Why Are the Leaves of a Mandarin Orange Tree Turning Yellow? Causes and Solutions

During the Spring Festival, many households would not be without a mandarin orange tree, but problems are inevitable during the growing process. For example, yellowing leaves. The causes of yellowing leaves on a mandarin orange tree are many and varied. Such as insect pests, infectious diseases, nutritional imbalance disorders (physiological diseases), etc. Below, the editor will introduce several causes of yellow leaves on mandarin orange trees and their solutions.

I. Insect Pests

1. Red spider mite: April to June and September to November are the peak periods of occurrence; it mainly damages citrus leaves, twigs and fruit. When the damage to leaves and fruit is severe, they turn grayish-white and lose their luster, until they wither and yellow and drop off.

Control of red spider mite:

① You can choose 75% Propargite 3000x solution, or 15% Saomanjing 1000x solution for spraying (also controls rust mites);

② It is recommended to use 24% Envidor suspension concentrate produced by Bayer of Germany at 5000x, with outstanding results.

2. The leaves of the mandarin orange suddenly turn yellow, mainly caused by insects in the roots. If the roots are damaged by insects, the root bark is damaged, nutrients and water cannot be absorbed and transported, and the leaves will turn yellow because they cannot get a normal supply of water and nutrients.

II. Symptoms of Nutritional Imbalance in Citrus

1. Nitrogen deficiency in citrus.

Damage symptoms: Leaves turn yellow, the leaves uniformly become light green, accompanied by nitrogen being transferred out of the leaves to maintain new growth.

① Measures: Strengthen management, apply more farmyard manure, and in combination with foliar topdressing, spray 0.2% urea plus 0.3% monopotassium phosphate or nutrient solution fertilizer at intervals of about 10 days; for mountain citrus orchards, add 0.2% borax spray.

2. Iron deficiency disease in citrus.

Damage symptoms: Generally the young shoots show symptoms first, the leaves become thin and yellowed, light green to yellow-white, with green veins, showing a distinct green network pattern on the yellowed leaves, and the leaves at the tips of small branches are even more obvious. The branches of diseased plants are weak, and the leaves on young branches drop very easily, often leaving only sparse leaves. After the leaves on small branches drop, normal branches and leaves only grow on the larger branches below, but the top branches die off one after another. When the disease is severe, all the leaves of the whole plant turn orange-yellow.

Citrus iron deficiency disease is mainly caused by insufficient iron supply. According to reports, insufficient nitrogen fertilizer can also produce iron deficiency symptoms. Iron deficiency often occurs together with zinc deficiency, manganese deficiency, magnesium deficiency, etc.

Control methods:

① Improve the soil and fertilize. The fundamental method of preventing and treating iron deficiency disease is to improve the soil and do a good job with the drainage and irrigation system, to prevent drought and waterlogging disasters. For alkaline soil, more organic fertilizer should be applied, with special attention to applying more green manure, soil and miscellaneous manure, and other acidic fertilizers.

② Apply ferric sulfate. On the basis of improving the soil and doing a good job with the drainage and irrigation system, apply ferric sulfate or spray a 0.1% ferric sulfate solution; the method of cutting off part of the roots can be used to make it absorb ferric sulfate. However, using ferric sulfate for control without improving the soil has no obvious effect.

③ Apply 15-20 jin of sulfur powder per mu, plus ammonium sulfate, potassium sulfate, etc., to acidify the soil.

3. Zinc deficiency disease in citrus.

Damage symptoms: The leaves of new shoots become yellowed, appearing yellow or yellow-green, while the main and lateral veins and the tissue near them remain green. The main and lateral veins of old leaves have irregular green bands, while the rest is light green to orange-yellow. Some leaves show only yellow and light yellow spots between the green main and lateral veins, and some leaves become smaller, narrower and lose their green color. When zinc deficiency is severe, the top twigs become slender and short, the leaves stand upright, and the whole plant appears as an upright dwarf bush, after which the small branches die off.

Control methods: Spraying 0.4%-0.5% zinc sulfate solution (with 1%-2% lime and 0.1% sticker) before the growth of spring shoots can effectively treat zinc deficiency disease. In addition, spraying 0.1%-0.2% zinc oxide can also correct it. For slightly acidic soil (pH 5.5-6), applying a small amount of zinc sulfate can also achieve good results, but it is ineffective on alkaline soil. If zinc deficiency is induced by magnesium deficiency or copper deficiency, applying zinc salts alone will have little effect; compounds containing magnesium, copper and zinc must be applied at the same time to achieve good results. Secondly, applying more organic fertilizer is also very important.

4. Boron deficiency disease in citrus.

Damage symptoms: Mature leaves and old leaves begin to yellow from the veins, eventually the whole leaf yellows, the mesophyll is relatively thick, and the leaf tips mostly curl backward; the veins swell, the main and lateral veins become corky, and in severe cases they crack. Young leaves produce irregular water-soaked yellow spots, twisted and deformed; some have water-soaked black spots at the base of the main vein on the underside of the leaf, and drop easily. On the rind of young fruit, milky-white slightly raised small spots appear, and in severe cases sunken black spots appear, and the mesocarp and fruit core are filled with gum; this often causes a large number of young fruit to drop. The remaining fruit is small and hard, with tumors on the rind surface, thick rind, little juice, and abortive seeds, and the mesocarp and fruit core fill with gum. When a citrus orchard is severely boron deficient, a large number of leaves drop early, branches wither and die, and sometimes the whole plant dries up.

Control methods:

(1) Apply boron fertilizer. Mix boron fertilizer into human excrement and urine, dig a trench under the tree canopy and apply it, cover with part of the organic fertilizer and then cover with soil. Apply 0.1-0.15 kg of boron fertilizer each time for adult trees, and reduce the amount appropriately for lightly diseased trees. Generally apply once every 2-3 years.

