Research into the root lesion nematode has shed new light onto the precise consequences for a wheat plant of being attacked by the microscopic worm.
By growing wheat on high and low populations of nematodes and monitoring the rate of water use, CSIRO researcher Jeremy Whish has gained detailed insight into the lifecycle of the tiny worm that is found on more than 75 per cent of farms and causes $38 million damage in the north alone.
Dr Whish, who is midway through four years of research, said the old adage “know your enemy” is even more relevant when you are dealing with something too small to see.
“I had always assumed, as had many other researchers, that nematodes ate into the roots, stunting their growth so that the plants could not reach water in the deeper soil,” Dr Whish said.
However, his research is finding that the root systems have developed almost normally and Dr Whish instead suspects the nematodes inflict their damage by penetrating the roots’ outer layer and restricting their ability to transport water.
The next stage of his research, which is being supported by the Grains Research and Development Corporation (GRDC), will test this hypothesis.
“The nematodes penetrate the root, lay eggs inside the cortex of the roots and eventually the outer wall of root collapses causing the lesion and we believe this breaks the pipe that transports water to the plant,” Dr Whish said.
“It is basically like putting a rubber band around the roots so they collapse in on themselves. That is why we were receiving reports of plants being water-stressed and of water being left in the soil after harvest.”
Dr Whish said the information was relevant because having a detailed knowledge of nematodes’ lifecycle offered the greatest hope of finding a way to beat them in the future.
“The best chance we have of finding a way of beating nematodes, or at least reducing their impact lies in gaining a thorough understanding of their life cycle and identifying their most vulnerable stages, if indeed there is one,” Dr Whish said.
“At this stage you couldn’t really say they have a particularly vulnerable stage but you need to know your enemy.”
Nematodes, most commonly Pratylenchus thornei (Pt) in the north, as well as Pratylenchus neglectus (Pn), are microscopic worms in the soil that swim in free water and largely live inside root systems.
They feed on root cells causing extensive lesions that can also allow fungi or bacteria diseases to attack the susceptible plants, and have been shown to reduce yields in wheat by up to 50 per cent.
At his research site at Formartin west of Toowoomba, Dr Whish installed aluminium neutron probe access tubes down to 1.8m into the soil and each week lowers a sensor down the tubes to measure precisely how much moisture is in the soil at each level.
In addition, an EM38 is run across the soil every week to give an indication of the total amount of moisture in the soil.
Every four weeks gravimetric samples provide an additional measure of soil water and provide samples to assess how the nematode population is changing during the season.
Dr Whish found that the nematodes damage the crop very early on in the plant’s life cycle and the impact is permanent.
“Because plants can’t take up as much water as they want early on, they reduce their canopy and set themselves up to be a much smaller crop,” Dr Whish said.
“Many farmers may intentionally do the same thing, to reduce the plants’ canopy early in the season to spare water and nitrogen for later.
“But the nematode pressure is constant and the plants never get the chance to recover, even if nutrients are added or there is good rain late in the season.
“So the restriction never stops and the effects on the plant can be devastating, with the reduced canopy resulting in reduced demand, growth and yield potential.”
Interestingly, over the two years he has conducted his research, including both a drought year and a year with above average rainfall, both resulted in the same yield loss of 66 per cent caused by nematodes in susceptible wheat varieties.
“Not all wheat varieties will lose yield and Australian wheat breeders have produced tolerant varieties that maintain their yield despite being under high nematode pressure,” he said.
“However, the ultimate goal is to breed resistant varieties that will reduce the nematode population. It is important that growers test to see what species of nematode they have and then select the most appropriate crop varieties to manage the nematode population.”
The next stage of Dr Whish’s mission to “know the enemy” will involve learning and describing the different stages of the pests.
Each time the gravimetric samples are taken measurements are also taken of exactly how many nematodes were in the soil and their stage of development and this information will help determine exactly how they grow.
Dr Whish said a difficulty with managing nematodes is their broad host range, which makes it difficult to manage their population by rotating to a different crop.
“Once you have got them you can’t get rid of them, however one positive is that they don’t move quickly.
“So if you are cautious and have good practices you can delay the rate of infection into clean areas.”
For those farmers already dealing with the pest on a wide scale, his research, combined with that of fellow nematode researchers John Thompson, looking at resistant wheat varieties, and Kirsty Owen, looking at the impact of crop rotations, offers significant hope.
“With any pest problems you are never going to have a silver bullet, so together we will hopefully find a crop rotation model that will be profitable and will reduce nematode populations or keep them at a manageable level,” Dr Whish said.
“Nematodes require plant roots to reproduce, which is why fallowing and growing non-host crops is really good. If you don’t provide them with a site to reproduce then populations will decline.
“If you can reduce the ability of nematodes to have young then you reduce the ability of the population to explode.”
Dr Jeremy Whish, researcher, CSIRO
07 4688 1419
Michael Thomson, Senior Consultant, Cox Inall Communications
07 4927 0805, 0408 819 666
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