What are the control methods for apple silver leaf disease?
Apple silver leaf disease is distributed in all apple-producing areas of China, occurring mainly in the apple-producing areas along the old course of the Yellow River in Jiangsu, Anhui and Henan provinces, and rarely seen in other apple-producing areas. It is a devastating disease that, under suitable conditions, can cause the entire apple tree to die.
[Identification features] The typical symptom of apple silver leaf disease is leaves that are silver-gray with a metallic luster. Its symptoms fall into two types: typical silver leaf and latent silver leaf.
(1) Typical silver leaf: the leaf surface looks as if coated with a layer of silver-gray film; the leaves are small, thick and brittle, and the epidermis separates easily from the leaf mesophyll. In the later stage the edges of the diseased leaves become scorched, rust spots appear along the main veins, and the diseased leaves split and fall off easily.
(2) Latent silver leaf: occurs mainly on newly diseased trees or on trees whose condition has improved after treatment. The leaves fade in color and develop uneven fish-scale-like markings alternating gray, green and yellow. The tree does not bloom or blooms little, sets very little fruit, and the fruit is small and pale in color.
Apple silver leaf disease generally spreads from small branches to main branches, gradually affecting the whole tree. It usually occurs on trees with weak growth; in severe cases the xylem of the main trunk and roots turns brown, leading to root rot and death and to the death of the whole tree.
[Occurrence characteristics] Apple silver leaf disease is a disease caused by fungi. The pathogen overwinters as mycelium in the xylem of diseased branches, and can also overwinter as fruiting bodies on the outside of the bark. In the apple-producing areas along the old course of the Yellow River, diseased branches produce fruiting bodies twice each year, in May to June and September to October. After maturing they produce basidiospores, which spread with wind and rain and invade through pruning and saw cuts or other wounds, and can also invade through leaf scars and lenticels. When the weather is humid, the spores germinate and infect the xylem, producing a toxin that turns the xylem brown and gradually spreads to all parts of the tree; silver leaf symptoms appear after the apple tree leafs out. The largest number of diseased trees occurs in April to May, and after June diseased trees successively show symptoms. Its occurrence is closely related to orchard terrain, variety and tree age. Generally, orchards on level ground with heavy, sticky soil, poor drainage and weak tree vigor have severe disease.
[Integrated control]
(1) Eliminate the source of inoculum: clean up the orchard in good time, remove severely diseased trees and dead trees, remove dead branches, and burn them in a centralized manner. After rainfall, strengthen inspection, scrape off and centrally burn the fruiting bodies produced on diseased trees and on other trees near the orchard. Then apply 40% asomate 100-fold solution or 1% copper sulfate solution to the diseased parts to reduce the spread of the pathogen.
(2) Reduce wounds: when training fruit trees, pay attention to developing strong scaffold branches, control the crop load, and prevent branches from splitting and breaking due to bearing too much fruit; minimize mechanical wounds as much as possible, and apply 5 degrees Baume lime sulfur mixture or 40% asomate 100-fold solution to pruning and saw cuts and wounds for chemical protection.
(3) Strengthen cultivation management: apply more organic fertilizer, improve the soil, drain water in good time, and prevent waterlogging in the orchard, so as to strengthen tree vigor and improve the tree's disease resistance.
(4) Chemical control: for lightly diseased trees discovered early, sulfuric acid-8-hydroxyquinoline pellets can be buried in the diseased branches and trunk for treatment. The specific method is to use a punch to make holes 1.5 cm in diameter and 3 cm deep at the base of the diseased branch, insert the pellets, and seal them well with cork or wax. When the branch is relatively thick, holes can be made in a spiral staggered arrangement at a distance of about 10 cm, with 1 gram of medicine buried in each hole.