Crown rot – does cereal crop or variety choice matter?

Take home message

• Barley and bread wheat varieties do vary in their yield response to crown rot infection
• Variety choice can provide a 20-50% yield benefit over growing the susceptible variety EGA Gregory in the presence of high levels of crown rot infection
• However, all varieties are susceptible to crown rot infection and will not significantly reduce inoculum levels for subsequent crops. Variety choice is NOT a sole solution to crown rot
• Crown rot tolerance should not be the only consideration in variety choice, impacts on other pathogen populations, especially Pratylenchus thornei, resistance to other pathogens, grain quality and delivery should all be considered along with relative grain prices.

Background

Crown rot, caused predominantly by the fungus Fusarium pseudograminearum is a significant disease of winter cereal crops in the northern NSW and southern Qld. All winter cereal crops host the crown rot fungus. Yield loss varies between crops and the approximate order of increasing loss is oats, barley, triticale, bread wheat and durum. Barley is very susceptible to crown rot infection and will build up inoculum but tends to suffer reduced yield loss through its earlier maturity relative to wheat. Late planted barley can still suffer significant yield loss especially when early stress occurs within the growing season.

Yield loss trials conducted across 11 sites in northern NSW in 2007, in collaboration with the Northern Grower Alliance (NGA), found that the average yield loss from crown rot was 20% in barley (4 varieties), 25% in bread wheat (5 varieties) and 58% in one durum (EGA Bellaroi). In 2007, a yield benefit of only around 5-10% could be demonstrated in bread wheat varieties between choosing the best and the worst entries in the presence of high levels of crown rot infection. However, recent research highlights that some newer bread wheat varieties appear to differ significantly in their level of yield loss to crown rot with some in the northern region (Sunguard, Suntop, LRPB Spitfire, LRPB Lancer and Mitch) appearing to suffer less yield impacts compared to the widely grown EGA Gregory. NSW DPI trials from a total of 23 sites conducted across the northern region in 2013 and 2014 indicate that this can represent a yield benefit of around 0.50 t/ha in the presence of high levels of crown rot infection. In a relatively short period of time the yield benefit associated with bread wheat variety choice in the presence of high crown rot infection has grown to around 20-30% with some of these newer varieties.

Continued research in 2015

A further 12 replicated crown rot yield loss trials were conducted across northern NSW and southern Qld in 2015 with sites spread from Wongarbon in the south to Macalister in the north (Figure 1). There were two barley, 13 bread wheat and one durum entry evaluated across the trials in 2015 (Figure 2). The trials used an inoculated versus uninoculated trial design to evaluate the relative yield response of varieties to crown rot infection at each site. Each site was soil cored at sowing (separate bulk samples across each range) to determine background pathogen levels using the DNA based soil test PreDicta B®. Post-harvest soil cores were also collected from all plots at Wongarbon and Macalister in December and analysed by PreDicta B to determine the impact of varieties on the build-up of populations of the root lesion nematode, Pratylenchus thornei (Pt) and crown rot inoculum over the 2015 season. These two sites were targeted for post-harvest assessment based on PreDicta B analysis of all 12 sites at sowing.

Yield impact – site effects

Background crown rot inoculum levels existed at half of the 12 sites with medium background crown rot levels at Mullaley, Macalister and Merriwa while high background levels were measured at Coonamble, Wongarbon and Mungindi. All trials were conducted in grower paddocks and generally co-located with GRDC funded National Variety Trials (NVT). One criterion for selecting sites is that they are generally paddocks with a good crop rotation, with all 12 sites having a non-winter cereal break (chickpea, canola or sorghum) as the previous crop within the rotation sequence. The medium to high background crown rot inoculum levels still evident at half of the sites highlights the continuing difficulty of managing stubble-borne inoculum levels of the crown rot fungus across the region. Background crown rot levels potentially underestimate the yield impact associated with crown rot infection as varying levels of infection would have occurred in the no added crown rot (CR) treatment plots at these sites.

Yield varied across the sites which, when averaged across the 16 winter cereal entries, ranged from 4.60 t/ha at Mullaley down to 2.87 t/ha at Mungindi in the no added CR treatments (Figure 1). The addition of crown rot inoculum at sowing (added CR) significantly reduced yield at all 12 sites in 2015 when averaged across entries. Average yield loss (difference between no added CR and added CR treatments) ranged from 7% (0.29 t/ha) at Trangie up to 43% (1.22 t/ha) at Mungindi (Figure 1).

