Control methods for yellowing disease in citrus cultivation
In many places in our province, citrus yellowing disease occurs commonly in citrus orchards, causing weakened tree vigor, reduced yield, degraded quality, and premature tree decline. Some fruit growers believe it is all citrus greening disease and adopt the practice of cutting down trees, causing unnecessary production losses. In fact, as long as yellowing disease is not caused by citrus greening disease, scientific management can restore vigor.
The main symptoms of citrus yellowing disease are root rot in the underground part and sparse new roots; yellowing of autumn shoots above ground, smaller leaves, easily withered branches, small deformed fruit, hard peel and sour taste. Ponkan is more severely affected, while oranges, satsuma mandarins and pomelos are less affected.
Citrus yellowing disease is caused by physiological disorder factors and pest and disease damage. The main physiological disorder factors are poor site conditions, improper fertilization, lack of organic fertilizer, heavy use of herbicides, trifoliate orange rootstock being prone to premature decline, disastrous climate, lack of trace elements, unpruned tree canopy, and drought-induced loss of fertilizer and water. Pest and disease damage mainly includes citrus greening disease, anthracnose, foot rot, root-knot nematode, branch and trunk pests, cracked bark disease, broken leaf disease, resin disease, and decline disease.
Yellowing disease caused by physiological disorder factors can be corrected through the “four improvements”, namely: soil improvement, fertilizer improvement, tree improvement, and orchard improvement. Do well in expanding planting holes and improving soil, increasing application of organic fertilizer, applying more farmyard manure, emphasizing balanced fertilization, and spraying trace elements to increase soil organic matter content, so as to supply sufficient nutrients to citrus trees. Pay attention to bud wiping, shoot control, pinching, and pruning of citrus trees, and at any time prune out water sprouts, shaded branches, weak branches, pest and disease branches, and dead branches, so that the tree canopy takes on a natural open-center shape. Adopt methods such as branch pulling and twisting to dwarf the tree body and “open skylights”, which helps ventilation and light penetration. Improve the orchard's infrastructure and perfect the drainage and irrigation system, so that the citrus orchard can ensure stable yields despite drought or excessive rain.
Yellowing disease caused by pest and disease damage is controlled by strengthening integrated pest and disease control measures. Only trees diagnosed by experts as having citrus greening disease should be removed. For suspected cases, apply medicine according to different symptoms, such as: for anthracnose, use 600-fold carbendazim-plus-thiram solution for control; for foot rot, scrape off the diseased part and then apply 200-fold metalaxyl-copper solution; for root-knot nematode, use 1-2 liang of carbofuran per tree by furrow application; for branch and trunk pests, adopt the methods of “scraping, wiping, smearing, plugging, and spraying” for thorough control.