Southern root-knot nematode damage mainly occurs in the roots of plants, with lateral roots and whisker roots being the most vulnerable, and aboveground parts can also show obvious symptoms. Root hazards, one is direct mechanical damage, destroying the host epidermal cells; the other is to stab the host with a kissing needle, secrete saliva, destroy the normal metabolic function of the host cells and produce lesions, so that the roots are deformed or the internal tissues of the plant are damaged.
After the root system is damaged, nodular root knots of different sizes and shapes are formed, some of which appear beaded, white at first, soft in texture, and later become light brown, with cracks on the surface.
After the onset of the disease, the ability of the root system to absorb and transport nutrients and water decreases, forming weak seedlings, affecting the yield; the aboveground part of the severely diseased plant shows malnutrition, the plant size is different, the plant is not neat, the dwarf, thin, slow growth, the noon wilt, morning and evening recovery, the severe ones the whole plant dies; the leaves are small, the leaf color becomes light, yellow, and resembles deficiency; the flowers and fruits fall, and the fruits are small and deformed.
The complete life history of root-knot nematodes is required to go through three stages: egg-larvae-adult. Overwintering in the soil in eggs or other insect forms in the field can survive for up to 3 years without the presence of host plants in the soil.
When the temperature reaches more than 10 °C, the eggs can hatch, and the larvae are mostly active at 5 to 30 cm in the soil layer. Root-knot nematodes occur in greenhouses for about 10 generations a year, and each female lays 300 to 800 eggs. At a temperature of 25 to 30 °C, a generation can be completed in 25 days, suitable for soil moisture of 40% to 70%, suitable soil pH PH4 ~ 8.
Soil temperatures above 40 °C or below 10 °C are rarely active, and lethal temperatures are 55 °C for 10 minutes. The second-instar larvae are the infestation age of the root-knot nematode, usually invaded by the root tip of the plant, and move between cells by squeezing the gap between the cell walls, completing the infection of the plant, and stimulating the host cells to accelerate division, so that the injured site forms a nodule or root knot.
The larvae are mobile intraparasites, and the females are immobilized. The puncture of the second-instar larvae invades the roots of the host plant, forming feeding sites near the vascular bundle, and the cells around their head region fuse to form giant cells. After 3 molts, she develops into a female and is fixed in the root for feeding.
Females usually perform parthenogenesis. According to the test, it takes 30 days for southern root-knot nematodes to complete a life history on tobacco at 28 °C and 57 to 59 days on tomatoes at 20 °C.
It is also reported that the nematode also interacts with a variety of Fusarium species to form a compound disease of cotton, tobacco, tomato and alfalfa, and also interacts with parasitic mold, Phytophthora, Pliatus, melon and fruit rot mold, forming a compound disease and aggravating crop losses.
Female perineum pattern, with a high, square dorsal arch, a clear rotational pattern in the caudal end area, smooth to a wavy or "zigzag" shape, no obvious lateral lines, but in the lateral areas there are fractures and forks, sometimes curved towards the pudendal gate.
The most important feature of females is that the needle is curved towards the back. The anterior half of the needle cone is cylindrical, the posterior half is conical, and the posterior part of the needle stem is slightly wider. The base ball of the needle is constricted at the junction of the needle stem, the anterior part is serrated and transversely elongated (some specimens, the base ball is obviously serrated, it seems to have 2 balls), and the length of the needle is 15 to 17 μm.
The male's head area and mouth needle The lip is discular and large, higher than the middle lip. Viewed from the side, the lip disc is concave to flat, and the crown of the head is high. The head area often has 2 to 3 incomplete rings, which may also be smooth, and the head area and the insect body are not significantly constricted. The top of the needle cone is slightly wider than the middle, the tip is blunt and rounded, leaf-shaped, needle dry cylindrical, and often narrow near the basal ball.
The basal sphere is large, rounded to ovoid, sometimes serrated at the front, and constricted at the junction with the needle stem. The needle is 23 to 25 μm long, and the distance from the base of the needle to the opening of the dorsal esophageal gland is 2 to 4 μm.
The female of the northern root-knot nematode is pear-shaped or bag-shaped, the perineal pattern is rounded to oblate ovate, the dorsal arch is low and flat, the lateral line is not obvious, the caudal end area is often notched, and the excretory hole is located at a length of 1.5 needles from the end of the head. The male is linear, the crown is high and narrow, the head area has a clear boundary with the body, the lateral area has 4 lateral lines, the cephalic receptor is long and cracky, and the dorsal esophageal gland opening to the bottom of the needle base ball is 4 to 6 μm long. The 2nd instar larvae are linear, with an average length of 413 μm for Small Species 1 and 475 μm for Small Species 2. It can reproduce on disease-resistant tobacco, peppers, and peanuts, but does not infect watermelon and cotton.
The nematodes parasitize the roots, causing tumors. In the plot with large insect population density, many tumors are formed at the root, and when it is severe, the root rots and falls off, resulting in yellowing of the leaves, growth stopping, and the plant wilting and dying. In particular, the false planting beds and seedbeds with long creeping stems are more serious.
The hatching larvae invade the tissues from the tip of the root and absorb nutrients to live in a camp. Under suitable conditions, several generations occur in each year. However, it is not found to be parasitic in sweet potatoes, watermelons, maize, wheat and cotton. Therefore, these crops can be used for ecological control.
Route of transmission: Root-knot nematodes have a small range of activity in the soil and do not move more than 1 meter in a year. Therefore, the sources of initial infection are mainly diseased soil, disease seedlings and irrigation water. The long-distance movement and spread of nematodes is usually accomplished with the help of flowing water, wind, relocation of diseased soil and diseased residues and soils contaminated by agricultural machinery, diseased seeds and other nutritional materials, and various agricultural activities.
Pathogenesis conditions: soil moisture is an important condition affecting hatching and reproduction, the rainy season is conducive to the hatching and infestation of nematodes, but in dry or overly wet soils, its activity is inhibited. Suitable soil pH4-8, high terrain, loose soil texture, low salinity conditions are suitable for nematode activities, favorable disease, generally sandy soil is heavier than clay, continuous cropping disease is heavy
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