Diagnosing nematode problems in berry crops

Nematodes are microscopic eel-like worms that live in soil and water. Most soil dwelling nematodes are beneficial organisms that play a role in the breakdown and release of nutrients from organic matter.

Several species of nematodes live and feed on plant roots. These plant parasitic nematodes possess a hollow stylet, which is forced into plant cells. Enzymes are injected to decompose the cell content. The nematode withdraws the partially digested cell contents through the stylet.

Root lesion nematode is the most common nematode pest on strawberry and raspberry. During the growing season, root-lesion nematodes live and feed inside plant roots. When the plants and roots die in the autumn, they move out of the root into the soil.

On raspberries, dagger nematode is another important nematode pest, because it is a virus vector.

Signs of nematode injury: In raspberries, nematode feeding causes plants to decline over time. Canes get shorter and weaker. Plants are poorly rooted and can be easily pulled from the soil. Primocane growth becomes sparse.

Nematode-infested strawberry fields show uneven growth across the field. Plants are stunted or weak in patches, next to apparently healthy vigourous plants. Some varieties are more susceptible than others to nematode damage. New plantings decline more quickly than normal.

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Roots of infected plants may show brown flecking, larger brown lesions, or black root rot. (Figure 1) Nematode feeding can cause young white roots to be stubby and swollen, or excessively branched.

Figure 1: Strawberry roots showing flecking and brown lesions caused by root lesion nematode

Nematodes are often associated with soil borne diseases, such as verticillium, or black root rot. Damage caused by root lesion nematodes provides infection sites for disease causing fungi.

Sampling for nematodes: Nematode populations can be estimated by sampling soil, and plant roots. The samples can be processed at the Pest Diagnostic Clinic in Guelph, 519-767-6256. For submission forms and a fee schedule, visit their web site.

Late summer and fall are good times to sample soil for nematodes. Populations are generally highest in May-June and September-October. However, nematode samples can be collected any time as long as the soil is not frozen. Collect samples in time to arrange fall fumigation if necessary.

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 have already died or moved away from dead roots into the soil.

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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.

Interpreting sample results: Economic thresholds for nematodes are based on pre-plant soil populations that can build up to damaging levels during the growing season.

Strawberry: Control nematodes if populations exceed the economic threshold of 500 nematodes per kg/soil.

Raspberry: The threshold for root lesion nematode on is 1000 nematodes per kg of soil. The threshold for dagger nematode is 100 nematodes per kg of soil.

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Nematode control: A nematode control strategy could include

• crop rotation with non-hosts for several years
  
• planting a root lesion nematode suppressive cover crop such as some Canadian forage pearl millet and oilseed radish varieties
  
• cultivation
  
• fumigating soil with a nematicide.

When planting a nematode suppressing cover crop be sure to obtain a variety that suppress nematodes since some varieties can actually increase or maintain root lesion nematode populations.

Root lesion nematode suppressing cover crops do not eliminate nematodes completely but can reduce population levels when cropped for two or more years.

Fumigation with a nematicide is usually performed in mid fall or early spring. Fields should be cultivated several weeks before fumigation to allow organic matter to decompose that would otherwise tie up the chemical fumigant. The fall is a good time to fumigate provided the soil temperature is warmer than 4oC at a depth of 15 cm and the moisture content at a level that would permit seed germination. Dry soil will not seal in the fumigant and should be irrigated prior to fumigation.

For information on cover crops see OMAFRA publication 811, Agronomy Guide, or the Cover Crops Index page on the OMAFRA website.

Related Links

• Pest Diagnostic Clinic
• OMAFRA Agronomy Guide, Publication 811
• OMAFRA Cover Crops Index Page

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