nematodes
Beginner
Nematodes are microscopic eel-like worms living in soil and water. Most soil dwelling nematodes are beneficial organisms and play a role in the breakdown and release of nutrients from organic matter. Some beneficial nematodes prey on other nematodes and soil-borne insects, others feed on fungal and bacteria pests. Several species of nematodes are plant parasites feeding on or in roots of apple trees causing failure in the replant of new young orchards and significant yield reduction.
Scientific Name
The most common plant parasitic nematodes attacking fruit trees include root-lesion nematodes (Pratylenchus penetrans), dagger nematodes (Xiphinema spp.) and pin nematodes (Paratylenchus projectus).
Identification
Root-lesion nematodes cause tiny brown lesions on white lateral roots, and rotting of the fine feeder roots as they feed. The lesions eventually merge causing the entire root system to become discoloured. Feeding and damage immediately behind the root tip often cause the tip to die, stimulating the plant to produce many new feeder roots – resulting in a “witches’-broom” or “hairy root” effect. Feeder roots may be pruned off entirely in trees severely infested with root-lesion nematodes, resulting in premature tree death.
Young non-bearing trees infested with root-lesion nematodes exhibit abnormal, stunted growth with small chlorotic leaves, short internodes and thinner trunks. Above ground symptoms on established trees are less obvious than those on young trees. Young trees that are severely infested often die or become unproductive. Replacement trees planted in the same area become infested and die as well. Root-lesion nematode populations are distributed unevenly throughout fields, and new orchards planted in invested fields lack uniformity in establishment, growth and vigour. Nematodes are often associated with apple replant disease complex. Some species also transmit viruses. Dagger nematode can vector tomato ringspot virus which causes graft union necrosis in some apple cultivars such as Empire.
Period of Activity
Soil populations of most plant parasitic nematodes are highest in September and October when many have moved from the roots to the soil – this is the best time of year to sample soil for nematodes.
Scouting Notes
The best method of monitoring nematodes is to take root and soil samples. Take root and soil samples containing roots any time of the year as long as the soil is not frozen. The best time to sample soil for nematode population assessment is in the spring (May and June) after the soil has warmed up, or during the fall (September and October). Do not take nematode samples when soils are very wet. Sampling in the early fall allows time to make decisions on whether to fumigate during the fall or spring, or whether to plant the site to young apple trees. It also allows time to implement an integrated management strategy prior to establishing the orchard at that site. Sampling in the spring prior to planting a crop may also be reliable but the time it takes to receive, interpret and act upon the soil nematode analysis limits the time to make an informed decision and implement appropriate action before planting.
The location where soil samples are collected depends on the purpose for taking the sample. If the purpose of sampling soil for nematodes is to diagnose a problem in trees with low vigour during the growing season, be sure to get the feeder roots of the crop in the soil sample since this is where many nematodes live. For individual fruit trees suspected of being infested with nematodes, take soil samples from just below the drip line and in the area between the outer branch tips and the tree trunk.
If the purpose of sampling is to determine whether the nematode population has reached an economic threshold at a site prior to planting, walk in a Z, W or M pattern across the field while taking a soil sample every 10-25 paces. Be sure the soil sample represents no more than 2.5 ha. If the orchard to be established at the site is more than 2.5 ha, divide the site into 2.5 ha parcels and sample each parcel separately.
Nematodes are rarely distributed evenly throughout a field or orchard, and populations fluctuate throughout the growing season. For more information on sampling for nematodes see Publication 310, Integrated Pest Management for Apples or Publication 360, Fruit Crop Protection Guide.
Thresholds
Economic thresholds for nematodes are based on initial soil population levels that build up to damaging levels during the growing season.
Economic thresholds refer to the number or population of nematodes in a kilogram of soil that multiply over the growing season and cause economic damage to the crop. The economic threshold is often different for each crop and each nematode species. If the nematode soil analysis report indicates populations higher than the threshold, implement an integrated nematode management strategy. For more information, consult the OMAFRA Factsheet 06-099, Sampling Soil and Roots for Plant Parasitic Nematodes.
