Analysis of the Advantages and Disadvantages of Square Bud Grafting of Walnut in Xianyang and Its Technical Measures

Advantages and Disadvantages of Square Bud Grafting of Walnut in Xianyang City and Its Technical Measures

Xianyang City has a walnut planting area of 51,200 hm2, a preserved area of 30,400 hm2, and a preservation rate of 60%. The survival rate and preservation rate are low, the varieties are mixed and chaotic, and the quality is poor. The transformation of low-yield and low-efficiency forests has become an important measure for improving quality and efficiency in the local walnut industry. Starting with varieties, through square bud grafting technology and timely post-grafting management, the goal of high quality and high yield can be achieved.

1 Advantages

1.1 Saving scion wood

The collection of square bud grafting scions is relatively convenient. A single scion stick generally retains 5-6 buds, whereas branch grafting requires the use of one complete scion stick per grafting, so square bud grafting saves more scion wood.

1.2 High survival rate

The factors that most affect the survival rate of walnut grafting are temperature and rain. Grafting is generally chosen from mid-May to late June before the rainy season arrives. At this time, the temperature and humidity are suitable, and the rainy season can be avoided, so the survival rate can reach more than 95%.

1.3 Saving labor and time

For a skilled grafter, from taking the bud from the scion stick to grafting it onto the rootstock and wrapping it well takes only about 40 s.

2 Disadvantages

2.1 High temperature requirements

Square bud grafting has very high temperature requirements. If the temperature is too low, it is not conducive to the healing of the graft interface, and a long healing time makes mildew likely; if the temperature is too high, it may mildly burn the grafted bud or, in severe cases, burn the grafted bud to death, affecting the grafting survival rate [Jishan Huayao].

2.2 Scion sticks cannot be stored for a long time

Scions should be collected and grafted immediately; otherwise, attention should be paid to maintaining the moisture of the scion sticks to prevent water loss from affecting the grafting survival rate. The maximum storage time does not exceed 3 d.

3 Technical measures

3.1 Selection of mother trees for scion collection

Mother trees for scion collection should be selected from robust-growing, pest- and disease-free plants of superior varieties, and after selection they should be marked in a timely manner. The scion sticks used for square bud grafting should be current-year developmental branches with a high degree of lignification, and the collected grafted buds should be mature and plump.

3.2 Variety selection

In recent years, the main cultivars promoted in Xianyang City include Xiangling and Luguang bred by the Shandong Walnut Research Institute, Shaanhe No. 2 and Shaanhe No. 5 bred by the Shaanxi Walnut Research Institute, Liaohe No. 1, Liaohe No. 3 and Liaohe No. 4 bred by the Liaoning Economic Forest Research Institute, and Xilin No. 2 and Xilin No. 3 bred by the Northwest Forestry College, all of which are early-fruiting varieties with early fruiting and high yield.

3.3 Collection, preservation and transportation of scion sticks

3.3.1 Collection. The collection time is generally from mid-May to late June. After collecting in the early morning, the compound leaves should be cut off immediately (leaving the petiole 2-3 cm long). The scion sticks should be grafted as soon as possible after collection to prevent water loss from affecting the grafting survival rate.

Select current-year new developmental branches with a high degree of lignification and mature, plump buds. After the scions are cut, remove the compound leaves, retain 5-6 buds on each scion branch, bundle every 20 or 30 sticks into one bundle, label the variety, and place in a cool place for later use. When bundling, prevent the petioles from rubbing and damaging the young tender epidermis.

3.3.2 Preservation and transportation. The collected scions should be stored for no more than 3 d. Before grafting, cut the top ends of the scions crosswise and bundle them, stand them upright and soak them in a special walnut growth regulator solution to a soaking depth of 5 cm, and use them once a water film appears on the upper cut surface of the scions. For transporting scions, if the distance is short, wrap the scions in wet burlap and transport them quickly; if the distance is long, wrap them well with plastic film, pay attention to ventilation, and when arriving at the grafting site, open the film promptly and place them in a moist, cool place.

3.4 Rootstock selection

First, low-yield forests. Select seedling trees in low-yield walnut forests, surviving seedlings from rootstocks, low-quality trees, and iron walnut and Mian walnut mixed in walnut forests for comprehensive low-yield transformation. Second, seedling trees. Graft seedling trees that are old, of unclear variety, and low-yielding. Third, nursery stock. Select robust-growing, pest- and disease-free nursery seedling stock with 2-year roots and 1-year shoots for grafting.

3.5 Grafting

Square bud grafting must be carried out during the growing season, and the rootstock is required to be a one- to two-year-old branch or a current-year new shoot. When grafting, the rootstock cut is required to be the same size as the square bud piece. First, take the bud. Use a square knife to cut around the grafted bud so that the bark separates from the branch. Use a blade to pry up the bark at the interface and take out the bud piece [4]. Second, open the rootstock. At a smooth, semi-lignified spot on the current-year seedling, use the same square knife to remove a piece of rootstock bark of the same size, then place the bud piece into this interface. Third, wrap. Use special plastic strips to tie the bud tightly, ensuring close contact between the bud and the rootstock and avoiding air leakage.

3.6 Post-grafting management

First, watering. For seedlings grafted in the field, water promptly after grafting to ensure the growth of the walnut seedlings. Second, cutting the rootstock. For bud-grafted seedlings, in order to promote healing, the top is generally cut off. Cut the rootstock leaving 2 compound leaves above the graft interface. Third, bud removal. Promptly remove seedling buds sprouting from the rootstock to maintain sufficient water and nutrients for the grafted bud to sprout and grow and to improve the grafting survival rate. Fourth, weeding. Promptly clear weeds and crop seedlings within 1.5 m around the tree trunk to facilitate seedling growth and reduce the breeding of pests and diseases, and be careful not to break off the young shoots sprouting from the scion. Fifth, fertilizing.

After the grafted bud grows to 30 cm, spray the leaves once every 10 d or so with phosphorus-magnesium essence, phosphorus-potassium essence, or sodium humate. In autumn, appropriately increase phosphorus and potassium fertilizer to prevent excessive vegetative growth of the seedlings. Sixth, loosening the binding. When the grafted bud grows to 15-20 cm (when the bud piece has healed), loosen the binding. When loosening, use a knife to make a vertical cut on the opposite side of the bud and gently remove the film to untie it.

3.7 Pest and disease control

On the basis of sufficient investigation, it is determined that the common walnut diseases in the Xianyang area include anthracnose, black spot, rot, and brown spot, and the insect pests include the mealybug moth, cloud-patterned longhorn beetle, yellow slug moth, and aromatic carpenter moth. In pest and disease control, fully implement the plant protection policy of "prevention first, integrated control," give priority to agricultural control and ecological regulation measures, fully utilize the natural control role of natural enemies, and mostly adopt manual and physical control methods. In addition, chemical control should use low-toxicity, high-efficiency, low-residue chemical pesticides.