultivation of winter cereal stubble harbouring the crown rot fungus effectively breaks the inoculum into smaller pieces and spreads them more evenly through the cultivation layer across the paddock. Consequently, the crown rot fungus has been given a much greater chance of coming into contact with the major infection sites below ground as the next winter cereal crop germinates and develops. In a no-till system the crown rot fungus becomes confined to the previous cereal rows and is more reliant on infection through the outer leaf sheathes at the soil surface. This is why inter-row sowing with GPS guidance has been shown to provide around a 50% reduction in the number of plants infected with crown rot when used in a no-till cropping system. Cultivation or harrowing negates the option of inter-row sowing as a crown rot management strategy.

Expression: extensive research has shown that cultivation dries out the soil to the depth of cultivation and reduces the water infiltration rate due to the loss of structure (macropores etc). The lack of cereal stubble cover can also increase soil evaporation. With poorer infiltration and higher evaporation, fallow efficiency is reduced for cultivated systems compared to a no-till stubble retention system. Greater moisture availability has the potential to provide buffering against crown rot expression lat in the season. Like crown rot management and all farming practices, cultivation is a balancing act between perceived benefits and costs.

Was crown rot worse on worked or no-till paddocks in 2012? If you had heavy rain on worked paddocks at the start of the 2012 season did water sit on top of the soil because the profile was full or it could not infiltrate?

Soil type?

Soil type does not differentially affect the survival or infection phases of crown rot. However, the inherent water holding capacity of each soil type interacts with expression by potentially buffering against moisture stress late in the season. Hence, yield loss can be worse on red soils compared to black soils due to their generally lower water holding capacities. Any other sub-soil constraint e.g. sodicity, salinity or shallower soil depth effectively reduces the level of plant available water which can increase the expression of crown rot.

Were whiteheads worse in red paddocks, red patches within black paddocks or rocky or shallow ridges in 2012?

Rotations?

Growing non-host break crops remains an important tool for managing crown rot as they allow time for decomposition of winter cereal residues that harbour the crown rot inoculum. The canopy density and rate of canopy closure can impact on the rate of decomposition and varies with the different break crops (i.e. faba bean and canola better than chickpea). Row spacing and seasonal rainfall during the break crop also impact on decomposition and hence survival of the crown rot fungus. Break crops can also further impact on the expression of crown rot in the following winter cereal crop in terms of both the amount of soil water they use and therefore leave at depth and their impact on the build-up of root lesion nematodes.

Stubble burning?

Burning removes the above ground portion of crown rot inoculum but the fungus will still survive in infected crown tissue below ground so it is not a ‘quick fix’ for high inoculum situations. Removal of stubble residues through burning will increase evaporation from the soil surface and impact on fallow efficiency. A ‘cooler’ autumn burn is therefore preferable to an earlier ‘hotter’ burn as it minimises the negative impacts on soil moisture storage whilst still reducing inoculum levels.

Sowing time?

Earlier sowing within the recommended window of a given variety for a region generally brings the grain-fill period forward where the probability of moisture stress during grain-fill is reduced. Earlier sowing may also increase the extent of root exploration at depth that could provide greater access to deeper soil water later in the season, which buffers against crown rot expression. This has been shown in previous NSW DPI research across seasons to reduce yield loss from crown or see Gardner and Simpfendorfer 2012 paper for current Walgett findings.

Did you have more whiteheads (i.e. expression) in 2012 in early or late sown wheat crops?

How do new varieties look?

A variety trial was conducted at Gurley in northern NSW in 2012. The site had faba beans in 2011, barley in 2010 and was cultivated using kelly-chains prior to sowing in 2012. The trial site was soil cored (0-30 cm) for PreDicta B testing of background pathogen levels at sowing in 2012. The site had high levels of the root lesion nematode Pratylenchus thornei (9,183 Pt/kg soil) and medium risk of crown rot (140 pg Fusarium/kg soil). Ten bread wheat varieties and one durum variety were sown in replicated plots and evaluated for their relative tolerance to both Pt and crown rot. The tolerance and resistance rating of each variety to Pt or crown rot and yield outcomes are outlined in Table 1.

Table 1: Pratylenchus thornei tolerance, crown rot resistance ratings and yield of 10 bread wheat and 1 durum variety evaluated at Gurley in 2012. Yield values followed by the same letter are not different at the 95% confidence level.

When looking at the yield of each variety it is difficult to determine the relative contribution of both diseases to the final outcome as a lack of tolerance to Pt can also increase the severity of crown rot expression (yield loss). However, it is evident that varieties which have good levels of tolerance to Pt combined with some level of resistance to crown rot (even MS) had a significant improvement in yield in the presence of both of these diseases.

Within the bread wheat varieties Suntop was 1.54 t/ha (109%) higher yielding than Ellison which has poorer resistance/tolerance to both Pt and crown rot. Suntop and EGA Gregory are both moderately tolerant (MT) of Pt but the improved resistance of Suntop (MS) to crown rot over EGA Gregory (S) appears to have provided a 1.05 t/ha (55%) yield advantage at this site in 2012.

Both Pt and crown rot had significant impact alone and in combination with the clear message that varieties which are very susceptible or intolerant of either of these two pathogens should not be grown in medium to high risk situations. This was painfully obvious with Caparoi which was the only durum variety in the trial which yielded only 0.89 t/ha and was the most susceptible (VS) to crown rot as evidenced further by around 60-70% whiteheads during grain-fill in mid October.

Conclusion

Soil water is more important than inoculum load in reducing yield loss from crown rot. All management strategies need to have a balanced consideration of the impact they are potentially having on both crown rot inoculum levels and soil water.

Acknowledgments

The authors would like to thank Scott Carrigan, “Murray Cummumualah” Gurley for providing the 2012 trial site. This paper includes some older information conducted in collaboration with Northern Growers Alliance as acknowledged in the text. This information has been presented in greater detail at previous GRDC Updates with full reports available at www.grdc.com.au. Technical assistance provided by Robyn Shapland, Kay Warren, Rod Bambach, Peter Formann, Stephen Morphett and Jim Perfrement are gratefully acknowledged.

Contact details

Steven Simpfendorfer
NSW DPI
Ph: 0439581672
Email: steven.simpfendorfer@dpi.nsw.gov.au