Root lesion nematodes cereal variety and rotational crop impacts on yield and nematode populations

Take home messages

Choose tolerant, rather than intolerant, wheat varieties when P. thornei is present at damaging levels, or risk reducing your yields by 50%.

One resistant crop in sequence may not be enough to decrease damaging populations of P. thornei.

P. thornei survived after a sequence of five resistant crops, but in very low populations.

Management of P. thornei by growing several resistant crops is effective, and populations can be reduced to very low levels. However, on-going vigilance by testing soil for nematodes is essential when susceptible crops are planted.

Background

Root-lesion nematodes are microscopic thread-like animals that live in soil and plant roots.  These nematodes infest a broad range of plant species.  Plant roots are damaged by the nematodes and become inefficient at taking-up water and nutrients, causing up to 65% yield loss in intolerant wheat varieties.  Pratylenchus thornei is found in approximately 70% of fields in the northern grain region.

Management of the root-lesion nematode Pratylenchus thornei requires:

  • Growing tolerant wheat varieties so that yields are maximised
  • Rotating with two or more successive resistant crops so that populations of the nematodes decrease.

Summer crop rotation trial

Two summer crop rotation trials were planted in adjacent cropping strips in December 2011.

1) The first strip had low P. thornei populations (<125/kg soil or 0.125/g soil from 0–90 cm).  The previous cropping history was five resistant crops since 2004 (cotton, maize and sorghum) (Figure 1).

2) The second strip had moderate P. thornei populations (between 2000 to 3000/kg soil (or 2-3/g soil) from 0–90 cm soil depth).  The previous cropping history was wheat, sorghum, wheat (Figure 1).

Several cultivars of mungbean, soybean, sunflower, maize and sorghum were planted in each strip in December 2011 in a replicated design with sufficient plots to plant both wheat cvv. EGA Wylie (tolerant) and Strzelecki (intolerant) in 2013.  There was also an unplanted bare fallow treatment.  After harvest of the summer crops, nematode populations were assessed to 120 cm soil depth.

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Figure 1.  Starting populations of Pratylenchus thornei in the summer crop rotation experiments.

Results

Moderate P. thornei site

P. thornei was found to 90 cm soil depth and populations were greatest at 0–15 cm soil depth (Figure 2).

Populations of P. thornei after growing sorghum, sunflower and maize were similar to bare fallow (range of 2,900–4,500/kg soil at 0–15 cm (Figure 2).  There were no significant differences between varieties within each of these crop species (Figure 3).

In contrast, populations of P. thornei increased after growing soybean or mungbean compared to sunflower, sorghum, maize or clean fallow (Figure 2).  There were also differences between varieties of soybean and mungbean (Figure 3).

Soybean cv. Soya791 was moderately resistant (4,800 P. thornei/kg soil at 0–15 cm) and its effect did not differ significantly from the fallow treatment.  However, all other soybean varieties were very susceptible.  Populations of P. thornei increased 4–6.7 times to 12,000–20,600 P. thornei/kg soil at 0–15 cm (Figure 3).

Mungbean cv. Emerald was moderately resistant (3,400 P. thornei/kg soil) and its effect did not differ significantly from the fallow treatment.  However, all other mungbean varieties tested were susceptible and P. thornei populations increased 2.2–3.8 times to 6,700–11,700 P. thornei/kg soil at 0–15 cm (Figure 3).

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Figure 2.  Populations of Pratylenchus thornei increased after growing mungbean and soybean.  After growing sunflower, maize and sorghum, populations were similar to the bare fallow (grey line).  Means of varieties within each crop are presented for the moderate Pt site.


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Figure 3.  After harvest of the summer crops, Pratylenchus thornei was found in largest populations at 0–15 cm soil depth.  Varieties of sunflower, maize, sorghum, mungbean cv. Emerald and soybean cv. Soya791 were not significantly different to the fallow treatment. 
* indicates significantly higher P. thornei populations than the fallow treatment (P<0.05).  Green Dia is mungbean cv. Green Diamond.

Low P. thornei site

Pratylenchus thornei was detected to 60 cm soil depth; below that depth populations were very low or zero (Figure 4).

There were no significant differences in P. thornei populations after the summer crops or the varieties.  Populations remained less than 250/kg soil (Figure 4).

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Figure 4.  Pratylenchus thornei populations after harvest of the summer crops remained very low at the site that started with low Pt populations (black line) (Note the differences in population size compared to the other site shown in Figure 2 and 3).  Mean data for all crops presented; there were no significant differences between crops or varieties.  Letters indicate significant differences (P≤0.05) between depth intervals after harvest of the summer crops (black line).  The grey line is the pre-plant populations.

Summer crop biomass and yield

There were no differences in biomass or grain yield of the summer crops between the low and moderate P. thornei sites.

The summer crops grown were tolerant to P. thornei; they did not suffer yield loss.  Soybean and mungbean were susceptible but did not show symptoms of infestation by P. thornei or suffer yield loss.

Impact on the next wheat crop

At the site that started with moderate P. thornei populations, the yield of the intolerant wheat cv. Strzelecki was reduced by 49% compared to the tolerant wheat cv. EGA Wylie (1.9 t/ha for Strzelecki compared to 3.7 t/ha for EGA Wylie).  In contrast, at the site that started with low P. thornei populations there was only a 4% difference in yield between cv. Strzelecki and EGA Wylie (3.6 t/ha and 3.7 t/ha respectively).  The yield of cv. Strzelecki increased 47%, or 1.7 t/ha, at the low P. thornei site compared to the moderate P. thornei site (Figure 5).

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Figure 5.  Yield of the intolerant wheat cv. Strzelecki was reduced by 47% at the site that started with moderate P. thornei populations (Mod Pt; 2000–3000/kg soil before the trial started) compared to the low P. thornei site (Low Pt; <125 /kg soil before the trial started).  Columns with the same letter are not significantly different (P>0.05).

