Written by: Nick McKenna | Agronomist | 0427 681 574
Growing lupins is a lot like flossing your teeth. You know its the right thing to do, but you probably don’t enjoy doing it, take any excuse to avoid it, and only realise that you should have done it a lot more diligently when everything comes unstuck. (In a metaphorical sense it’s when your paddock profitability starts to decline, or in a literal sense when your precious teeth fall out!)
They aren’t sexy, and it takes a pretty special year to make them stack up on a one year gross margin. But they do have a place as a break crop for your profitable wheat and canola rotation. I have seen a number of paddocks show up with RLN this year, and its easy to see why.
- Lupins are falling out of favour (a great break for RLN),
- In 2022 over half of the WA wheat area was sown to Sceptre (rated as S for RLN),
- Canola is getting sown in larger and larger areas (an excellent host of RLN),
- Last year had a soft finish to the season (a great chance for RLN to build up),
This year had a cold late start (the crop is growing more slowly making it easier for RLN to affect the crop.
With any luck, we will get an earlier break to next season, the price for lupins will sky rocket convincing you to grow them, and RLN wont be an issue. But outside of making significant changes to your rotation the issue will likely be present next year.
Root Lesion Nematode (RLN) are a microscopic pest, less than 1mm in length. They can cycle many times in a single season, leading to a rapid buildup of population if the conditions permit. They cause damage to crops by feeding on and damaging roots, so they can be especially damaging in soils with lower fertility, in drier conditions, and when root growth is compromised by other factors.
The four common species of RLN are Pratylenchus neglectus, Pratylenchus thornei, Pratylenchus penetrans and Pratylenchus quasiteroides. P neglectus and P. quasiteroides are the two major species in WA, and P. penetrans are very damaging but uncommon. For diagnosis of which species is present, consult your agronomist for a PredictaB test. Best practice management varies slightly between species, so if
you suspect RLN is causing issues on your farm, do a test and be informed.
The symptoms caused by RLN are general crop stunting, obvious nutritional deficiencies, and a ‘waviness’ to the crop growth. The nutritional deficiencies will be well explained when you realise that crop may not be accessing what’s in the soil on account of a hamstrung root system! See below an 8 week old wheat plant from a fertile red loam soil near Geraldton. Despite kind winter rainfall, good nutrition both
banded and top-dressed and good background fertility, the affected patches are insipid and pitiful.
Figure 1. Wheat sample taken from a confirmed high load of RLN and Cereal Cyst Nematode. The variety is Sceptre, coming off a canola stubble last year
If you see stunted areas of crop and suspect RLN, dig some plants out of the ground and soak or wash the roots to remove the soil. Beware that the roots can be very fragile so exercise care!
Once you have removed the soil, look at the roots for signs of disease. RLN will cause brown lesions or even the entire root to appear brown as below.
Figure 2. Roots are looking unhealthy, with brown colour to the root system and very few root hairs.
Before jumping to conclusions, a differential diagnosis for insipid looking cereal crops is rhizoctonia, which will be characterised by dark, necrotic tip to the roots, which typically won’t be as prevalent if grown after a canola crop.
If in doubt talk to your agronomist.
Figure 3. Ranking of species resistance to Pratylenchus Neglectus.
A few management factors have conspired to give the increased numbers of RLN I am seeing at the moment.
- The mainstay wheat varieties have incrementally decreased in resistance to RLN. Wyalkatchem is rated MRMS, Mace is rated MS, Sceptre is rated S. Spot the trend…
- Lupins have fallen out of favour, with more consecutive cereals and canola being grown!
- Canola is being pushed into a tighter rotation, and with the market prices for lupins and canola its no wonder.
Figure 4. Variety ratings for resistance to disease. Note Brumby as a standout for Powdery Mildew and nematodes.
Figure 5. Barley varietal RLN ratings.
Thankfully there are still a few management factors in your control. You can make the conscious choice to grow more lupins or serradella, chase a bit less canola area every year, and look at getting a cereal variety that has a better defence against RLN.
Unlike foliar diseases, RLN is graded in relation to cereals both by resistance and tolerance. Resistance measures how well the species will resist infection and therefore how well the pathogen will survive during the rotation. Tolerance is a measure of how much yield is affected by the presence of the disease. So, if a variety is not resistant but is tolerant, RLN numbers will increase but the crop will not show symptoms.
Though there aren’t many great options, Brumby is one standout for the wheat varieties. It is rated as MRMS and TMT- about as good as it gets for wheat. As a nice bonus its also rated as R for powdery mildew. Alternatively, most barley varieties are more tolerant than wheat, so if you are happy to grow barley in your rotation there is a good enough reason to do so.
Outside of this, committing to growing a lupin crop every once in a while, will give the soil a break from RLN and give the cereal crops a fighting chance.
Just like flossing, all too often the wake-up call to stick more rigorously to your rotations can come too late. Many farmers made handsome profits in the last few years by increasing canola area at the expense of lupins. But it can come at a cost, and cereal yields are sure to be constrained if you have high levels of RLN preventing root growth in your dependable wheat crop. Certainly, the ever decreasing resistance to RLN of our mainstay wheat varieties isn’t helping! Have a look around your farm this winter for signs of RLN and make the changes before it gets really painful.
Further reading
Root Lesion Nematode – Western
Sources:
DPIRD variety guide 2023