Root lesion nematodes – the hidden yield thief
With thousands of hectares of WA soils estimated to be infiltrated by the relatively unknown microscopic parasite, yields across the grainbelt are being compromised
A microscopic parasite present in a high percentage of Western Australian grainbelt soils has been found to be significantly reducing yields in paddocks where nematode densities are medium or high. This confirmation from research by the Department of Agriculture and Food, WA, highlights the need for growers to know what the situation is in their paddocks.
Root lesion nematodes (RLN) cannot be seen with the naked eye, but their impact on crops can be extreme.
The parasite uses a syringe-like stylet to penetrate and extract nutrients. The damage can impair the ability of the plant to achieve its growth potential.
PreDicta® B tests are the best way to identify paddocks at risk and can now identify Pratylenchus neglectus and P. quasitereoides, the two species of RLN found in WA, which are suspected of reducing grain yields.
Dumbleyung grower Glenn Ball, who has been hosting on-farm tests, believes RLN could be costing him cereal yields upwards of one tonne per hectare, and, if not managed, will be the biggest issue facing him and other growers in the next decade.
Glenn says, like many growers, he mistakenly believed his yield reductions were a result of a potassium deficiency, so he initially applied high rates of lime and potash to affected paddocks.
He says that while this acted as a short-term fix in areas where the nematodes were rife, the problem returned.
“We applied 2t/ha of lime and 100 kilograms/ha of potash, with 90kg of K-Till® to areas we had identified on the yield maps as having yield penalties, and this did seem to get the plants through that early growth stage, but those rates would be pushing the boundaries in terms of being economical,” Glenn says.
“So in the early stages, we were putting the problem down to something else. Impacted plants look unhealthy, wilt easily and are nutrient deficient. Visually, it is very similar to a potash deficiency, so it would be easy to mistake it for that. We had also put it down to seasonal variability and perhaps even soil acidity.
“Last year we early-sowed some wheat, and these crops should have been the best wheat crops on the property since they were following canola, but they ended up being 0.5 to 1t/ha down on where they should have been.
“We have now put this result almost entirely down to nematodes.”
Glenn says wheat or barley plants appear to be growing well until the four to six-leaf stage, when the lateral roots begin their formation, and the nematode attack becomes obvious.
“All of a sudden the plants just don’t grow. They will get through this period eventually, but each plant will only have one tiller, and by then it has missed the boat to set itself up to yield well,” he says.
The Balls farm on productive red and grey loams in the southern wheatbelt, with wheat, barley, oats, canola and lupins making up 70 per cent of their business profits.
They also run 4500 ewes plus a wether flock, but from his experience, Glenn doesn’t believe leaving paddocks to pasture is the solution to reducing RLN numbers.
“We took paddocks to pasture for three years, but after soil testing, the nematode numbers don’t seem to be significantly reduced,” Glenn says.
And while canola is normally a good break crop for most diseases, he says it hasn’t given his paddocks the break from the nematodes that he had hoped for.
“What we are seeing with the P. quasitereoides species is that there is in fact a spike in numbers after canola. We tend to have the worst problem when we have an extended cereal rotation, then canola, then back to wheat,” Glenn says.
However, he has noticed that lupins and field peas don’t appear to be good hosts of nematodes, so a rotation of lupins/oats/wheat is a strategy being considered.
Early test results on Glenn’s property are also indicating windrowing might be exacerbating the problem.
While there is not yet any scientific explanation, Glenn says that under the windrow, nematode numbers are very low – about 300 per gram of root, but just a metre from the windrow they are up to 13,000/g of root.
ConsultAg Agronomist Garren Knell, who has been working closely with the Balls to find solutions to the nematode problem, says investing in the expensive but necessary PreDicta® B soil tests over the summer months is critical.
Like Glenn, he believes many growers are suffering significant yield losses to RLN, but are probably attributing the losses to something else.
“There is so much we don’t know about nematodes; what conditions they like or, conversely, don’t like,” he says.
GRDC Project Code DAV00128, DAW00209