Latest Causes of Black-Skinned Fruit and Detailed Prevention Measures

Many farmers who have grown citrus, lemons and pomelos have had this experience: in summer and autumn, these fruits develop blackened rinds, commonly known as “black-skinned fruit”. Some orchards are even devoured by this black-skinned fruit, with black fruits hanging all over the branches; these fruits are small, sour, and black-skinned, completely losing their value as a commodity. All of this is caused by a pest called the citrus rust mite. This citrus rust mite can be called the killer of citrus appearance: it not only affects the look of the fruit, but more seriously affects the taste, leading to lower returns for fruit growers. What is even more frightening is that this pest can also cause damage for several consecutive years, hence the saying “one year of copper, three years of poverty”. Below, taking citrus as an example, let us understand the causes of this “black-skinned fruit” and the control measures.

I. Causes

Citrus black-skinned fruit is caused by damage from the citrus rust mite (also known as the citrus rust spider, rust mite, or leaf-piercing eriophyid mite). The adult mites, nymphs and larvae of the citrus rust mite pierce the fruit and leaves with their piercing-sucking mouthparts to suck the sap. When the oil cells are destroyed, the overflowing oil is oxidized and turns brown to dirty black, which fruit growers call “oil-skin lump” or “oil-skin citrus”. On damaged leaves, the underside is dark brown, as if smoked, and the stain cannot be wiped off (the mold layer of sooty mold disease, by contrast, can be wiped off). Once the citrus fruit (or leaf underside) turns dark brown, the best time for control has already been missed. Damage to citrus trees by rust mites also causes a large number of leaves to turn yellow and drop, weakening the tree vigor and seriously affecting the next year's yield.

II. Development Characteristics

The host range of the citrus rust mite is limited to citrus fruit trees only, and at the peak of occurrence, larvae, nymphs and adults can be seen at the same time. Adults overwinter on summer and autumn shoots, and the following year move to spring shoots and fruits to cause damage. Generally, damage begins in May on the undersides of tender spring leaves; the undersides of damaged leaves show dark brown net-like patterns and trigger a large number of fallen leaves. From late May to early and mid-June, it begins to damage the fruit. From July to September is the peak period of rust mite damage, with fruit suffering the most severe damage. During this period, if there is hot and humid weather, it is unfavorable to rust mite reproduction; if there is high temperature and drought, rust mite damage worsens, and it can still cause disaster in October.

III. Control Measures

Having understood the causes and development characteristics of this “black-skinned fruit”, next we introduce the control measures that fruit growers care about most. Because rust mites are very small, initially white and later turning pale yellow, and when the infestation is severe, mites and eggs appear in patches on leaves and fruits, initial observation requires using a magnifying glass to examine the leaf and fruit surfaces, with particular attention to the leaf undersides. If individual fruits are found to have dust-like mite bodies on their surface, chemical control should be carried out immediately. For chemical control of rust mites, spray the inner and lower parts of the canopy first, then the outer and upper parts, focusing on the leaf undersides and the shaded side of the fruit. The best spraying times are before 10 a.m. and after 5 p.m. Spraying should be careful and thorough; the spray volume should be such that leaves and fruits are moist but no liquid drips off. Generally, spray 2 to 3 times consecutively, with an interval of 7 to 10 days between sprays.

As for effective chemicals for controlling rust mites, here we introduce one called lime sulfur mixture. Different concentrations can be used for different seasons: generally 0.2 to 0.3 degrees Baume in spring, 0.1 degrees Baume in summer, 0.2 to 0.4 degrees Baume in autumn, and 0.6 to 0.7 degrees Baume in winter. In hot seasons, the concentration applied must be chosen with care. Other effective chemicals include 75% propargite, 50% cyhexatin, 20% fenpropathrin, 50% fenbutatin oxide, 20% deltamethrin, 40% omethoate, 1.8% abamectin emulsifiable concentrate, 63% zineb wettable powder, Hirsutella thompsonii powder (70,000 colonies per gram), etc. At present, citrus growers in Yuanjiang City, Hunan Province, use Daconil (80% mancozeb wettable powder) to control citrus rust mites, with good results and a long residual period. Spraying once with a 600-fold dilution of Daconil gives a residual period of 30 to 60 days. In ordinary years, 1 to 2 consecutive control applications are enough; in years of severe outbreak, 2 to 3 consecutive applications can also control citrus sooty mold and other diseases and pests that easily infect and affect the appearance and color of the fruit.

The above is a brief introduction to the occurrence of citrus “black-skinned fruit” and its control methods. In the control process, biological control can also be used, making use of the natural enemies of rust mites, mainly including predatory ladybirds that feed on mites, predatory mites, long-haired mites with tubercles, thrips and Hirsutella thompsonii, etc., which can also have a certain inhibitory effect on rust mites.