Pests and Diseases
Nematodes are vermiform invertebrates that colonize any terrestrial, aquatic or marine ecosystems. About 80% of all multicellular organisms on earth are considered to be nematodes. In soil ecosystems nematodes play a significant part within the food web where they will feed and being fed by other soil organisms. Most nematodes utilize bacteria as a food source (bacteria-feeding), others fungi (fungal-feeding), small invertebrates including nematodes (predatory) or plant roots (plant-parasitic). The first three groups are often summarized as free-living nematodes and are favorable for agriculture, while the last group represents a major threat for crop yield and food safety. In agricultural production systems nematode numbers can range from a few hundred in heavily managed and depleted soils to more than 10,000 specimens per 100 ml in sustainable and biologically active soils. Build-up of plant-parasitic nematodes is fostered by good food availability such as growing host plants in narrow rotations.
1. Plant parasitic nematodes,
2. Nematodes and organic farming,
3. Nematodes and minimum tillage and
4. Free-living and beneficial nematodes.
Plant parasitic nematodes
Out of almost 30,000 nematodes species described so far, about 5,000 species are plant-parasitic. Of these, 30-50 species are economically important pathogens, such as members of the root-knot nematodes (Meloidogyne), root lesion nematodes (Pratylenchus), cyst nematodes (Heterodera, Globodera), pin nematodes (Paratylenchus) and stubby root nematodes (Trichodorus, Paratrichodorus). Depending on geographic region and climatic conditions additional specimens might need attention. Some nematodes can be considered generalists having a broad host spectrum (e.g. Meloidogyne, Pratylenchus, Paratylenchus, Trichodorus), others are more specialists with a narrow host range (e.g. Heterodera, Globodera). For members of the first group food availability is usually high and juveniles hatch spontaneously. Members of the second group have to withstand periods where no host plant is grown and therefore juveniles only hatch after a period of dormancy and following stimulation by root exudates of host plants.
Plant parasitism has evolved at least three times independently represented by plant-parasitic specimens within three orders, i.e. Tylenchida, Dorylaimida and Triplonchida. Out of these, tylenchid nematodes are by far the largest group of plant-parasitic nematodes in agricultural soils representing more than 90% of this group. Depending on their feeding type, plant-parasitic nematodes can be grouped as ectoparasites, migratory endoparasites and sedentary endoparasites.
Ectoparasitic nematodes (e.g. Paratylenchus) feed from the outside on root epidermal cells resulting in death of the cell. As only individual cells are killed, overall damage to the plant is usually small and it takes several hundred specimens to cause a significant impact on plant growth. Threshold levels are generally above 500 nematodes/100 ml soil. Exceptions are ectoparasites that feed on the root tip destroying the meristematic tissue inhibiting further root growth (e.g. Trichodorus). Threshold levels for those specimens are less than 10 nematodes/100 ml soil.
Migratory endoparasites (e.g. Pratylenchus) enter the root near the root tip and then migrate with the help of their stylet and secretion of cell wall degrading enzymes through the root cortex. They feed on cortical cells that are more nutritious than epidermal cells. Direct damage caused by the nematode during migration in combination with plant defense mechanisms will result in necrosis of the root tissue. The damage potential is higher than for ectoparasitic nematodes and threshold levels usually start at about 80-100 nematodes/100 ml soil.
Sedentary endoparasites (e.g. Meloidogyne, Heterodera, Globodera) are the most damaging plant-parasitic nematodes. Juvenile stages enter the root near the root tip and migrate towards the nutritious vascular system where they establish a feeding site. With initiation of the feeding site the juveniles become sedentary and from now on fully depend for the rest of their life on the plant’s delivery of nutrients to the feeding site. The plant nutrient flow is redirected towards the benefit of the nematode and the disadvantage of the plant. Threshold levels for this group of nematodes often start around 50 juveniles/100 ml soil.
Plant-parasitic nematodes destroy root tissue, interfere with the plants water and nutrient household and support secondary pathogens (e.g. soil-borne bacterial and fungal pathogens). Resulting symptoms vary depending on nematode species and host plant. In general, above-ground symptoms are less specific including growth depression, deformation, wilting and chlorosis. Below-ground symptoms are often more specific such as root galls, cysts attached to roots, root lesions, stubby roots, biforking, excessive side root formation (root beard) or rots.
Once above-ground symptoms are noticed, yield reduction has already exceeded 10% but can be much higher if the quality of root and tuber crops is affected. First above-ground signs for a nematode damage can be patches of poor growth, heterogeneous plant stands or failure of the crop to respond to a fertilizer application.
