South Australian cereal root disease update 2019

Take home messages

• Drought in 2018 and low summer rainfall are likely to increase the risk of rhizoctonia root rot and crown rot in 2019. Identify paddocks at risk prior to sowing using PREDICTA® B to implement strategies to minimise yield loss.
• Cereal cyst nematode (CCN) levels have increased in SA with detections in 24% of paddocks tested in 2018, compared to 1% in Victoria. Monitor numbers in paddocks sown with susceptible cereals.
• Adding stubble to PREDICTA® B samples improves crown rot risk assessment; crown rot and root lesion nematode (RLN) PREDICTA® B yield loss risk categories were updated and testing for sclerotinia stem rot was added to PREDICTA® B for 2019. PREDICTA® B manual (version 10.2) is available for download.
• Consult current cereal and pulse disease guides for the latest RLN resistance ratings as there is variation between crops and crop varieties for resistance to RLN species.

Background

The severity of soilborne disease is significantly affected by seasonal conditions in both the preceding crop that produced the inoculum and, in the crop, exposed to the inoculum.

The 2018 season was characterised by below average rainfall in autumn to mid-winter, reasonable rainfall in August, followed by low rainfall during spring. These conditions favour:

• Build-up of Rhizoctonia solani AG8, the cause of rhizoctonia root rot.
• Production of whiteheads caused by crown rot.
• Reduced breakdown of cereal stubble in break crops.
• Root damage caused by root lesion nematodes.

Losses from crown rot and RLN were recently estimated to be $125 million and $16 million per/year, respectively.

Managing cereal soilborne diseases in 2019

It is advised that the disease risk profile of paddocks is determined using PREDICTA®B testing well ahead of sowing in 2019. PREDICTA®B is a DNA-based soil testing service which enables identification of pathogens posing the greatest threat to crops. For further information on PREDICTA®B, head to the PIRSA website.

Rhizoctonia

Inoculum levels of Rhizoctonia are likely to be a concern in 2019 as dry conditions favour a build-up of the pathogen. Forty-four percent of paddocks in SA that were tested in 2018 using PREDICTA®B had medium to high risk levels of Rhizoctonia prior to sowing (Table 1) and with the dry 2018 season, inoculum level is likely to have increased during the year. The impact on crops during 2019 will be greatest if the summer remains dry, the season breaks late and temperatures drop quickly after sowing. In areas where good summer rainfall occurs, Rhizoctonia levels will be lower. The impact of the pathogen will be reduced if the season breaks early and crops establish in warm soil. Rhizoctonia is most damaging when root growth is restricted either by cold soils, compaction layers or lack of moisture. Crops that establish well can still be affected in mid-winter when soil temperatures drop below 10°C when Rhizoctonia can attack the crown roots causing uneven growth and reduced tiller number, rather than classic bare patch symptoms.

In paddocks with Rhizoctonia present at high levels, control summer weeds and autumn green bridge and consider rotating to a non-cereal crop. If cereals are to be grown, wheat is more tolerant than barley and early sowing in the seeding window with banding of nitrogen (N) below the seed can facilitate rapid root growth in the soil profile. Ensure good crop nutrition, with particular attention to trace elements, and increase seeding rates to reduce impact of lost tillers from Rhizoctonia damage to crown roots. If growing cereals in 2019, a PREDICTA® B test can be used to identify paddocks at risk.

Consider fungicides to protect the roots. Rainfall is needed to move fungicides into the root zone as roots outside the fungicide zone are not protected. Seed treatments tend to protect the seminal roots, whereas liquid streaming Uniform® above and below the seed can protect crown and seminal roots and tends to produce larger yield responses in above average rainfall seasons.

Crown rot

Crown rot has become the most important soilborne disease affecting cereal crops nationally — 38% of SA grower samplestested in 2018 by PREDICTA®B had medium to high risk levels of crown rot prior to sowing (Table 1). The dry finish to the season in 2018 favoured whitehead development in wheat and high inoculum in infected plants. Where there is a medium to high crown rot risk, it is best to avoid growing durum or bread wheat. Yield loss in barley will be lower, but inoculum levels will increase. Generally, a two-year break from cereals is required to reduce medium/high levels to a low inoculum level. However, the dry season of 2018 will have slowed the breakdown of stubble and a three-year break may be required. A PREDICTA® B soil test prior to sowing can identify at risk paddocks — make sure one piece of stubble, from 15 locations, each 5cm long and from the base of the plant, is added to that sample

Cereal cyst nematode

Cereal cyst nematode was the most important soilborne disease in SA and Victoria. However, it is generally under control, following two decades of using resistant varieties. Over the past five years, numbers have been slowly increasing in SA, but not in Victoria. In 2016, CCN was detected in 15% of paddocks tested in SA, increasing to 24% in 2018 (Table 1). While only 5% of paddocks had a medium to high risk (Table 1), the detection of CCN should be noted due to its ability to rapidly increase. PREDICTA®B can be used to monitor CCN levels in paddocks sown to susceptible cereals. Use of resistant varieties or non-host crops is recommended in paddocks where CCN is detected.