Figure 1. Average yield of 16 winter cereal entries in the absence and presence of added crown rot (CR) inoculum at 12 trial sites in 2015

Did cereal crop type and/or variety make a difference?

An across site analysis was conducted to assist in summarising the general trends in varietal performance across the 12 sites in 2015. Only yield results from the barley variety Commander at Mungindi were excluded from the analysis due to severe damage from the herbicide Topik® which was applied across the predominantly wheat trial site. Significant lodging of both barley varieties occurred at North Star due to delayed harvest of the trial site waiting for some of the bread wheat entries to mature. Unfortunately a significant rain event occurred during this period which severely lodged the barley and caused some sprouting which differentially impacted on barley yield at this site with La Trobe appearing to be more disadvantaged than Commander. Barley yellow dwarf virus (BYDV) was evident in the Merriwa trial site with the yield impact appearing to be greater in La Trobe (2.78 t/ha) than in Commander (3.44 t/ha). Based on Western Australian data Commander is rated MR-MS to BYDV while La Trobe has a provisional rating of S. The impact of BYDV on yield is generally greater in barley than in wheat, but as appears to have occurred at Merriwa, varieties can differ significantly in their levels of resistance. It is notable that the NVT trial conducted at this site was all treated with the seed treatment Hombre which contains a fungicide and the insecticide imidicloprid. Imidicloprid has been shown to provide early season control of aphids which transmit BYDV. No BYDV symptoms were evident in the NVT trial while interveinal yellowing/reddening of leaves characteristic of BYDV infection was obvious throughout the crown rot trial which was all treated with the same fungicide as a seed treatment (Dividend® M). However, yield results from the two barley varieties at both North Star and Merriwa were still included in the across site analysis. 

Figure 2. Impact of crown rot on the yield of two barley, 13 bread wheat and one durum entry averaged across 12 trial sites in 2015

Averaged across sites, yield in the no added CR treatments (grey bars) ranged in the barley from 4.07 t/ha (La Trobe) to 3.86 t/ha (Commander), in the bread wheat from 3.86 t/ha (Beckom) to 3.36 t/ha (EGA Gregory) and was 3.36 t/ha with Jandaroi, the only durum variety included in the trial series (Figure 2). Remember, yield in the no added CR treatments was potentially impacted by background crown rot inoculum levels at half of the sites. The addition of crown rot inoculum at sowing (black bars) significantly reduced the yield of all entries compared to the no added CR treatments (grey bars). Yield loss associated with high levels of crown rot infection when averaged across the sites was 9% (0.37 t/ha) in La Trobe and 14% (0.53 t/ha) in Commander. In bread wheat, yield loss ranged from 11% (0.38 t/ha) in Sunguard up to 30% (1.04 t/ha) in EGA Gregory(Figure 2). Yield loss averaged 25% (0.83 t/ha) in the durum variety Jandaroi.

Another way of comparing the relative impact of crown rot on the yield of varieties, which is not complicated by differential background inoculum levels present at the different sites, is to concentrate on the absolute yield achieved under high disease pressure in the added CR treatments (black bars; Figure 2). Under high crown rot pressure average yield ranged from 3.70 t/ha in the barley variety La Trobe down to 2.45 t/ha in the widely grown bread wheat variety EGA Gregory. Only the advanced bread wheat line V07176-69 and the durum variety Jandaroi were not significantly higher yielding than EGA Gregory when averaged across the 12 sites. The average yield benefit over growing EGA Gregory under high crown rot infection ranged from 1.25 t/ha (51%) with the barley variety La Trobe down to 0.13 t/ha (5%) with the recently released bread wheat variety LRPB Flanker. However, the relative yield benefit compared to EGA Gregory was considerably greater with other bread wheat varieties such as LRPB Lancer (0.51 t/ha; 21%), LRPB Gauntlet (0.52 t/ha), Sunguard (0.54 t/ha), Mitch (0.54 t/ha), LRPB Spitfire (0.61 t/ha), Suntop (0.72 t/ha) and Beckom (0.82 t/ha; 33%). Commander, the second barley variety in the trials, averaged a 0.88 t/ha (36%) yield benefit over EGA Gregory under high levels of crown rot infection across the 12 sites in 2015 (Figure 2).

