Symptom Identification and Control Methods of Carrot Root-Knot Nematode Disease
Carrot, also known as red radish, clove radish, bottle gourd root, and also called carrot, red beetroot, yellow radish, etc., is an umbelliferous biennial herbaceous plant, with its fleshy root eaten as a vegetable. Every 100 grams of fresh carrot contains 1.67-12.1 mg of carotene, 5-7 times higher than that of tomato. After eating, it is digested and decomposed by the stomach and intestines into vitamin A, which can prevent night blindness and respiratory diseases. It can be stir-fried, boiled, eaten raw, pickled in sauce, salted, etc., and is resistant to storage. It is distributed all over the world, and is cultivated in both northern and southern China, with its output ranking second among root vegetables. It can fight cancer and is known as an underground “mini ginseng”.
Requirements of carrot for the growing environment
Temperature: Carrot is a semi-cold-resistant vegetable, and its temperature requirements differ between the vegetative growth period and the reproductive growth period. Seeds can germinate at 4-6℃ and begin to grow at 8℃; at 18-20℃, seedlings can emerge in about 10 days. Seedlings can endure short-term low temperatures of -3 to -4℃, and can also grow normally under high temperatures above 27℃. The optimum temperature for the germination stage of carrot is 20-25℃; the suitable temperature for the vigorous leaf growth stage is 20-25℃ during the day and 15-18℃ at night; for the fleshy root swelling stage, 20-22℃ is suitable, and growth stops below 3℃. Carrot root stocks can be planted when the soil temperature reaches 8-10℃. The suitable air temperature for carrot flowering, pollination, and seed setting is 22-28℃ during the day and 15-20℃ at night. When the daytime temperature exceeds 35℃, pollination and seed setting cannot occur.
Light: Carrot is a light-loving, long-day plant, especially during the vegetative growth period, requiring light of medium intensity or above.
Water: Keep the soil moist during sowing to promote seed germination and uniform emergence. During the seedling stage and vigorous leaf growth stage, alternate between dry and wet conditions, ensuring normal aboveground growth while increasing soil aeration to promote good taproot development. The fleshy root swelling stage is the period with the greatest water demand; water evenly to meet the needs of fleshy root swelling, and also prevent drastic changes in irrigation amount from causing root cracking. Carrots generally stop being watered 10-15 days before harvesting the fleshy roots, reducing cracking and facilitating storage and transportation.
Soil: Carrot prefers sandy loam and loam with deep soil layers, loose texture, good drainage, and high porosity. In poorly aerated, heavy clay soil, the fleshy root is lighter in color, has many fibrous roots, is prone to developing tumors, and is of poor quality; in low-lying areas with poor drainage, the fleshy root is prone to cracking, often causing rot and an increase in forked roots. Carrot has a wide adaptability to soil pH, growing well in soil with a pH of 5-8, but growing poorly in soil with a pH below 5.
Introduction and control of carrot root-knot nematode disease
What is carrot root-knot nematode disease?
Called peanut root-knot nematode. Among them, the northern root-knot nematode (Melotidogyen hapla Chitwood) is distributed north of the isotherm with an average July temperature of 26.7℃. The perineal pattern of the northern root-knot nematode larva consists of smooth striae, nearly circular, with some striae extending in one or both directions to break the wing-like shape, and punctations are commonly seen near the tail end; the larva has a blunt and bifurcated tail.
Disease diagnosis of carrot root-knot nematode disease:
It damages the underground roots. The aboveground leaves of diseased plants turn yellow, the outer leaves of the plant extend horizontally, and growth is weak. When the fleshy root is dug out, many swollen semicircular galls can be seen scattered on the taproot, mostly on the upper half of the galls, initially white, later turning brown and hardened. On the fibrous roots, nodular irregular small galls form, or some are strung together in a bead-like shape. The affected main root often forks into branched roots. The pathogenic nematodes are mostly the southern root-knot nematode and the northern root-knot nematode, and the peanut root-knot nematode may also be mixed in. The pathogenic nematodes are sexually dimorphic; the male adult is linear, slightly rounded at the tail end, colorless and transparent. The female adult, apart from the short head and neck, has a body swollen to nearly spherical, pear-shaped, milky white; the area near the vulva at the posterior end of the body is the perineal region, and special annular ridges form on its cuticle, called the perineal pattern, which is an important basis for species identification. The larva is small and linear. The female adult lays eggs in an egg sac formed by gelatinous secretion from the vulva, with each female laying 300-500 eggs. Females are mostly buried within the host tissue.
Occurrence pattern of carrot root-knot nematode disease:
The pathogenic nematode overwinters in the soil as eggs with diseased residues, or as larvae in the soil, or as larvae and female adults inside the host. The following year, overwintering eggs hatch into second-stage larvae; overwintering second-stage larvae resume activity, move and gather at the tips of host roots, invade from the young root tips, then migrate inward and upward, settle and feed in the root cell elongation zone until developing into adults. While piercing and feeding with the stylet, the stylet injects secretions, stimulating the proliferation and enlargement of host cells near the feeding site. Several multinucleate giant cells form around the larval head, serving as nutrient cells that transfer nutrients within the invaded root to supply food for the nematode, and at the same time causing galls, commonly called root knots, to appear on the diseased roots. Nematodes mostly live and move within the 5-30 cm soil layer, and completing one generation takes about 25-30 days, with 4-5 generations occurring per year. In the soil, apart from the nematode (second-stage infective larva) moving short distances by wriggling, it is mainly spread by diseased soil, diseased seedlings, and irrigation water. Nematodes begin activity when the soil is above 10-15℃, and the optimum temperature for growth and development is 25-30℃. Soil that is too dry or too wet is unfavorable to nematodes; soil moisture suitable for vegetable growth is also suitable for nematode activity. The suitable pH is 4-8; well-aerated sandy soil is favorable for nematode survival and activity, so nematode damage is often more severe in sandy soil than in clay soil.
Control methods for carrot root-knot nematode disease:
1) For seedling raising and transplanting, use disease-free soil for seedling cultivation. Cultivate disease-free strong seedlings. Strictly remove diseased seedlings when planting.
2) Severely affected plots should be rotated for 2-3 years with grain crops or asparagus, and rice-upland rotation is even more effective. Planting disease-resistant vegetables such as Welsh onion, garlic, Chinese chives, and chili in severely diseased fields can significantly reduce the number of nematodes in the soil and lessen damage to the following crop.
3) Where conditions permit, the summer fallow period can be used to plow and sun the field, plowing once every 7-10 days for 2-3 consecutive times, which can kill nematodes in the surface layer. Flooding diseased fields for 1 month can also kill most nematodes in the surface layer.
4) For chemical control, 50% chuxiante powder can be used, with 2.3-3.8 kg of the agent per 1000 sq m mixed with 30-37.5 kg of fine soil to make medicated soil, scattered in the sowing furrow, then sowing and covering with soil. Alternatively, at sowing or planting, apply 10% limanku granules in holes, using 5 kg per 667 sq m.
5) For scattered occurrence, in the early stage of disease, drench the roots of diseased plants; the agent can be 50% phoxim emulsifiable concentrate at 1500 times dilution, or 80% dichlorvos emulsifiable concentrate at 1500 times dilution, with 250-500 ml of solution per plant. Generally, watering before drenching can improve efficacy.