Most plant-parasitic nematodes prefer sandy soils where particle size and water/air ratio support optimum nematode activity and movement. Heavy soils tend to compactness and anaerobic conditions deteriorating nematode mobility. The main factor causing nematode damage is a narrow rotation that leads to build-up of nematode numbers. Soils with low antagonistic potential are more suitable to nematode damage than suppressive soils. Heavy use of synthetic fertilizer and plant protection agents with negative impact on the antagonistic potential should therefore be avoided. Organic amendments and cover crops that stimulate soil life will increase soil suppressiveness towards plant-parasitic nematodes. In general, any factor causing plant stress (e.g. low pH, nutrient deficiency, draught, diseases, weeds, soil compactness) will also favor nematode damage. Cool and wet conditions after planting will extend the period where the plant is susceptible to nematode attack. Warm conditions will enhance nematode development and dry conditions plant losses due to competition for water.
The biggest helpers for the farmer controlling plant-parasitic nematodes antagonistic are probably the soil microorganisms. Thus, any effort should be taken to keep the antagonistic potential of the soil as high as possible. This can be achieved by maintaining the soil organic matter at high levels applying organic amendments, green manure, or plant residues. Since nematodes are obligate plant parasites, absence of food will famish them. However, the time needed to kill 90% of a given population varies a lot and ranges from few months (e.g. Meloidogyne), to several months (e.g. Pratylenchus) or even more than a year (e.g. Trichodorus). The most common practice for managing plant-parasitic nematodes is growing a non host crop. If the main damaging nematode species has been identified, non host plants can be selected that do not allow reproduction of this species. If available, resistant cultivars or resistant green manure crops are valuables tool, too. In cases where nematode densities are high and crop production cannot pause, a tolerant crop should be selected. Tolerant crops help saving the yield for the farmer but on the other side will increase nematode numbers and therefore should only be used exceptionally. Other control options include trap crops, antagonistic crops, biological control or physical measures (e.g. flooding, steaming).
Nematodes and organic farming 
Plant-parasitic nematodes occur in organic as well as in conventional farming with little difference in species spectrum und population dynamics between both systems. However, species with a broad host spectrum (e.g. Meloidogyne, Pratylenchus) tend to be the main pathogens in organic farming while cyst nematodes with a narrow host range (e.g. Heterdodera, Globodera) tend to be the main pathogens in conventional farming. However, more important than the farming system are factors such as narrow cropping frequency of host plants and unsatisfactory weed control favoring nematode build-up or organic amendments and green manures suppressing plant-parasitic nematodes.
Nematodes and minimum tillage
Minimum tillage means less disturbance of soil life and if combined with continuous soil cover better food conditions for plant-parasitic nematodes. In principle, both factors promote plant-parasitic nematode numbers. On the contrary, minimum tillage systems maintaining soil organic matter content at high levels will reduce plant-parasitic nematode numbers due to enhanced soil life and increased antagonistic potential. In conclusion, negative and positive effects of minimum tillage are often balanced. As a result of the OSCAR project plant-parasitic nematodes numbers were slightly higher at minimum tillage compared to ploughing, and free-living nematodes (non plant parasites) were much higher. The latter indicates high biological activity in the soil. Furthermore, the use of subsidiary crops, green manure crops and mulch did not affect total numbers of plant-parasitic nematodes. However, differences were observed at species level depending on the main crop.
Free-living and beneficial nematodes
Free-living nematodes feed on bacteria, fungi or small soil invertebrates and form a significant part of the soil food web. Agricultural soils are dominated by bacteria-feeding nematodes often contributing to 90% of the free-living nematodes. Organic amendments and green manure crops with C:N ratios ranging from 10-18:1 stimulate bacterial decomposition. As a consequence, bacterial numbers will increase and thus will bacteria-feeding nematodes. To fulfill their carbon need, bacteria-feeding nematodes have to consume a certain amount of bacterial biomass. Since the C:N ratio (8-12:1) of nematodes is higher than for bacteria (3-4:1), they take up more N than needed. The excessive N is excreted as NH4+, urea, peptides and amino acids providing nutrients for plants and soil organisms. Overall contribution of bacteria-feeding nematodes on nitrogen mineralization can range from 10 to more than 100 kg N per hectar and year. In comparison, fungal-feeding nematodes play a minor role in agricultural soils and only appear in higher numbers when cellulose-rich plant material is applied (e.g. straw). Predatory nematodes are sensitive to soil disturbance and usually occur at levels less than 2%. However, higher levels can be reached under long-term minimum tillage.
Beneficial nematodes describe a subgroup of free-living nematodes used for biological control of insects . Species of Steinernema and Heterorhabditis are nowadays widely used for insect control and represent a real success story in biological control. Beneficial nematodes can be applied with commercial sprayers as long as the nozzles are wide enough to let the nematodes pass. Following penetration of the host insect, beneficial nematodes release their symbiotic bacteria that kill and digest the insect.
Johannes Hallmann 03.02.2016