The symptoms of the disease are primarily caused by the lodging of the root system. The lateral roots and main roots are most affected, with the formation of numerous spherical root galls of varying sizes, resembling a string of beads. When cut open, these galls reveal a milky white appearance under a magnifying glass, or sometimes a pale yellow color, indicating the presence of nematodes. Above ground, the plant's growth is stunted, showing yellowing leaves, chlorosis, and overall dwarfing. Leaves may wilt late in the season, and the roots can rot, eventually leading to the entire plant drying out.
The primary pathogen responsible for this disease is the root-knot nematode, specifically species such as *Meloidogyne incognita* (southern root-knot nematode) and *M. javanica*. These nematodes are microscopic worms that parasitize the roots of plants, causing significant damage.
Root-knot nematodes overwinter in the soil in the form of eggs, oocysts, or second-stage larvae, remaining viable in infected plant residues. They can survive in the soil for 2–3 years when conditions are unfavorable. In the following growing season, the eggs hatch into larvae, which then spread through infected soil, diseased seedlings, water movement, and agricultural practices. Once they locate a host, the larvae invade the roots, stimulating cell multiplication and forming galls or tumors.
As the larvae develop, they go through several instars. By the third instar, they begin to mature, and by the fourth instar, they become adults capable of laying eggs. Males die after mating, while females remain within the galls, where their eggs hatch. The first-stage larvae appear as "8"-shaped, curled structures inside the eggshell. The second-stage larvae then enter the soil, either invading new hosts or overwintering.
The optimal temperature for *Meloidogyne incognita* is 28–32°C, while *M. northern* prefers 16–25°C. Temperatures above 40°C or below 5°C inhibit nematode activity. Soil moisture levels above or below 40% promote rapid development, and the rainy season aids in egg hatching. However, after four months of continuous flooding, nematode larvae die in large numbers, although eggs can still survive. After 22–23 months of flooding, both larvae and eggs are completely eliminated.
Soil conditions also play a role: high, dry, loose, and sandy soils with low salinity favor nematode activity and lead to higher infection rates. Continuous cropping increases the likelihood of infestation, especially in greenhouses or plastic tunnels where the same crop is grown repeatedly. This leads to weakened plant resistance and an accumulation of pathogens, worsening the damage.
To manage the disease, several control methods are recommended:
1. **Field sanitation**: Before harvest, thoroughly plow the field, remove all celery roots from the area, and burn them to reduce nematode populations. During the summer fallow period, plow and dry the soil every 10 days to kill surface nematodes using heat.
2. **Use disease-free seedlings**: Plant only 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 the surface with plastic film, and seal the greenhouse to use high-temperature anaerobic conditions to kill nematodes.
4. **Crop rotation**: Fields that have been heavily infested should be rotated with non-host crops like corn, wheat, or rice for more than three years to break the nematode life cycle.
5. **Soil disinfection**:
- **Steam sterilization**: During the fallow period, close the greenhouse and apply steam to raise the temperature to 100–132°C, ensuring 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, and cover with plastic for 5–7 days before planting.
- **Chemical control**: At planting time, apply 1.5% avermectin (1.5–2 kg per acre) mixed into the soil. For adult nematodes, use 50% phoxim emulsion diluted 800–1000 times and apply as a root drench.
These integrated strategies help reduce nematode populations and protect crops from future infestations.
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