Varietal impact on final soil populations of Pratylenchus thornei

The root lesion nematode, Pratylenchus thornei (Pt), has been demonstrated in repeated studies to be widespread across the northern region and at moderate to high populations it appears to interact with the expression of crown rot which can exacerbate yield loss from both pathogens. While consideration needs to be given to the relative yield of cereal type and variety in the presence of crown rot infection in the current season, potential consequences of these choices on the build-up of Pt for subsequent crops within the rotation should not be overlooked. Final Pt populations developed by the 16 different winter cereal entries was determined after harvest at two sites (Wongarbon and Macalister) in 2015 to determine potential residual impacts on the differential build-up of Pt populations within a rotational sequence.

Both sites had similar average starting Pt populations across the trial area at sowing of around 5.5-5.6 Pt/g soil in the 0-30 cm soil layer. This level is generally considered a medium risk for yield loss in intolerant wheat varieties (low = <2.0, medium = 2-15 and high =>15 Pt/g soil). Final populations established by the varieties during the 2015 season varied markedly between the two sites but significant differences between varieties were evident. Final Pt populations varied from 0.9 Pt/g soil after Suntop, up to 19.8 Pt/g soil after Mitch at Wongarbon.At Macalister populations varied from 11.8 Pt/g soil after Commander up to 105.0 Pt/g soil after Mitch (Table 1). There was generally a fair consistency between the ranking of varieties between the two sites with Mitch clearly being at the more susceptible end and Suntop at the more resistant.  Both barley varieties and the durum variety Jandaroi were generally towards the mid to lower end of final Pt populations relative to the bread wheat entries. The two barley varieties appear to vary in their resistance to Pt with La Trobe leaving approximately double the Pt population of Commander at Macalister. The difference between the barley varieties at Wongarbon was not significant even though La Trobe similarly trended towards a higher final Pt population than Commander (Table 1).

Further research across sites is required to confirm differences in resistance of barley and wheat varieties to Pt as this can have significant implications for the build-up of Pt populations within a paddock and hence following rotational choices. For instance, while it appears that Mitch has a useful level of tolerance to crown rot (average 0.54 t/ha higher yielding than EGA Gregory in 2015), its increased susceptibility to Pt resulted in it taking nematode populations from a medium risk level at sowing to a high risk level (arguably extreme at Macalister) by harvest at both Wongarbon and Macalister in 2015 (Table 1). Hence, Mitch should only be considered for production in paddocks known to be free of Pt as its increased susceptibility to Pt is likely to override the yield gain in the presence of crown rot when considering the whole rotational sequence.

Table 1. Impact of selected barley, bread wheat and durum entries on final post-harvest soil populations of the root lesion nematode, Pratylenchus thornei (Pt/g soil) at two sites in 2015

Values within sites followed by the same letter are not significantly different (P=0.05) based on transformed data (ln (x+1)). Back transformed values presented in table. Sowing Pt soil populations averaged across ranges were 5.6 Pt/g soil at Wongarbon and 5.5 Pt/g soil at Macalister at 0-30 cm. Final Pt numbers post-harvest were from 0-30 cm at Wongarbon and due to drier soil conditions 0-15 cm at Macalister.

What about the build-up of crown rot inoculum?

Post-harvest soil cores collected from all plots at Wongarbon and Macalister were also analysed using PreDicta B for residual crown rot inoculum levels established by the different varieties. Crown rot risk is a sum of the DNA levels of all three Fusarium species known to cause crown rot expressed on a log scale where <0.6 is below detection, 0.6-1.4 is low, 1.4-2.0 is medium and >2.0 is high risk. At Wongarbon all entries left low inoculum levels (0.6 to 1.4) in the uninoculated plots and high levels (2.0 to 3.0) in the inoculated plots with no significant difference between entries. A similar outcome occurred at Macalister with crown rot inoculum levels across entries in uninoculated plots lower (0.5 to 1.8) but a high risk (2.0 to 3.0) remained with all inoculated plots with no significant difference between entries. Although varieties appear to significantly differ in their yield in the presence of crown rot infection, differences in the levels of partial resistance, which limits the rate of spread of the crown rot fungus through the plant during the season, do not appear to result in significant variation in inoculum levels at harvest. Partial resistance does not actually prevent the plant from being infected but rather slows the rate of fungal growth in the plant arguably delaying expression of the disease which can translate into a yield and grain quality (reduced screenings) benefit. However, the crown rot fungus, while being a pathogen when the winter cereal plant is alive, is also an effective saprophyte once the plant matures and dies. This saprophytic colonisation of infected tillers late in the season as the crop matures is the likely reason why limited practical differences in residual inoculum levels are created between varieties and winter cereal crop types.