(2) Foliar boron spraying. Generally spray boron fertilizer (boric acid or borax solution) once each in early spring and at full bloom, which can effectively prevent and treat boron deficiency disease. In early spring, use a concentration of 0.5%-0.8%, afterward use a concentration of 0.3%-0.4%, but in summer and autumn the concentration should be changed to 0.2%-0.3%. To prevent phytotoxicity, about 0.5% quicklime can be added. Spraying should be done on cloudy days, or in the early morning or evening of sunny days when the temperature is lower and the humidity is higher.

(3) Avoid applying too much nitrogen, phosphorus and calcium fertilizer. Especially for soil with low organic matter content, even more attention should be paid not to apply too much nitrogen, phosphorus and calcium fertilizer; composted stable manure, or farmyard manure and green manure with higher boron content, should be applied. In acidic soil, too much lime should also not be applied.

(4) In dry seasons, retain water and water in time; in rainy seasons, pay attention to drainage to prevent the loss of available boron.

5. Manganese deficiency disease in citrus.

Damage symptoms: The leaves of young branches are normal in size, yellow-green, with the main and lateral veins and the tissue near them green, and the rest yellow-green, rather similar to the symptoms of zinc deficiency disease. But the yellowed part of zinc deficiency is very yellow, while that of manganese deficiency is with a green tinge; the young leaves of zinc deficiency are small and narrow, while the leaf size and shape of manganese deficiency are basically normal, and old leaves also show symptoms. In severe cases, the leaves age and drop early, the growth of new shoots is severely inhibited, and some wither and die. If manganese deficiency is accompanied by zinc deficiency, even more small branches wither and die.

Control methods:

(1) Apply manganese sulfate. For acidic soil, manganese sulfate can be mixed with other fertilizers and applied. Lemon is more prone to phytotoxicity from manganese sulfate, so the number of sprays should be fewer and the concentration should be lower; attention should be paid to this. For alkaline soil, a mixed liquid of manganese sulfate and quicklime (0.2%-0.6% manganese sulfate plus 1%-2% quicklime) can be sprayed, neutralized and adjusted to (pH 6-6.5), and the control effect is good.

(2) From May to August, 0.6% manganese sulfate and 0.3 degrees Baume lime sulfur mixture can be sprayed; or 0.3% manganese sulfate plus an equal amount of lime can be sprayed; in August, 0.4% manganese sulfate mixed with rosin mixture can also be sprayed.

(3) Apply more organic fertilizers such as composted stable manure or fermented green manure.

6. Magnesium deficiency disease in citrus.

Magnesium deficiency disease is a micronutrient deficiency disorder. The leaves are yellow but the veins are green. You can get some soil from the field, preferably stir it thoroughly with water from a rice paddy and then let it settle, extract the trace elements from the soil, and water the mandarin orange tree with this water; this is a relatively good method to solve the nutrient deficiency problem.

III. Tips for Caring for Mandarin Orange Trees

Fertilization precautions

1. Apply heavy winter fertilizer

Before and after fruit harvest, in combination with deep plowing and soil improvement, apply heavy organic fertilizer, which has a notable effect on restoring tree vigor and promoting flower bud differentiation. The specific method is: in early to mid-November to December, dig deep trenches along the outer edge of the tree canopy and apply heavy base fertilizer (peanut bran, bone meal, soil and miscellaneous manure, pig and cow manure, compost, stable manure, phosphate rock powder, superphosphate, etc.). Applying heavy winter fertilizer is the foundation for a bumper harvest the following year. But it should be noted that: winter fertilizer must be applied early and heavily (accounting for 50% of the annual fertilization amount), and should be mainly organic fertilizer.

2. Apply spring fertilizer early

Applying quick-acting nitrogen and phosphorus fertilizer before the spring buds of the mandarin orange tree sprout (late February to early March) has the effect of strengthening shoots and improving the quality of flowering branches (increasing the number of leaves and leaf area of flowering branches), and can promote the normal sprouting of new shoots and supply the nutrients needed for fruit development.

3. Skillfully apply fruit fertilizer

Applying quick-acting nitrogen fertilizer in mid to late May can compensate for the nutrients consumed by flowering and allow young fruit to get enough nutrition, reduce fruit drop, and increase the fruit-setting rate. From July to September, the fruit grows rapidly and a large number of autumn shoots are produced, which serve as the fruiting mother branches for the next year; vegetative growth and fruit development often come into conflict over nutrients and water. At this time, topdressing with quick-acting nitrogen fertilizer, together with phosphorus and potassium fertilizer, can accelerate fruit growth, improve fruit quality, and promote the occurrence of autumn shoots. But it must be noted that: the type, concentration and method of fertilization should be determined according to tree vigor growth and soil and weather conditions. In dry seasons, watering should be done first to keep the soil moist, and then fertilizer should be applied, which helps root absorption and achieves the effect of "water promoting fertilizer." Applying fertilizer together with water is a method of skillfully applying fertilizer, which can make the same fertilizer play a greater role. For fruit trees with vigorous growth, too much nitrogen fertilizer should not be applied; otherwise, when spring and summer shoots grow too abundantly, it will also cause fruit drop.

4. Foliar fertilization

Foliar fertilization is a topdressing method that uses the stomata and cuticle of leaves to directly absorb nutrients, and has the advantages of quick effect and high absorption and utilization rate. During the bud stage, flowering stage and young fruit stage, use water-soluble quick-acting fertilizer for foliar fertilization, which can increase the fruit-setting rate and help fruit expansion and development.