Are Soil Nematodes Insects? Full Analysis of Their Harm and Control Methods
Nematodes in soil are abundant in number and diverse in species. Soil nematodes are widely distributed in all types of soil, and the number of nematodes varies with soil depth, with the largest number of soil nematodes distributed within the 10-20cm soil layer. As one of the number one killers among common soil-borne diseases, nematodes cause enormous harm to agriculture. With a body length of only 0.3-6mm, they are nearly “invisible”, yet they are related to more than 50% of soil diseases. Burrowing nematodes, root-knot nematodes, cyst nematodes, stem nematodes……the wide variety of nematodes causes a 5%-20% reduction in yield for agricultural industries in related fields, with localized yield reductions reaching 60% or even total crop failure. Are soil nematodes insects? What harm do they cause? How to kill and control them?
1. The harm of nematode disease
“The biggest characteristic of nematode disease is that it has no characteristics.” said Wang Xinrong, professor at the Plant Nematology Laboratory of the College of Agriculture, South China Agricultural University. Saying it has no characteristics is actually because insufficient attention causes nematode disease to often be confused with other pathogenic diseases, and without the right remedy it is naturally impossible to fully prevent. The neglect of nematode disease and improper response methods have brought severe challenges to the agricultural economy.
The harm of plant nematodes to plants comes on one hand from the mechanical damage to plant roots, stems and leaves caused by the mouthparts during parasitism; these physical attack wounds promote infection of the plant body by pathogenic organisms such as fungi, bacteria and viruses, making it a “susceptible host” for many plant diseases. On the other hand, the “biochemical skills” of nematodes also make plants suffer greatly; the digestive fluids and various enzymes secreted during feeding enter the plant's circulatory system, disrupting and destroying the plant's normal metabolic functions and affecting its growth and development.
2. Symptoms of nematode disease are not obvious
Most plant nematode diseases have no obvious specific symptoms, appearing in the field similar to nutrient deficiency, mainly as weak plant growth, stunted and yellowing, and reduced yield. But if you observe carefully, the “ghost” of nematodes has nowhere to hide. Plant-parasitic nematodes have limited activity ability and weak independent migration ability during parasitism; without interference from human factors, their maximum radius of influence in one year is only 1 meter. Plant communities parasitized by nematodes are distributed in the field in small patches and wither and yellow, which is an important manifestation distinguishing them from plant nutrient deficiency and soil water and fertilizer deficiency, since the latter's wilting and yellowing is distributed in whole patches or whole rows and columns.
Nematodes can not only harm plant roots, but also ruin stems, flowers and seeds awaiting harvest. Typical plant nematode disease symptom types are divided into aboveground and underground parts. Among them, the aboveground part mainly manifests as leaf distortion (narcissus, strawberry, rice, etc.); stem swelling, distortion and rot (coconut, potato, sweet potato, etc.); flower discoloration and rot, shortened inflorescence, and increased number of sterile florets (chrysanthemum, etc.); seed gall formation, seeds failing to form starch (wheat, etc.), and even whole plant death (pine, etc.). The underground part symptoms mainly appear as root knots (cucumber, peanut, citrus, etc.) and root necrosis and rot (banana, etc.). These seemingly common disease symptoms often make growers lower their guard, thinking that spraying pesticide is enough, but if the nematodes are not killed, no amount of investment is of any use.
3. How to simply distinguish whether it is nematode disease?
Taking the most frequently occurring root-knot nematode disease as an example, the plant roots and underground organs harmed by parasitic nematodes form tumors, called root knots, distributed irregularly. On the root knots, adventitious fibrous roots can also grow, and after the fibrous roots are infected they form root knots again; with repeated infection, a tangled mass of fibrous roots forms. The aboveground part manifests as stunted growth, yellowing and other decline symptoms. If you cut open a root knot and take a sample, a large number of nematodes can be observed under a microscope. Compared with root knots, the root nodules commonly seen on leguminous plants are caused by symbiosis between rhizobia and the plant; root nodules have a regular shape, often grow on one side, and when cut open exude a red-green bacterial fluid.
4. Early prevention is key
Wang Xinrong said that once nematode disease truly reaches the late stage of onset, it is very difficult to control, so prevention should be the main approach. “Water contaminated by nematode eggs and farm tools can both become transmission vectors.” At the same time, non-standard daily planting management can also become an “accomplice”. Unintentionally bringing soil, seedlings and farm tools from disease-infected areas to healthy plants can cause the disease to spread rapidly. Therefore, before sowing crops, the land should be treated and disinfected by sun-drying, fumigation, closing the greenhouse, and mixing in tea seed cake and lime; permafrost in Northeast China also has a similar effect. If conditions permit, after disinfection add microbial nematode-killing organic fertilizer. The most critical thing is still to strictly control water sources and farm tools, while ensuring that incoming seedlings and seeds are disease-resistant and pest-free.
5. How to kill soil nematodes? Nematode control methods
Plant quarantine: prevent invasion of foreign nematodes;
Agricultural control: disease-resistant varieties, disease-resistant rootstocks, crop rotation, soil improvement;
Physical control: high-temperature greenhouse sealing, flooding treatment, calcium cyanamide, ultrasound, radiation;
Biological control: plant-derived, microbial-derived, predatory nematodes, insects, mites;
Chemical control: the most effective method, divided into fumigant and non-fumigant pesticides.
Among these, chemical control is still the most important control method, with a usage rate of more than 80% of all control methods. The commonly used avermectin type is a contact-type nematicide with fast nematode-killing speed and low dosage, but it is easily adsorbed by soil aggregates in the soil and rapidly degraded by microorganisms. The advantage is low pollution, while the disadvantage is a short duration of nematode control, and it can only kill nematodes in the rhizosphere soil, having no effect on those that have invaded plant roots. Thiazophos, in addition to contact action, can also be systemically conducted into the plant body to kill nematodes in the root system, but an excessive dose can cause root burn. Generally, compounding avermectin and thiazophos can complement the efficacy while avoiding nematode resistance. Chitin, which can induce crops to produce chitinase, is also often compounded with the two; chitinase can decompose the chitooligosaccharides in nematodes and eggs, dissolving the nematode wall and eggshell. Since the life cycle of nematodes is about 20 days, statistics on pesticide efficacy must pass through one generation before the value is accurate.
In addition to chemical control, environmentally friendly biological control based mainly on microbial nematicides has also become a hot topic year by year. Safe, growth-promoting, highly sustainable and also improving soil, biological agents have obvious advantages, but high cost, incompatibility with fungicides, and high technical requirements are also testing the pioneers within the industry.