Why do fruit trees need boron and zinc fertilizer in spring

What symptom does fruit tree lack boron zinc have? How to treat? For fruit farmers, fertilization is very important. If it is not done well, it will lead to many symptoms and greatly reduce the quality of fruit.
Nutritional deficiency symptoms
Boron deficiency usually occurs in early spring. At first, the flower and leaf buds delayed germination, then withered, and the branches also withered. By identifying the bark cambium, it can be concluded that the cambium caused by boron deficiency is white compared with that caused by overwintering branches.
Symptoms of boron deficiency fruit
At the top of the new shoots, the apple trees lack boron and present whorled leaflets, which sometimes form leaf clusters. With the increase of boron deficiency, the leaves become smaller and smaller from the bottom to the top, and the trunk and bark of some varieties become necrotic. When the pear tree is lack of boron, it forms the top bud very early, stops growing, and has no round leaflet symptom. The most common flowers wilt before they open, some of the short branches of the seated flower buds start to be normal, but gradually wilt after flowering.
The small leaves of zinc deficient fruit trees are often called "lobular disease" or "clustered leaf disease". The new shoots are seriously shortened, which makes the leaves cluster, which is called "cluster leaf". In the case of "small leaves", the leaf spacing is normal, but the leaves become smaller, sometimes the whole tree new shoots come on, sometimes local.
Symptoms of zinc deficiency in Fuji apple
The obvious symptom of zinc deficiency is withered leaves. In early spring, there are flower spots on the top of new shoots. Other parts of the new shoots are likely to have no leaves and look like light branches. Later, new shoots appear at the lower part of the injured branches. When the leaves open, they grow normally. Later, they gradually narrow and grow longer, and there are also flower spots. In the first year of the injured branches, there may be withered shoots, few flower buds, small fruits and deformities.
Methods of boron and zinc application
After the diagnosis of boron and zinc deficiency in fruit trees, the method of furrow application and hole application was used once a year. In spring, the boron and zinc fertilizer were applied to 50 grams of each tree which was put into production. First dig a 30 cm wide circular semicircle ditch around the tree crown, apply boron and zinc fertilizer and organic matter in the ditch at the same time, water after application, and cover the soil after water drying.
During the growth period of fruit trees, it was found that the fertilizer could be applied at any time when boron and zinc were deficient. Stop one year after two years of application, and stop applying fertilizer after normal growth. At the flowering stage of fruit trees, the quality and yield of fruit trees can be improved by spraying a picture of trace elements on the flowers and leaves when the flowers are 50%.