Title: Nematodes – an overview
Author: Kathi Hertel
Last saved by: Craig Bratby
Company: NSW Department of Primary Industries

Understanding root lesion nematodes – A hidden problem – (Transcript)

Introduction
Root lesion nematodes are tiny microscopic worms about half a mm in length, that feed and reproduce in plant roots. This can lead to large yield losses in intolerant cereal and pulse crops.

Two important species of root-lesion nematodes are Pratylenchus thornei and Pratylenchus neglectus.  Either species may be found separately or together in mixed populations. 

Why are nematodes a problem?
Nematodes invade the roots of growing plants. When nematode numbers are high, this can cause damage to the roots and effect nutrient and moisture uptake.

Crops with damaged root systems may suffer nutrient deficiency and become quickly moisture stressed as soil moisture becomes limiting, as often occurs towards the end of the growing season or in dry years.  Low nematode populations and favourable growing conditions may mask the effects of the injury to plant roots during the growing season, but show up at harvest as decreased crop yields. 

These lower yields are often attributed to other causes and nematodes remain a hidden problem. 

Research has shown intolerant wheat varieties can lose between 50 and 70% of their yield when nematode populations are high. Intolerant chickpea varieties have been known to lose 20% of their yield. More recently, NSW DPI research in 2011 recorded yield losses of up to 43% in intolerant wheat varieties.

Life cycle of RLN
They must survive in the soil between crops and different tillage practices will affect their survival rates.  Findings in research has shown that nematode numbers may build up rapidly in wheat crops and decline slowly during fallow periods.  

When the soil is dry and no living roots are available, nematodes become dormant and may survive in the root tissue of old crops. As soils become wet again after rain, nematodes become active again and move to growing roots nearby. Juvenile and adult nematodes penetrate and feed in the root cells.

Nematodes are migratory feeders and they move from outside cells towards the middle of the root. They search for fresh and young roots to feed on as the older roots die. Female nematodes lay eggs in the roots and three to four generations of nematodes may develop during a single cropping season. A single female can lay up to 20 eggs several times through the growing season. 

These generations can all occur within a root system so populations of nematodes can be low in soil and yet high in roots at the same time. The nematode numbers on good hosts may increase to more than 100,000 nematodes per gram of dry root. As soils start drying at the crop harvest stage, nematodes become inactive again as they dry out. They remain dormant until the soil next becomes wet.

RLN were able to survive in low numbers for 8 years without a crop and build-up to damaging numbers in the first wheat crop.  After 2 wheat crops followed by 30 months without a host crop, RLN were still at levels that would reduce yield of intolerant wheat varieties.

Research in Queensland has confirmed that once high populations of P. thornei are established in a paddock, they can survive very long fallow periods, in some cases 8 years without a crop, particularly in the subsoil. These numbers can rapidly increase to damaging levels on susceptible host crops.

Signs and symptoms
Plant symptoms caused by nematodes can be difficult to identify.  

Symptoms of nutrient deficiency may be due to poor root function, despite adequate available soil nutrients. Plants may show poor vigour, stunted growth and poor tillering. Affected crops may wilt even in moist soils. 

Damaged plants may occur in large or small patches within a crop.

Digging up plants and washing the roots can show black and brown-coloured root lesions, indicating tissue death.  Whole sections of the root system may be dead. When nematode numbers are high, roots are often thin with little branching. It can be very difficult to distinguish between damage from nematodes and fungal root diseases like rhizoctonia or take- all.

Soil testing
Populations of nematodes of 2000 nematodes per kilogram of soil and higher is the threshold for yiled loss in intolerant crops. At these levels or greater, growers need to choose a tolerant crop variety or rotate to a resistant crop, according to the identified nematode species. 

To test a paddock for the presence of nematodes it is important to follow recommended procedures.  This includes taking a minimum number of soil samples from different locations in the paddock as well as at different soil depths. 

Handling and storage of the soil samples too is important for reliable results, avoiding high temperatures in particular.

Soil samples may be sent to a testing laboratory at either the Leslie Research Centre at Toowoomba in Queensland or to SARDI in Glen Osmond in South Australia

Details on sampling procedures are available at their web sites.

Send samples to:
Soil Microbiology Section
Leslie Research Centre
13 Holberton Street or P.O. Box 2282 
Toowoomba Qld 4350 
Phone: 07 4639 8888 
FAX: 07 4639 8800

To download a sample submission form, go to: 
http://www.dpi.qld.gov.au/26_14803.htm
Click on Root-lesion nematode sampling submission form (PDF, 234 kB)
SARDI (South Australian Research & Development Institute)
Predicta B soil test
Send PreDicta B samples to:
C/- SARDI RDTS
Locked Bag 100
Glen Osmond SA 5064

Contact: Alan McKay
Email alan.mckay@sa.gov.au
Ph 08 - 8303 9375

For further information go to: http://www.sardi.sa.gov.au/pestsdiseases/diagnostic_service/predicta_b_sample_registration

Concluding comments 
Root lesion nematodes are a hidden problem in many paddocks.  

Soil testing is the first step in identifying the magnitude of the problem and identifying which species of root lesion nematodes are present. This will allow strategic crop and variety choices to be made in local farming systems to reduce their impact.

Testing for root lesion nematodes should be considered before the next cropping season.

Do you have root lesion nematodes in your paddocks?

For further information, contact your local NSW DPI District Agronomist.