Table 1. Summary Southern RegionPREDICTA® B results showing percentage of samples returning medium/high disease risk (non-bracketed) and total percentage of samples returning positive detection of inoculum (bracketed) from 2016-2018.

(X%) Total percentage of samples infected.

Root lesion nematode

Pratylenchus neglectus levels before sowing in 2018 were at medium to high levels in 18% of paddocks tested in SA (Table 1). This was 11% greater than in 2016 due to the exceptional conditions during 2017. The dry conditions in 2018 are expected to reduce P. neglectus multiplication and potentially reduce the risk to crops sown in 2019. To keep RLN densities below yield limiting thresholds, grow varieties/crops with a moderately resistant-moderately susceptible (MRMS) or better resistance rating. If susceptible varieties are grown, rotate with resistant crops/varieties.

P. neglectus yield responses

In 2018, 60% of varieties in a P. neglectus tolerance trial located at Pinery, SA, had significant yield losses ranging from 8% to 15% (Figure 1). The trial was established in 2017 to produce paired plots of low (5 P. neglectus/g soil) and high (80 P. neglectus/g soil) to be over sown in 2018. Low growing season rainfall limited yield potential. The trial established well on 30mm of rainfall prior to sowing (11 May), followed up with 45mm rainfall in June, while the rest of the season received only 100 mm, most in August.

Emu Rock, Razor CL Plus and Corack had the largest losses with 12.1%, 12.6% and 14.6%, respectively, equating to about 0.2t/ha. Scepter, DBA Spes, Spartacus CL, Hindmarsh and Chief CL Plus had yield losses of 8%.

Severe seminal root damage was observed on a majority of varieties in the spring (Figure 2) in the high treatments. Plants in these plots appeared to rely on the crown roots to finish the season.

A review of losses caused by P. neglectus estimated significant yield losses occurred in 30% of seasons in the Southern Region. Preliminary studies indicate seasons with good early/mid-season rainfall, followed by low rainfall and high evapo-transpiration in spring, are most conducive to yield loss. However, there are significant genotype x environment (GxE) interactions that result in different groups of varieties producing yield responses in different seasons. Therefore, growers should grow the best adapted varieties and use resistant varieties/crops when practical to control high populations.

Figure 1. Yield responses of cereal varieties to P. neglectus in 2018 at Pinery, SA. * Yield responses significant at p=0.05

Figure 2. Root damage on Corack wheat at 21 weeks from sowing.

Root lesion nematode yield loss categories

Following the review of yield loss data collected nationally from 51 field trials (2011-2015), the PREDICTA®B RLN medium to high risk threshold for the Southern Region was increased from 60 to 100 nematodes/g soil (Table 2).

Table 2. Southern RegionPREDICTA® B RLN yield loss risk categories.

Resistance to RLN in cereals and pulses

It is important to know which RLN species are present, as each species has a different host range. For P. neglectus, triticale is the most resistant cereal crop followed by barley and durum. Wheat is the most susceptible crop, with useful variation in ’resistance’ between varieties. The more resistant wheat varieties, Mace and Trojan, rated MS. It is important to note that in good seasons all varieties will increase nematode densities except the triticale varieties.

For P. thornei, durum, barley and triticale are more resistant than bread wheat (Figure 3b). Wheat varieties vary in susceptibility of P. thornei, so it is important to consult a Cereal Variety Disease Guide or NVT Online.

Figure 3. Multi-site analysis of P. neglectus A) and P. thornei B) multiplication in cereal varieties in field trials conducted in the Southern Region between 2011-2017 and 2016-2017, respectively.

The relative resistance classifications for cereal, pulse and oilseed varieties are summarised in Figure 4 for P. neglectus and P. thornei. Wheat is a susceptible crop for both species. For P. neglectus, canola is susceptible. For P. thornei, chickpea, faba bean and vetch are susceptible in seasons with good spring rainfall. Lentil, lupin and field pea are reasonable break crops for both species.

Figure 4. Resistance of cereals, oilseeds and pulses to P. neglectus and P. thornei.

Conclusion

Proactive disease control is required as yield losses due to soilborne diseases can be greater than 20%. Therefore, it is important that the disease risk profile of paddocks is determined using PREDICTA®B testing well ahead of sowing in 2019 and plans are developed and implemented to manage disease risks in 2019.

Useful resources

PREDICTA®B website

2018 Cereal Variety Disease Guide

2018 Pulse Disease Guide

GRDC Tips and Tactics Root Lesion Nematode 

GRDC Tips and Tactics Cereal Cyst Nematode

GRDC Tips and Tactics Rhizoctonia 

GRDC Tips and Tactics Crown Rot 

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC — the authors would like to thank them for their continued support.

Contact details

Katherine Linsell
SARDI Plant Research Centre
Gate 2b Hartley Grove, Urrbrae, SA, 5064
0417 889 253
Katherine.linsell@sa.gov.au
@klinsell