What are the control methods for nutrient deficiency diseases in citrus

Zinc deficiency causes mottled leaf disease. Citrus mottled leaf disease is a common disease, with the typical symptom being mottled small leaves. When new leaves begin to turn green, the main veins and lateral veins appear markedly green, while the remaining tissue is light green to yellowish white and glossy. In severe cases, only the area near the main vein or thick lateral veins is green, and the new leaves are narrow, small and mottled, mostly upright, with short internodes and clustered branches. Mottled disease is caused by zinc deficiency. Many soils are deficient in zinc, such as soils formed from granite, gneiss and serpentinite, which contain little zinc; acidic sandy soils lose zinc easily; in alkaline soils zinc readily forms insoluble compounds, thus causing zinc deficiency. In addition, soil that is too dry or too wet, low humus content or excessive application of organic matter, which hinders contact between roots and soil, as well as heavy pruning, all make zinc deficiency likely to occur. Foliar fertilization can achieve fairly good results in preventing and controlling zinc deficiency. It is advisable to spray 0.2%-0.3% zinc sulfate solution (with an equal amount of lime) once during the young shoot stage, and spray 2-3 times where zinc deficiency is severe.

Copper deficiency or boron deficiency causes dieback disease. In the early stage of copper deficiency in citrus, new shoots are long, soft and slightly curved, the leaves are large and dark green, and fruiting is low, so it is easily mistaken for excessive growth. In severe cases, the tips of the shoots turn tea-brown and die, short weak clustered branches appear, and they dry out and drop early and exude gum; when the branches mature, the wounds are reddish brown, and parts of the leaves are reddish brown. Citrus needs very little copper, and ordinary spraying with Bordeaux mixture is already sufficient. However, copper deficiency easily occurs in peat soil, acidic sandy soil, soil with high organic matter content, as well as on abandoned grazing land and abandoned cemetery land; excessive application of barnyard manure and phosphate fertilizer also causes copper deficiency; newly reclaimed citrus orchards that have not been sprayed with Bordeaux mixture or other copper preparations for several years are prone to copper deficiency. Boron deficiency in citrus: the growing points of the buds die continuously, forming clustered buds; the leaf veins swell, become corky or split; the fruit is small and deformed, with thick hard rind, and brown resinous deposits in both the fruit core and the spongy layer. Boron is easily lost from acidic soil, hence boron deficiency. In soil parent materials such as granite, gneiss and schist, the boron content is extremely low; alkaline soil has poor solubility, so it is also deficient in boron. Excessive application of phosphate fertilizer or lime, or soil drought, all affect the availability of boron and the absorption of boron by roots; sour orange has a weak ability to absorb boron, so citrus using sour orange as rootstock is generally deficient in boron. For copper deficiency, Bordeaux mixture or other copper fertilizers can be sprayed. To supplement boron, soil application can be used, or 0.05%-0.1% boron fertilizer solution can be sprayed at the bud stage, flowering stage and small fruit stage.