What are plant parasitic nematodes and why should we care?
If you work in agriculture, I am sure you have heard about nematodes, nonetheless now is a good time to review what they are and how they affect crops.
Nematode are eel-like worms that live in a wide range of environments including soils, oceans, rivers, hot springs, arctic ice, tree canopies, and the desert. They also have diverse feeding behaviors!
Nematodes can feed on bacteria, fungi, protozoans, animals, and plants. Therefore, nematodes are very important for soil nutrient cycling and can serve as an indicator of soil health.
Out of the known species of nematodes, just 10 to 15 percent feed on plants and are called plant parasitic nematodes (PPN). In soil ecosystems, the number of PPN can be variable, but may reach 50 percent of the total nematode population.
Plant parasitic nematodes are microscopic and will feed mainly from plant roots, however, some species feed from plant and tree leaves, seeds, bulbs, stems, and tree trunks.
According to Nicol et al (2011), PPN cause worldwide losses of 80 billion dollars per year, although this number could be an underestimation since many farmers may not be aware that they have losses associated with PPN.
HOW DO PPN
FEED FROM PLANTS
Plant parasitic nematodes have a very specialized mouth part called stylet, a spear-like structure they use to puncture the cells, digest their contents, and kill them. In terms of feeding strategies, nematodes can be classified as follows:
∫ Ectoparasites: These nematodes will not enter the root, and they will feed from the outermost part using their long stylets to reach plant cells deep in the root. Sedentary ectoparasites become attached to the root, for example, the ring nematode (Criconemella spp.). Migratory ectoparasites will not become attached to the root. Examples of these include the dagger (Xiphinema spp.), sting (Belonolaimus spp.) and stubby root (Trichodorus spp. and Paratrichodorus spp.) nematodes.
∫ Endoparasites: Nematodes will enter the root and feed from within. If the nematodes move inside the tissues as they feed, they are called migratory endoparasites. Examples of migratory endoparasites include the lesion (Pratylenchus spp.), burrowing (Radopholus spp.), lance (Hoplolaimus spp.) and spiral (Rotylenchus spp.) nematodes. If the nematodes become attached to the inner parts of the root, they are called sedentary endoparasites. The sedentary ectoparasites include two of the most economically important nematodes worldwide: the cyst (Heterodera spp., Globodera spp.) and the root knot nematodes (Meloidogyne spp.). Both the cyst and the root knot nematodes will feed from the inner parts of the roots without killing the cells, instead, they turn them into food factories to get their food delivered to where they are.
Above ground symptoms may not be obvious until nematode populations have exceeded economic threshold.
Nematodes impair root growth and water and nutrient uptake. Therefore, plants affected by nematodes may express a wide range of symptoms including those that look like nutrient deficiencies, water deficits, and root rots.
Plants will look stunted and yellow, accompanied by a yield decline. In the case of soybean cyst nematode (SCN), signs of infections are the yellow-lemon-shaped females attached to the roots that are visible 4-6 weeks after planting. Root knot nematodes will cause a characteristics root galling.
Plant parasitic nematodes spread through anything that moves soils and infected plant parts, for example, water, ag-machinery and implements, wind, and shoes.
INTERACTION WITH OTHER PATHOGENS
Plant parasitic nematodes can have a synergistic interaction with other fungal and bacterial pathogens that may lead to a greater disease severity than the effect of the nematode or the pathogens alone.
Some nematodes species also have the ability to transmit plant viruses.
There are many nematodes that affect crops, and a single crop can be affected by different nematode species.
However, the number of nematodes required to cause crop losses varies according to the nematode and plant species. Any integrated pest management strategy starts with the accurate diagnosis of diseases, insects, and nematodes!
The only way to confirm the presence of nematodes and populations levels is by sending soil samples to a professional laboratory.
If you have questions about nematodes sampling on agronomic crops, feel free to contact me via email or at the Cooperative Extension Office at the Willowbank County Office Building, 420 Holmes St. in Bellefonte.
Adriana Murillo-Williams is a Penn State Extension Agronomy Educator. She can be contacted by email at firstname.lastname@example.org or by calling 814-355-4897.