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Figure 6.  Yield of the 2013 intolerant wheat cv. Strzelecki and tolerant cv. EGA Wylie at the site that started with moderate P. thornei populations (2000–3000/kg soil at 0–90 cm soil depth) before planting the summer crops in 2011-12.  Strzelecki columns with the same letter are not significantly different (P>0.05).

Yield of wheat cv. Strzelecki was lowest following soybean (1.6 t/ha) and greatest following maize and sunflower (2.1 t/ha).  An unexpected result was that there were no significant differences in yield of cv. Strzelecki after fallow, sorghum and mungbean.  This result may be partly due to dry conditions during the 2011-12 summer and following winter season which limited nematode multiplication, particularly after the susceptible mungbean.  Additionally and importantly, the results support that one resistant crop in sequence was not enough to sufficiently reduce populations of P. thornei.  Nevertheless, there was a strong negative relationship between populations of P. thornei after the summer crops and yield of the following intolerant wheat cv. Strzelecki.  In contrast, there was no relationship between populations of P. thornei and yield of the tolerant wheat cv. EGA Wylie (Figure 7) which is an expected result because of the good level of tolerance of EGA Wylie to P. thornei.

At the low Pt site, there was no relationship between yields of the tolerant and intolerant wheat and populations of P. thornei after growing the summer crops.  Populations were below the damage threshold for wheat cv. Strzelecki.

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Figure 7.  There was a strong negative relationship between populations of Pratylenchus  thornei after the 2011-12 summer crops and yield of following wheat cv. Strzelecki (***, P=0.01, n = 60).  In contrast there was no significant relationship between yield of cv. EGA Wylie and populations of P. thornei.  Data is from the low and moderate P. thornei sites for summer crop varieties and a fallow treatment.

What did we learn from this experiment?

Grow a tolerant wheat variety when populations of P. thornei are at damaging levels or risk losing up to half of your yield.

Growing one resistant crop, such as sorghum, maize or sunflower did not provide a quick fix in the field that started with 2,000–3,000 P. thornei/kg soil.  Populations of P. thornei did not fall below damaging levels and the next intolerant wheat lost 44–51% in yield compared to a tolerant wheat variety.

P. thornei did not die out completely at the low P. thornei site, even after five successive resistant crops.  Management of root-lesion nematodes is on-going and requires regular soil tests to monitor nematode populations.

What happens to P. thornei populations in a 4-year rotation with resistant and/or susceptible crops?

In another crop rotation experiment, P. thornei populations before starting the experiment peaked at 4,000/kg soil at 30–45 cm soil depth.  We grew a resistant or susceptible winter crop in Year 1, followed by summer crops in Year 2 and then after a long fallow, the intolerant wheat cv. Strzelecki in Year 4.  Yield of the Year 4 wheat cv. Strzelecki was greatest (3.1 t/ha) when the two previous crops were both resistant to P. thornei, for example canaryseed followed by sorghum.  Populations of P. thornei decreased to less than 900/kg soil (see Figure 8).

In contrast, the poorest yield of cv. Strzelecki (1.4 t/ha) followed two susceptible crops, for example, wheat followed by mungbean.  Populations of P. thornei increased in Year 2 and again in Year 3 to 10,000/kg soil (Figure 8).

There was a strong, negative relationship between the yield of the Year 4 wheat cv. Strzelecki and populations of P. thornei after the Year 2 summer crops (P<0.001, R2 =0.82).  Yield loss of up to 68% was predicted by this relationship.

The impact of growing a P. thornei-susceptible crop in Year 1, such as wheat, was still evident in Year 4, even when the summer crop in Year 2 was resistant (see Figure 8, wheat followed by sorghum).  One resistant Year 2 summer crop, such as sorghum could not reduce all of the damage potential due to P. thornei.  The Year 4 wheat lost 33% of its yield if sorghum followed a susceptible winter crop compared with sorghum following a resistant winter crop.

The message to take away is that when P. thornei is present in high populations, two or more sequential resistant crops are needed to reduce populations to low enough levels to avoid yield loss in intolerant: susceptible wheat crops that follow.

What is the impact of chickpea on P. thornei populations?

Chickpea is generally susceptible to P. thornei but there are differences between varieties, for example, cv. PBA HatTrickis moderately resistant and cv. Amethyst is susceptible (see Figure 9).  Most chickpea varieties will cause populations of P. thornei to increase, but the degree of increase will depend on the susceptibility of the variety, initial P. thornei populations at planting, cropping history of the site as well as seasonal conditions.  The message to take away is to repeat soil tests for nematodes and monitor their populations during crop rotation sequences.


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Figure 8. When Pratylenchus thornei is present in high populations more than one resistant crop in sequence needed to reduce populations.  The grey line indicates P. thornei populations before the experiment began.  In Year 1, wheat (susceptible) or canaryseed (resistant) was planted, followed in Year 2 by mungbean (susceptible) or sorghum (resistant).  In Year 4 the P. thornei-intolerant wheat cv. Strzelecki was planted.  Note the change in P. thornei populations compared to the grey line during the differing crop sequences.

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Figure 9.  Populations of Pratylenchus thornei generally increased after growing chickpea compared to a fallow (dashed line) however, there were differences between varieties and cv. PBA HatTrick was moderately resistant.

Contact details

Kirsty Owen
Department of Agriculture, Fisheries and Forestry Queensland
Leslie Research Centre, PO Box 2282, Toowoomba, 4350
Ph: 07 4639 8888

Email:  Kirsty.Owen@daff.qld.gov.au

Reviewed by

Stephen Neate and John Thompson

GRDC Project Code: DAV00128, ICN00014,