Barley is very susceptible to infection by the crown rot fungus. It does not have improved resistance to crown rot infection. Barley tends to yield better in the presence of crown rot infection due to its earlier maturity relative to bread wheat, providing an escape mechanism which reduces its exposure to moisture stress during the critical grain filling stage. This is often referred to as tolerance. It is CRITICAL that growers do not continue to confuse tolerance with resistance when considering crown rot. Barley is likely to provide a yield advantage over wheat in the presence of high crown rot infection but it will not reduce inoculum levels for subsequent crops. This is similarly true with any bread wheat variety choice. Variety selection can improve yield in the presence of crown rot, though all varieties still suffer yield loss, which can maximise profit in the current season but this will not reduce inoculum levels for subsequent crops.

Implications

Interestingly, the better options across sites in 2015 appeared to be the reduced biomass plant types La Trobe barley and the new bread wheat variety Beckom. These reduced growth habits may provide a yield advantage under lower yielding situations in the northern region as they potentially conserve soil water usage for grain-fill. This may also reduce the expression of crown rot and the impact of this disease on yield. Variety maturity and consequently sowing date can also have a large impact on the expression and therefore yield loss associated with crown rot infection. Earlier sowing or quicker maturity can result in grain-fill occurring under reduced evapotranspiration stress relative to delayed sowing or longer season varieties sown on the same date. This interacts with the expression of crown rot which is strongly influenced by moisture/heat stress during grain filling.

The barley variety La Trobe appeared quite promising for maximising yield in the presence of high crown rot infection across the 12 sites conducted in 2015. La Trobe on average had a significant yield benefit over Commander barley (0.37 t/ha) and the best bread wheat varieties Beckom (0.43 t/ha) and Suntop (0.53 t/ha) in the presence of high crown rot infection in 2015. La Trobe is malt accredited but relative grain price (malt vs feed barley; wheat vs barley), the increased susceptibility of La Trobe to BYDV, impact on Pt populations, segregation by grain accumulators, and performance of other barley and bread wheat varieties not included in these trials (NVT Online) should be considered as part of potential variety choices. Unfortunately, grain quality data was not available at the time of writing this update paper which should also be a consideration in variety choice.

If forced into planting a cereal crop in a high crown rot risk situation then some barley varieties may provide a yield advantage over bread wheat in that season, as long as early stress does not occur. Some of the newer bread wheat varieties do appear to be closing this gap to some extent. However, a key message is that this decision is only potentially maximising profit in the current season. Growing barley over bread wheat will not assist with the reduction of crown rot inoculum levels as barley is very susceptible to infection. Significant yield loss is still occurring in the best of the barley and bread wheat varieties in the presence of high crown rot infection. Crop and variety choice is therefore not the sole solution to crown rot but rather just one element of an integrated management strategy to limit losses from this disease.

Acknowledgments

The research undertaken as part of project DAN00175 is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support. The project is co-funded by the NSW state government through the NSW DPI who are also thanked for their support in fully funding my position and laboratory and other infrastructure costs. Technical assistance provided by Robyn Shapland, Tim O’Brien, Finn Fensbo, Patrick Mortell, Carla Lombardo, Chrystal Fensbo, Kay Warren, Karen Cassin and Rachael Bannister is gratefully acknowledged.  Soil-borne pathogen levels were determined using the DNA based soil test service PreDicta B provided by the South Australian Research and Development Institute. We are also extremely thankful to NVT operators Peter Matthews (NSW DPI), Douglas Lush (DAF Qld) and Research Agronomist Rick Graham (NSW DPI) and their staff for sowing, managing and harvesting the trials and co-operating growers for use of their paddocks.

Contact details

Dr Steven Simpfendorfer
NSW DPI
Mb: 0439 581 672
Email: steven.simpfendorfer@dpi.nsw.gov.au

​Varieties displaying this symbol beside them are protected under the Plant Breeders Rights Act 1994

Reviewed by: Dr Guy McMullen, NSW DPI