THE ADDITIVE YIELD IMPACT OF ROOT LESION NEMATODE AND CROWN ROT?

Background

NGA have been involved in 22 field trials since 2007, in collaboration with Steven Simpfendorfer I&I NSW, evaluating the impact of crown rot (CR) on a range of winter cereal crop types and varieties. This work has greatly improved our understanding of crown rot impact and variety tolerance, but also indicates we may be suffering losses from another ‘disease’ that often goes unnoticed. Although the trials were not designed to focus on nematodes, a convincing trend was apparent after 2008 that indicated the root lesion nematode Pratylenchus thornei (Pt) may be having a wider and larger impact on actual yield than previously expected.

Nematode counts

Routine trial site characterisation included soil tests for background CR level as well as a range of other pathogens including nematodes. PreDicta B (DNA extraction) was used on all samples with DEEDI also conducting ‘manual’ nematode counts on soil samples collected in 2008 and 2009. The correlation between the two tests has been good with both tests always indicating Pt presence at the same site (Figure 1 shows the 2009 data). One difference is that the DNA assessment is on a 0-15 cm soil fraction whilst the manual count is in layers from 0-90 cm. Deeper sampling may be useful at sites with prominent nematode ‘bulges’ at depth (e.g. North Star site seen in Figure 2).

NB although both techniques provided a similar count of nematodes/kg soil, the ‘risk ratings’ were very different. Even at the Bellata site, PreDicta B indicated only a ‘low risk’ of yield loss.

Apparent yield impact of Pt

During 2007 & 2008 a common series of 5 bread wheat varieties, 4 barley varieties and 1 durum were evaluated in all 18 trials. Variety yield as a % of a ‘standard’ was calculated to investigate the performance across sites. EGA Wylie  and Skiff were used as standards. (EGA Wylie  has the highest level of available tolerance to Pt in commercial wheat varieties but will still suffer yield loss in the presence of high Pt populations. There is little information available for barley varieties).

Figure 3 shows the variety yield performance compared to EGA Wylie  over all 18 trials. 11 trials had no Pt detected with 7 sites having Pt present (Pt were present at 5 of 11 sites in 2007 and 2 of 7 sites in 2008, PreDicta B Pt levels ranged from 1000-7000/kg soil in 2007 and 4000-5000/kg soil in 2008). 2007 was a high CR yield loss year with average bread wheat yield loss of ~25% and at the worst site even EGA Wylie  lost 50% yield to CR. In contrast 2008 was a very low CR yield loss year with average bread wheat yield loss of only ~1%.

The four lines show the yield performance of each variety as a % of EGA Wylie  with:
1.  ‘no constraint’ (no added CR, no Pt present)
2. ‘CR alone’ line (added CR, no Pt present)
3. ‘Pt alone’ line (no added CR, Pt present)
4. ‘CR + Pt’ line (added CR, Pt present)

Key points:
1. The difference between ‘no constraint’ and ‘CR alone’ indicates relative CR tolerance between varieties e.g. Bellaroi showed the largest fall, followed by EGA Gregory . NB this is not a true representation of actual yield loss to CR (EGA Wylie  can still lose up to 50% yield to CR) rather it reflects variety performance in the presence of CR relative to EGA Wylie .
2. The difference between ‘no constraint’ and ‘Pt alone’ indicates relative Pt tolerance. All varieties fell compared to EGA Wylie  with most impact on Lang , Sunco and Bellaroi . 
3. The ‘CR + Pt’ line shows the performance when both constraints were present. This data suggests an additive effect of variety CR loss PLUS Pt impact
4. Generally a flat response in yield between bread wheat varieties under CR pressure alone (all were 91-97% of EGA Wylie  yield)
5. Wider yield range from bread wheat varieties in response to Pt tolerance alone (ranged from 78-102% of EGA Wylie  yield)

Over these two years, varieties such as Lang  and Sunco performed well when CR was added in the absence of Pt (~90-95% of EGA Wylie  yield). However when CR was added and Pt were present they only recorded~70-80% of EGA Wylie  yield.

Barley varieties are generally considered to have increased levels of Pt tolerance. Comparison of barley performance over the same trials is seen in Figure 4.

Figure 5 shows the variety yield performance compared to EGA Wylie  for all varieties evaluated in both 2008 and 2009. There was no Pt detection at 6 sites, with 5 sites having Pt present (NB site selection biased to Pt presence in 2009). 2008 was a very low CR yield loss year with average bread wheat yield loss of ~1% with low to moderate CR losses in 2009 with an average bread wheat yield loss of ~8%

Key points:

1. Less varietal CR tolerance difference evident, as expected, in lower CR yield loss years
2. Poor relative yield performance from Strzelecki , Bellaroi  and Jandaroi  at sites with both Pt presence and CR added

Summary

CR impact and variety choice

There are clear differences in levels of CR tolerance between bread wheat varieties. However the relationship between CR tolerance rating and actual yield loss has been poor i.e. inherent yield potential and local adaptation has been more important than CR rating.

The performance of EGA Gregory  is a good example. Under CR pressure, EGA Gregory  will certainly show more disease symptoms than Sunco but has still significantly out yielded Sunco in 10 of 22 trials. There was no situation when Sunco significantly out yielded EGA Gregory . EGA Gregory  is NOT better than Sunco for CR tolerance but is a higher yielding, widely adapted option. This demonstrates the relatively small progress and the difficulty of breeding for CR tolerance. Bread wheat variety choice cannot be your major CR management tool. However, changing from durum, which are highly susceptible to CR, to a bread wheat or barley variety will assist in limiting losses.

Pt impact and variety choice

The pattern of yield results obtained at sites with Pt presence, generally supports the published variety Pt ratings. There appears to be a much stronger relationship between variety Pt rating and final yield under Pt pressure than exists for CR. Bread wheat variety choice on the basis of Pt rating appears a useful tool in Pt management and is likely to impact actual yield and economic results.

The performance of Strzelecki  appears a good example. Over the last 2 years at sites without Pt, it averaged 6% higher yield than EGA Wylie . However at sites with Pt present, it has averaged 24% lower yield (NB there may be other factors than just Pt responsible for this difference). The apparent loss in yield ‘potential’ has equalled ~1 t/ha. A poor variety choice, in the presence of Pt, could be costing a grower more than $180/ha.

Conclusions

These trials were designed to evaluate the impact of CR on variety yield and quality. However they strongly suggest that Pt is also having a significant impact on yield performance. The results do not compare the actual levels of yield loss due to the two diseases but indicate there is a greater range in variety Pt tolerance than currently exists for CR tolerance. Put simply, variety choice appears a more valuable tool when under Pt pressure than as a tool for CR management. It may be a co-incidence but four of the most widely adopted and successful varieties in the north (EGA Wylie , EGA Gregory , Baxter  and Sunvale) are the varieties with the highest currently available level of Pt tolerance.

Improved understanding of Pt status in individual paddocks should assist in developing sound hygiene practices to help limit further spread. This knowledge is also likely to provide economic gains from both sound varietal and rotation choices. Soil testing for nematodes may also provide benefits in the identification of other plant parasitic species not covered in this paper e.g. Pratylenchus neglectus and Merlinius brevidens.

Acknowledgements

Thanks to the many growers and consultants involved in this trial work, the support and advice of Steven Simpfendorfer and his pathology team, the support and advice of John Thompson and his pathology team together with Clare Felton-Taylor and Anthony Mitchell for field co-ordination.

Contact details

Richard Daniel  
Northern Grower Alliance
07 4639 5344
richard.daniel@nga.org.au