Causes and Control Methods of Black-Skinned Citrus Fruit

Many farmers who have grown citrus, lemons, and pomelos have experienced this: in summer and autumn, these fruits develop blackened peel, commonly known as “black-skinned fruit”. Such fruit is small, sour, and black-skinned, greatly costing growers their profits. Below we will look together at how black-skinned citrus fruit forms and how it should be prevented and controlled.

I. How does black-skinned citrus fruit form?

Black-skinned citrus fruit is caused by damage from the citrus rust mite (also known as the citrus rust spider, rust mite, or prickly leaf gall mite). The adult mites, nymphs, and larvae of the citrus rust mite pierce fruit and leaves with their piercing-sucking mouthparts to feed on sap. When the oil cells are destroyed, the overflowing oil is oxidized and turns brown to dirty black, which growers call “oil-skin lump” or “oil-skin citrus”. On affected leaves, the underside turns dark brown, as if smoke-blackened, and the dirty color cannot be wiped off (while the mold layer of sooty mold disease can be wiped off). Once citrus fruit (or the leaf underside) turns dark brown, the best time for control has already been missed. When the rust mite damages citrus trees, it also causes large numbers of leaves to yellow and drop, weakening tree vigor and seriously affecting the next year's yield.

II. How to prevent and control black-skinned fruit?

Having understood the causes of this “black-skinned fruit” and its developmental characteristics, next we will introduce the control measures that growers care about most. Because the rust mite is very small, initially white and later turning pale yellow, and when outbreaks are severe, mites and eggs appear in patches on leaves and fruit, early observation requires using a magnifying glass to examine the leaf and fruit surfaces, with particular attention to the leaf underside. 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 the rust mite, spray the inner and lower parts of the canopy first, then the outer and upper parts of the canopy, focusing on the leaf underside and the shaded side of the fruit. The best spraying times are before 10 a.m. and after 5 p.m. Spray carefully and thoroughly; the spray volume should be just enough to wet the leaves and fruit without dripping. Generally spray 2 to 3 times consecutively, with an interval of 7 to 10 days between each spray.