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Ministry of Agriculture, Food and Rural Affairs

Nematodes

Nematodes
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Beginner

Scientific Name: Pratylenchus penetrans and Xiphinema americanum

Identification

  • Root-lesion nematodes live inside raspberry roots or in the soil.
  • Mildly infected plants exhibit cane stunting and small root lesions.
  • Feeder roots are destroyed and replaced with many new, fine roots, giving a witch’s broom appearance.
  • Above ground symptoms include a decrease in the number and diameter of canes from year to year. .
  • Plants appear drought-stressed and yellowed with die-back. 
  • Plants are more susceptible to winter injury.
  • Dagger nematodes are found in the soil and do not invade raspberry roots.
  • Dagger nematodes  can transmit Tomato Ringspot Virus ( ToRSV)
  • Symptoms of ToRSV include mottled leaves, crumbly fruit and declining growth. 

Often Confused With
Root rot

Period of Activity
In Ontario, soil populations of nematodes generally peak between May and June and between September and October. Nematodes are less active in the winter when soil is frozen or during periods of drought.

Scouting Notes
Look for plants with weak, stunted, thin canes and sparse primocane growth. Mottled leaves and crumbly fruit are symptoms of nematode-vectored virus infections. Plants injured by nematodes have a very weak root system. Tugging on them should easily remove them from the soil and expose the small, brushy roots. Send soil samples to a diagnostic lab to confirm if nematodes are present.

Thresholds
Economic thresholds for nematodes are based on pre-plant soil populations that can build up to damaging levels during the growing season. Control root-lesion nematodes if preplant soil samples levels indicate 1,000 nematodes per kg of soil. Control dagger nematodes if preplant soil samples indicate 100 nematodes per kg of soil.

 

Advanced

Scientific Name: Pratylenchus penetrans and Xiphinema americanum

Identification
Nematodes are microscopic eel-like roundworms that live in soil and water. Most soil-dwelling nematodes are beneficial organisms involved in the breakdown and release of nutrients from organic matter.

A few nematode species are plant parasites that feed on raspberry roots or act as virus vectors. The two most important nematodes associated with raspberry roots are root-lesion nematodes, Pratylenchus penetrans, and dagger nematodes, Xiphinema americanum. These nematodes are most common in sandy or sandy loam soils.

Root-lesion nematodes may be found inside raspberry roots or in the surrounding soil. They feed on root cells by using their stylet, a hollow feeding tube that functions much like a hypodermic needle. Feeding causes small, elongate lesions on raspberry roots and these wounds may serve as an entry site for invading soil fungi.

Mildly infected raspberry plants are slightly stunted with small root lesions due to nematode feeding. Higher nematode populations cause more severe root damage. Feeder roots are destroyed and replaced with many new, fine roots, giving a witch’s broom appearance. As feeder roots die, nutrient and water uptake decreases. Plants weaken, die back, appear drought-stressed and are more susceptible to winter injury.  There is a decline in the number and diameter of canes from year to year.  Plant decline is gradual and can take several years depending on other stresses, varietal differences and nematode populations. In the final stages, all feeder roots die. Up to 25% of first year canes may be killed by severe infestations of root lesion nematodes.

Dagger nematodes are found in the soil and do not invade raspberry roots. They have a stylet for feeding on root tissue much like lesion-nematodes, but cause limited direct damage. However, dagger nematodes are vectors of Tomato Ring Spot Virus (TomRSV). The nematode picks up the virus as it feeds on a virus-infected plant. The virus remains active in the nematode for several weeks to several years. While feeding, the nematode transmits TomRSV to previously uninfected plants. Plants infected with TomRSV gradually lose vigour. Leaves appear mottled and canes may die back. Fruit is crumbly and yield is reduced.

Often Confused With

Root Rot
Root and crown diseases cause above-ground symptoms to similar to those caused by root lesion nematodes. To determine the cause of the problem, plants should be carefully dug up and roots examined.  Roots affected by root rot will show signs of reddish-brown discolouration. Roots affected by root lesion nematodes show signs of lesions,  lack feeder roots, or have a witch’s broom, brushy appearance. Use a diagnostic lab to confirm this diagnosis.