Economic Thresholds for Nematode Soil Population Levels |
|
|---|---|
Nematode |
Economic threshold (nematodes/kg soil) |
| Root-lesion | 1,000 |
| Pin | 5,000 |
| Dagger | 100 |
Advanced
Nematodes are microscopic eel-like worms living in soil and water. Most soil dwelling nematodes are beneficial organisms and play a role in the breakdown and release of nutrients from organic matter. Some beneficial nematodes prey on other nematodes and soil-borne insects, others feed on fungal and bacteria pests. Several species of nematodes are plant parasites feeding on or in roots of apple trees causing failure in the replant of new young orchards and significant yield reduction.
Scientific Name
The most common plant parasitic nematodes attacking fruit trees include root-lesion nematodes (Pratylenchus penetrans), dagger nematodes (Xiphinema spp.) and pin nematodes (Paratylenchus projectus).
Identification
Root-lesion nematodes cause tiny brown lesions on white lateral roots, and rotting of the fine feeder roots as they feed. The lesions eventually merge causing the entire root system to become discoloured. Feeding and damage immediately behind the root tip often cause the tip to die, stimulating the plant to produce many new feeder roots – resulting in a “witches’-broom” or “hairy root” effect. This abnormal root growth symptom is most striking on one- and two-year-old trees, but can also occur in older trees. Feeder roots may be pruned off entirely in trees severely infested with root-lesion nematodes, resulting in premature tree death.
Young non-bearing trees infested with root-lesion nematodes exhibit abnormal, stunted growth with small chlorotic leaves, short internodes and thinner trunks. Above ground symptoms on established trees are less obvious than those on young trees. Young trees that are severely infested often die or become unproductive. Replacement trees planted in the same area become infested and die as well. Root-lesion nematode populations are distributed unevenly throughout fields, and new orchards planted in invested fields lack uniformity in establishment, growth and vigour. Nematodes are often associated with apple replant disease complex. Some species also transmit viruses. Dagger nematode can vector tomato ringspot virus which causes graft union necrosis in some apple cultivars such as Empire.
Often Confused With
Biology
Root-lesion nematode is considered the most economically important plant parasitic nematode in Ontario fruit production. It is widespread in Ontario soils, particularly in orchards planted on sandy (coarse-textured) soils where it causes the most damage. Its host range includes most fruit crops, vegetables and many native weeds.
In the presence of a good host, populations increase to several thousand per kilogram of soil during one growing season. It moves in and out of roots throughout the season as it feeds and causes damage. When infested roots die, root-lesion nematodes move out of the roots into the soil, particularly at the end of the growing season. Soil populations are most numerous in May and June, and September and October each year, which are preferred times for sampling.
The dagger nematode and pin nematode thrive in clay and clay-loam (fine textured soils), and do not invade tree roots but live only in the soil feeding on external tissue of roots. Like the root-lesion nematode, dagger and pin nematodes have broad host ranges. The dagger nematode prefers woody-rooted plants, including weeds such as lamb’s-quarters, and is often numerous around wild Prunus species, such as chokecherry, pincherry and wild plum. Dagger nematode vectors several viral diseases of woody plants, as well as grapes.
Female plant parasitic nematodes are fertilized by the male and lay eggs in the soil or roots. After the eggs hatch, juvenile nematodes swim to other nearby plant roots and feed on or in them depending on the type of nematode. Nematodes can go through several juvenile development stages before becoming adults. Research shows damage caused by nematodes in many crops also provides an infection site for other disease-causing organisms, further reducing yields. Most nematodes complete their life cycle within three to six weeks during the growing season, depending upon available moisture and temperature. During favourable environmental conditions and adequate availability of host roots, populations build up rapidly in and around roots. The mobile stages of nematodes move through soil pores, but can only move a few centimetres per year on their own. However, they can spread over greater distances if soil is moved by erosion, tillage, wind, livestock, etc. Excessive moisture and extreme temperatures kills nematodes.
Period of Activity
Soil populations of most plant parasitic nematodes are highest in September and October when many have moved from the roots to the soil – this is the best time of year to sample soil for nematodes.
