According to the meteorological department, in late December, the average temperature of the middle and lower reaches of the Yangtze River and the main citrus producing areas south of it was 2-4 lower than that of the same period in normal years, of which Jiangxi, Zhejiang, Fujian, Guangdong, Guangxi, southeastern Hunan, and southeastern Hubei were lower than 4. In order to prevent low temperature and reduce citrus frost damage, the Department of Planting Industry Management, together with the Fruit Tree Expert Steering Group and the Citrus Industry Technology System, has formulated technical guidance on citrus prevention and response to low temperature frost damage.
1. Characteristics and typical symptoms of frost damage
Citrus frost damage often occurs in late winter and early spring in high-altitude areas or low-lying mountainous areas, and has the characteristics of suddenness, complex causes, serious harm and difficult prevention and control. The occurrence of frost damage is closely related to the minimum temperature and the duration of the freeze. Generally, lemon-3 will be frost damaged, sweet orange and grapefruit-5 will be frost damaged, Wenzhou mandarin -6 to -7 will be frost damaged, and flowers below 5 will be frost damaged. Mild frostbite was manifested as frostbite in a small number of late autumn shoots or terminal shoots, and 40% of the leaves fell off, which had little impact on tree strength and yield. Moderate frost damage is manifested in the fact that more than 70% of the leaves fall off due to freezing, and the 1 to 3-year-old branches freeze to death, and the yield is greatly reduced. Severe frost damage is manifested by the freezing of all leaves, the freezing of the main stem or main branches, and the difficulty of recovery after freezing or the freezing of the whole plant.
2. Implement preventive measures before frost damage occurs
Before the arrival of the low temperature and cold wave, build a windproof net in the direction of harmful winds, coat the trunk white and tie it to prevent freezing, add 1 part of quicklime and 7 parts of water and add a tablespoon of salt on a sunny day, stir evenly and paint the large branches of the trunk and the lower part of the canopy, or tie the straw on the main trunk. In areas where frost is frequent, cover can be taken to protect against freezing. Large trees can be covered with a layer of straw or weeds 10-20 cm thick under the canopy, and young trees can be covered with grass fans or shade nets. The orange seedlings of the nursery can be built on the seedlings with a small arch shed, and a grass fan or shade net can be laid on the small arch shed to prevent freezingIf necessary, cover the canopy with shade netting or non-woven fabric. Before freezing, irrigation and trunk (root neck) soil cultivation are advocated, the orchard is irrigated once 7-10 days before the low temperature comes, and the ditch soil or guest soil is cultivated at the base of the main trunk of the orange tree after irrigation and before frost damage occurs, and the height is generally about 40 cm. From late February to early March, when the weather warms up, the soil is removed. Before the cold spell, foliar sprays are applied 03% urea and 02% potassium dihydrogen phosphate can be sprayed twice continuously at an interval of about 7 days, and a steam-inhibiting heat preservation agent can also be sprayed to inhibit the transpiration of leaf moisture. Citrus that has not yet been picked should be picked as soon as possible according to the situation, and the fruit should be stored and stored.
3. Strengthen emergency protection in the process of frost damage
The following measures should be taken according to local conditions: First, smoke. On sunny and windless nights when temperatures can drop to -4, smoke in orangeries from 12 p.m. to 8 a.m. with suitable materials to help lift the cold air and reduce cold damage. The second is heating. The best citrus cultivated in the greenhouse facilities can be heated in the shed in a suitable form to increase the temperature in the shed. The third is to clear the snow in time. Knock off the snow on the trees and remove the snow on the greenhouse in time to prevent the snow from crushing the branches or collapsing the greenhouse.
Fourth, take post-freezing recovery measures in a timely manner
The first is to remove cedar soil in time. After heavy snow or frost, clean up the snow or frost on the trees and under the tree tray in time, and carry out shallow plowing of the whole garden or tree tray to loosen the soil (avoid hurting the roots), improve soil aeration, and promote new roots. The second is to strengthen tree management. Remove the leaves that have not recovered from frost wilting or have not fallen as soon as possible, and cut off the frozen shoots after the temperature rises in the following spring, leaving as much as possible the inside of the canopy and the lower branches. The large scissors (saws) should be smoothed and the wound should be protected with appropriate protective agents. For backbone branches that are directly exposed to strong sunlight, they should be painted white or tied with a shade net to protect from the sun. The spring and summer shoots that have grown to 30-40 cm after frost damage should be topped as soon as possible to promote branching and early restoration of the canopy. Timely thinning of excess flower branches on frozen plants to reduce nutrient consumption of trees. The third is to strengthen fertilizer and water management. After frost damage, water should be watered as soon as possible to replenish moisture to reduce the secondary damage caused by drought. After the temperature rises, in accordance with the principle of "diligent application and thin application", apply 2-3 times of high nitrogen and low potassium or other high-quality commercial water-soluble fertilizers in time. For frost-affected trees that have not fallen their leaves, use 01% urea and 03%—0.Foliar spray of 5% potassium dihydrogen phosphate with 2-3 consecutive sprays at intervals of 15-20 days. Fourth, strengthen pest control. After the frost pruning is completed, the dead branches and fallen leaves should be cleaned up in time for destruction. Thoroughly disinfect the whole garden with stone sulfur mixture, sodium terpineate or mineral oil + acaricide + fungicide + insecticide to prevent pests and diseases. In the process of new shoots, spray pesticides in time to prevent pests and diseases and protect new shoots.