Biology
Root-lesion nematodes and dagger nematodes share a very similar life cycle. Root-lesion nematodes are endoparasites that spend a significant part of their life inside root tissue. Dagger nematodes, on the other hand, are ectoparasites that do not invade plant roots but rather remain within the soil.

Females lay one egg per day which may be deposited inside the root, in the case of root-lesion nematodes, or in the soil. Eggs hatch and several stages of larvae develop and feed on roots, before the nematode becomes an adult. The life cycle can be completed between 30 and 100 days, depending on soil temperature. Adults may live for 2 to 3 years and reproduce asexually. 

Period of Activity
In Ontario, soil populations of nematodes generally peak between May and June and between September and October. Nematodes are less active in the winter when soil is frozen or during periods of drought. Under such conditions, they tend to move deeper into the soil or into roots for protection.

Scouting Notes
Look for plants with weak, stunted, thin canes and sparse primocane growth. Mottled leaves and cane die-back are also symptoms of nematode infections. Plants injured by nematodes have a very weak root system. Tugging on them should easily remove them from the soil and expose the small, brushy roots.

Nematodes cannot be seen without the use of laboratory equipment. To determine if nematode populations have become a pest issue, send soil and root samples to a pest diagnostic lab. Late summer and fall are most ideal for soil sampling, but nematode samples can be taken as long as the ground is not frozen. 

Soil should be sampled approximately 8 inches deep using a 1inch-soil diameter soil core probe, or narrow-bladed shovel. Discard the top 1 - 2 inches of soil. Include the feeder roots of the crop in the soil sample, since this is where many nematodes live. Do not sample the roots of dead plants since the nematodes will already have died or moved away from dead roots into the soil.

Mix soil cores thoroughly but gently in a bucket. Place a sub-sample (1/2 - 1 litre) in a plastic bag. Keep cool and out of direct sunlight during transportation to the diagnostic lab.

To diagnose a problem during the growing season, take 8 to 10 soil cores from areas where plants are unhealthy, or along the margin of a severely affected area. Another 8 to 10 soil cores from areas of healthy growing plants should be sampled separately for comparison.
To estimate nematode populations in a field, soil cores should be taken within the row of actively growing plants to obtain samples that contain feeder roots. Walk in a Z, W or M pattern across the field. The soil sample should represent no more than 2.5 ha.

The chart below is a guide of how many cores are necessary to make up a representative sample. Separate samples should be taken from different soil types.

Area

Number of soil cores/sample

< 500m2

8 - 10

500 m2 - 0.5 ha

25 - 35

0.5 ha - 2.5 ha

50 - 60

Thresholds
Economic thresholds for nematodes are based on pre-plant soil populations that can build up to damaging levels during the growing season. An economic threshold marks the number of nematodes above which economic damage will be incurred.

  • Control for root-lesion nematodes when levels reach 1,000 nematodes per kg of soil.
  • Control for dagger nematodes when levels reach 100 nematodes per kg of soil.

Management Notes

Sample new fields for nematodes well before planting:

  • Sample sandy and sandy-loam soils for nematodes prior to planting raspberries.
  • Sample loam and clay loam soils for nematodes if raspberries or other tree fruit were previously grown in these fields.

If necessary, control nematodes before planting; it could take several years to do so. 

Rotate raspberry plantings with non-hosts for several years.
Use nematode- suppressing cover crops to reduce nematode populations. These include

  • certain oriental mustard cultivars, like Caliente, Cutlass, or Forge
  • some but not all cultivars of sorghum x sudan-grass hybrids
  • African marigold cultivars like Crackerjack or Creole
  • Canadian Forage Pearl Millet 101

Be aware that nematodes can be transferred in soil, water, soil covered machinery from infested fields to clean fields, and take steps to prevent this.  

Always start new plantings with virus and nematode -free planting stock.
Control weeds that can act as hosts to root-lesion nematodes.
Soil fumigation with a nematicide requires specific knowledge, licenses and equipment.   Soil fumigation is usually performed in mid fall or early spring, before planting.  For more information on fumigation methods and nematicides, refer to OMAFRA Publication 360: Fruit Crop Protection Guide (Chapter 3 Berry Crops).