How to prevent and control root-knot nematode disease when growing citrus?

Citrus root-knot nematode disease damages the citrus root system, causing root galls of various sizes to form on the roots, and fibrous roots to clump together into a cake-like mass, which hinders the root system's ability to absorb nutrients and reduces the number of absorbing roots. When citrus is first infected and damaged by root-knot nematodes, the canopy does not show obvious signs of decline. As the root-knot nematodes continue to multiply, the damaged absorbing roots gradually increase, and only then does the canopy show poorer and weaker growth than healthy trees, that is, fewer flushes, smaller leaves, curled and yellowed leaf margins, lack of luster, more flowers but fewer fruits set, and low yield. In severe cases, branches wither and leaves drop, and seriously affected trees may die completely. To prevent and control citrus root-knot nematode disease, the following measures must be carried out well.

1. Strictly implement quarantine. Quarantine should be strengthened when purchasing nursery stock. It is strictly forbidden to purchase nursery stock that may be infected with nematodes from diseased areas already harmed by root-knot nematode disease. Protection should be strengthened in disease-free areas, and strict precautions taken to prevent soil, fertilizer, water, farming tools and other nematode-carrying materials from diseased areas from being introduced into disease-free areas.

2. Select and breed disease-resistant rootstocks. Breeding rootstocks resistant to citrus root-knot nematode disease is currently a relatively effective way to develop citrus planting in diseased areas. According to local cultivation conditions, through testing various suitable rootstocks, cultivate and screen out rootstocks with strong resistance to citrus root-knot nematode disease. Through rootstock trials on citrus root-knot nematode disease, Yangcun Citrus Farm screened out that the Jiubing?{ rootstock is resistant to citrus root-knot nematode disease, while red lemon and sour orange type rootstocks are severely susceptible rootstocks.

3. Prune out damaged root masses. In winter, in combination with loosening the soil and exposing roots, suppressing soil moisture, and promoting flower bud differentiation, dig deeply into the soil near the roots under the tree canopy of diseased plants. From the excavated root system damaged by root-knot nematode disease, cut off the fibrous root masses with root galls and root knots damaged by root-knot nematode disease, and retain healthy roots without root galls or root knots, as well as horizontal roots and relatively thick roots. Be careful when digging soil, and never hoe or dig carelessly. Try not to damage the cortex of the main and lateral roots, and only prune the damaged roots. Even a small healthy root should be retained. The principle for digging soil is that it should be deep below the canopy drip line and gradually shallower near the trunk base. For backfilling, it is best not to use the original excavated soil, and lime should be spread. The amount of lime spread should preferably be 1%--2% of the amount of borrowed soil needed. The pruned diseased roots should be promptly removed from the orchard and burned in a centralized manner.

4. Strengthen fertilizer and water management. For diseased trees, apply more organic fertilizer, 15--25 kg per tree, and mix organic fertilizer and borrowed soil at a 1:1 ratio for soil dressing and fertilizing. This is best done in winter when digging soil and pruning diseased roots, preferably during the Great Cold period. At the same time, when expanding holes and improving soil in June, apply organic fertilizer deeply once more. Irrigation should follow conventional cultivation management procedures, and other fertilizer and water management measures should be strengthened to enhance tree vigor and achieve the goal of reducing the degree of damage from this disease.

5. Chemical control. When digging soil and pruning diseased roots, apply chemicals evenly mixed into the backfill soil. Or dig a ring-shaped trench 15 cm deep and 30 cm wide below the canopy drip line, irrigate, then apply chemicals (preferably mixed with fertilizer that will not chemically react with the chemicals) and cover with soil. Or open a hole every 20--30 cm within the tree canopy area, inject or place the chemicals at a depth of 15--20 cm, and promptly cover with soil and irrigate after application. The safe interval period for chemicals used to control citrus root nematode disease is three months. For safety, chemical application should be carried out in winter or in early spring from February to early March. The following chemicals may be selected for chemical control (application amount per tree). 1) 80% dibromochloropropane 250-fold solution, 10--15 kg. 2) 10% dibromochloropropane granules, 200 g. 3) 3% Miral granules, 200 g. 4) 10% cadusafos granules, 200 g. 5) 15% aldicarb granules, 100 g. 6) 3% carbofuran granules, 150--300 g. 7) 3% isofenphos-methyl granules, 400 g. 8) 10% isazofos granules, 100 g for chemical control. Note that when spreading chemicals in trenches and holes, they must be applied evenly and in equal amounts.