Plant Parasitic Nematodes

Several different types of plant parasitic nematodes can impact field and horticultural crops. They can be categorized based on if they feed inside (endo) vs outside the plant (ecto) and whether they move when they feed (migratory) vs stay in one place (sedentary). These include:

Ectoparasitic nematodes (feed outside the plant):

• dagger nematode (Xyphinema spp.)
• needle nematode (Longidorus spp.)
• pin nematode (Paratylenchus)
• spiral nematode (Helicotylenchus spp.)
• stunt nematode (Tylenchorhynchus spp.)

Migratory endoparasitic nematodes (move while feeding inside the plant):

• root lesion nematode (Pratylenchus penetrans and other species)
• bulb and stem nematode (Ditylenchus dipsaci)

Sedentary endoparasitic nematodes (stay in one place while feeding inside the plant):

• northern root knot nematode (Meloidogyne hapla)
• soybean cyst nematode (Heterodera glycines)
• oat cyst nematode (Heterodera avenae)
• carrot cyst nematode (Heterodera carotae)
• tobacco cyst nematode (Globodera tabacum)
• sugar beet cyst nematode (Heterodera schachtii)

Some nematodes have a limited host range (e.g. soybean cyst nematode) while others have a wide host range, impacting several different crops (e.g. root lesion nematode). Soybean cyst nematode is found across most of Ontario and is considered the most destructive soybean disease in the world. Cyst nematodes are difficult to manage using crop rotation alone because the hardened cyst protects the eggs, allowing them to survive for many years in the absence of a host plant. The root lesion nematode is very common due to its wide host range and can be found throughout Ontario. It is one of the most economically important plant parasitic nematode in Ontario fruit and vegetable production. They cause small streak (scratch-like) lesions on roots providing an entry point for soil borne pathogens to infect. Root lesion nematodes are a significant factor of Apple Replant Disorder (ARD). They can also cause a decline in vigour of perennial crops such as peach and cherry.

Soil type, texture and pH can also be important factors in nematodes infestations. Lighter soils (e.g. sandy and loam) can experience higher infestations of northern root knot nematode, root lesion nematode and soybean cyst nematode. Soybean cyst nematode population densities tend to be higher in soils with a pH of 7 or greater.

Nematode Signs and Symptoms on Crops

Symptoms of nematodes vary depending on the crop and type of nematode. Infestations can go undetected even while significant yield loss has occurred. Above ground symptoms can often be misdiagnosed for other problems such as nutrient deficiencies, herbicide injury, soil compaction, drought, flooding or root rots. Signs of the presence of nematodes may also be present, especially in the case of cyst nematodes.

Generally, signs and symptoms of nematode infestations include:

• yellowing of leaves
• stem twisting, crown and bulb bloating (caused by bulb and stem nematodes)
• stunting or uneven plant growth
• poor plant/stand establishment
• plants weakening over time
• poor root growth and discolouration
• cysts, knots, lesions or galls on roots, depending on the type of nematode
• excessive branching of roots, hairy root symptoms

The only way to confirm the presence and population density of plant parasitic nematodes in your field is by soil and root sample analysis by accredited laboratories. Sample soils for nematodes before planting where susceptible crops will be grown. Soil samples can also be collected in the fall, soon after harvest but before freezing. For more information, see OMAFRA Factsheet, Sampling Soil and Roots for Plant Parasitic Nematodes (gov.on.ca). Fresh root samples can also be used to diagnose nematodes, especially during the field season if you suscept nematode injury. Nematode populations above economic thresholds can significantly reduce yields. These are based on initial soil population levels that will multiply over the growing season and cause economic damage to the crop. The economic threshold is expressed as the number of nematodes in a kilogram of soil or per gram of dried root tissue and is often different for each crop and each nematode species. Refer to each specific crop and nematode pest for available economic thresholds.

Management

Nematode management requires an integrated approach, using a combination of strategies to prevent and reduce their populations. How effective these strategies are depends on the nematode species and crop. Use these as general guidelines and refer to more specific information under each crop and nematode pest in this application.

Genetic Resistance

• Genetic resistance is more common against endoparasitic nematodes like cyst or root knot nematodes.
• Plant resistant or tolerant cultivars where available.
• Rotate sources of resistance in varieties to reduce the risk of resistance development.
• Rotate resistant varieties with susceptible varieties to reduce the risk of resistance development especially if no other sources of nematode resistance are available.

Cultural Practices

• Plant good quality, certified seed free of soil peds or, transplants or bulbs free from nematodes that were grown by an accredited plant propagator.
• Avoid growing susceptible crops and varieties for consecutive years.
• Rotate susceptible crops with non-host crops for several years (not as effective for cyst nematodes).
• Destroy residual crop roots.
• Wash soil from farm equipment when moving between infested fields or orchards.
• Follow soil conservation practices to reduce soil movement between fields and orchards.
• Avoid using sources of irrigation water that are near infected fields.
• In orchards, choose ground covers for planting between the rows that do not support nematodes, such as annual or perennial ryegrass, or creeping red fescue.
• Control weeds, as they can be good hosts of nematodes.
• Grow nematode-suppressing cover crops prior to establishing crops (see Cover Crop for Nematode Suppression, below). Plan an intensive weed management strategy for the cover crop where nematode suppressing cover crops are grown.

Chemical Control

• Some soil fumigant products and seed and soil applied nematicides are available for use on some crops. Products, rates and other information on fumigants and seed-applied nematicides, can be found through a control search for the appropriate crop and nematode pest.
• Best to use soil fumigation before planting when nematode populations in soil reach or exceed thresholds. Follow the label. Some fumigants have long post-application planting restrictions, especially under poor weather conditions.

Cover Crops for Nematode Suppression

Cover crops may reduce populations of plant parasitic nematodes when properly managed before planting. However, this depends on the species of nematode present in the field. Some cover crop species will suppress some nematode pests, while acting as hosts for others thereby increasing these nematode populations. Clovers and buckwheat in particular, should be avoided in fields with root lesion nematodes as they can be excellent hosts.

Cover crops suppress nematodes in different ways:

Releasing toxins that kill nematode populations

• Nematode-suppressing cover crop cultivars of Brassica plants (e.g. oilseed radish, mustards) produce glucosinolates and an enzyme in their leaves, stems and petioles. When the cover crop is cut (mustards should be cut at flowering) and immediately incorporated into the soil, the glucosinolates are converted into isothiocyanates and other compounds, which are toxic to many plant parasitic nematodes. Certain mustard varieties have been developed to produce higher levels of glucosinolates. The exceptions are beet cyst nematode and northern root knot nematode for which Brassicas can be excellent hosts.

Suppressing or not sustaining nematode development

• Canadian Forage Pearl Millet 101 is a poor host and inhibits the ability of root-lesion nematodes to reproduce in its root-system.
• Many grass species are poor or non-hosts for most of the common nematodes in Ontario. Options include annual ryegrass, barley, cereal rye, oats and wheat.
• Specific sorghum × sudan-grass hybrids