Scouting Notes
The best method of monitoring nematodes is to take root and soil samples. Take root and soil samples containing roots any time of the year as long as the soil is not frozen. The best time to sample soil for nematode population assessment is in the spring (May and June) after the soil has warmed up, or during the fall (September and October). Do not take nematode samples when soils are very wet. Sampling in the early fall allows time to make decisions on whether to fumigate during the fall or spring, or whether to plant the site to young apple trees. It also allows time to implement an integrated management strategy prior to establishing the orchard at that site. Sampling in the spring prior to planting a crop may also be reliable but the time it takes to receive, interpret and act upon the soil nematode analysis limits the time to make an informed decision and implement appropriate action before planting.
The location where soil samples are collected depends on the purpose for taking the sample. If the purpose of sampling soil for nematodes is to diagnose a problem in trees with low vigour during the growing season, be sure to get the feeder roots of the crop in the soil sample since this is where many nematodes live. For individual fruit trees suspected of being infested with nematodes, take soil samples from just below the drip line and in the area between the outer branch tips and the tree trunk.
If the purpose of sampling is to determine whether the nematode population has reached an economic threshold at a site prior to planting, walk in a Z, W or M pattern across the field while taking a soil sample every 10-25 paces. Be sure the soil sample represents no more than 2.5 ha. If the orchard to be established at the site is more than 2.5 ha, divide the site into 2.5 ha parcels and sample each parcel separately.
Nematodes are rarely distributed evenly throughout a field or orchard, and populations fluctuate throughout the growing season. For more information on sampling for nematodes see OMAFRA Publication 310 Integrated Pest Management for apples or Publication 360, Fruit Crop Protection Guide.
Thresholds
Economic thresholds for nematodes are based on initial soil population levels that build up to damaging levels during the growing season.
Economic thresholds refer to the number or population of nematodes in a kilogram of soil that multiply over the growing season and cause economic damage to the crop. The economic threshold is often different for each crop and each nematode species. If the nematode soil analysis report indicates populations higher than the threshold, implement an integrated nematode management strategy. For more information, consult the OMAFRA Factsheet 06-099, Sampling Soil and Roots for Plant Parasitic Nematodes.
Economic Thresholds for Nematode Soil Population Levels |
|
|---|---|
Nematode |
Economic threshold (nematodes/kg soil) |
| Root-lesion | 1,000 |
| Pin | 5,000 |
| Dagger | 100 |
Management Notes
- Planting the least susceptible root stock helps reduce the impact of parasitic nematodes on the growth, vigour and productivity of trees. In general, M.7 and M.9 rootstock are most susceptible, M.26 is moderately susceptible, and MM.106 and MM.111 are least susceptible to nematodes.
- Cultural control techniques actively reduce soil nematode numbers, or relieve other stresses on the trees to lessen the deleterious effect of nematodes on growth.
- Soil nematode populations higher than threshold must be reduced well below threshold prior to planting trees. Several cover crops have been demonstrated to suppress root-lesion nematode populations. Grow these nematode suppressing cover crops prior to planting a new young orchard to reduce soil population levels of root-lesion nematodes. For information on nematode suppressing cover crops refer to Publication 360, Fruit Crop Protection Guide.
- Good weed control is essential for nematode reduction when nematode suppressing cover crops are used to reduce soil population levels.
- Keeping fields fallow or leaving fallow strips where trees are planted prior to the establishment year significantly reduce nematode soil population levels. Fallow fields or strips require occasional cultivation to reduce weeds but may reduce soil moisture and contribute to wind erosion.
- Plowing late in the fall – just before freezing – leaves some nematodes exposed to cold and reduces numbers somewhat. These practices are best used only to reduce moderate nematode numbers, and used with soil nematode testing to determine their effectiveness.
- Pre-plant soil fumigationis the preferred method of nematode control for orchard establishment, although post-plant treatment with a systemic nematicide is also possible. Nematode control with fumigation is expensive, and tree-row or tree-site application helps reduce costs.
- Pre-plant fumigation requires advance planning, particularly if the fumigant is applied in the tree row only. For information on soil fumigation refer to OMAFRA Publication 360, Fruit Crop Protection Guide.
