The symptoms of the disease are primarily caused by the infestation of the root system. The lateral roots and primary roots are most affected, with numerous spherical galls or nodules forming on the roots, resembling a string of beads. When cut open, these galls may reveal a milky white fluid visible under magnification, or in some cases, pale yellow nematodes can be seen. Above-ground plant growth is significantly hindered, leading to stunted development, yellowing leaves (chlorosis), dwarfing, and premature wilting of older leaves. In severe cases, root rot occurs, ultimately causing the entire plant to wither.
The pathogen responsible for this condition is the root-knot nematode, specifically species such as *Meloidogyne incognita* (southern root-knot nematode) and *Meloidogyne javanica* (Javanese root-knot nematode). These microscopic pests are parasitic and cause significant damage to a wide range of crops.
Root-knot nematodes overwinter in the soil in the form of eggs, oocysts, or second-stage larvae, surviving in infected plant residues. They can remain active in protected environments during winter and can persist in the soil for 2–3 years if conditions are unfavorable. When conditions become favorable, the overwintering eggs hatch into larvae, which spread through contaminated soil, infected seedlings, water movement, and agricultural practices. Once inside the host plant, the larvae invade the roots and stimulate abnormal cell growth, resulting in the formation of galls or tumors.
As the larvae develop, they progress through several instar stages. By the third instar, they begin to differentiate, and by the fourth instar, they mature into adults capable of laying eggs. Males typically die after mating, while females remain within the galls, laying eggs that hatch into new larvae. The first-stage larvae appear in an "8"-shaped curled position inside the eggshell. The second-stage larvae then move into the soil, either invading the host or overwintering.
The optimal temperature for *Meloidogyne incognita* is between 28–32°C, while the northern nematode prefers 16–25°C. Java nematodes fall somewhere in between. Nematode activity is inhibited at temperatures above 40°C or below 5°C. Soil moisture also plays a key role: nematodes thrive when moisture levels are above or below 40%, and the rainy season promotes egg hatching. However, prolonged flooding for more than four months can kill many larvae, although eggs may still survive. After 22–23 months of continuous flooding, both larvae and eggs will eventually die.
Soil conditions influence nematode activity. Sandy, well-drained, low-salinity soils are ideal for nematode survival, and heavy infestations are common in areas with poor crop rotation. Continuous planting of the same crop, especially in greenhouses or plastic tunnels, leads to a decline in plant resistance and an accumulation of pathogens, worsening the problem.
To manage root-knot nematode infestations, several control methods can be applied:
1. **Field sanitation**: Before harvest, thoroughly plow the land and remove all root remnants from the field. Burn the removed roots to reduce nematode populations. During summer, plow and dry the soil every 10 days to use high temperatures to kill surface nematodes.
2. **Use of disease-free soil**: Plant seedlings in clean, disease-free soil to prevent initial infestation.
3. **Water management**: After harvesting, drain the field for 2–3 months. Dig trenches, add quicklime, cover with plastic film, and seal the area to create a high-temperature, oxygen-deficient environment that kills nematodes.
4. **Crop rotation**: Rotate crops in fields that have been heavily infested. For example, plant corn, wheat, or rice for more than three years before returning to the original crop.
5. **Soil disinfection**:
- **Steam sterilization**: During the off-season, close the greenhouse and apply steam to raise the temperature to 100–132°C, ensuring that the top 25 cm of soil reaches 60°C for over 30 minutes.
- **Fumigation**: Apply methyl bromide, chloropicrin, or DD mixtures at a rate of more than 10 kg per acre. Mix into the soil, cover with plastic, and leave it sealed for 5–7 days before replanting.
- **Chemical control**: At planting time, apply 1.5% avermectin at a rate of 5 kg per acre. Alternatively, use products like Miller, killing line Dan, or Ke line Dan at 1.5–2 kg per acre. During the adult stage, apply 50% phoxim emulsion diluted 800–1000 times and pour it directly onto the roots.
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