How to prevent and control the main pests and diseases of rhododendrons?
(1) Rhododendron diseases ① Brown spot: This is the main disease of rhododendrons. In the early stage of the disease, small brown spots appear on the leaf surface, then gradually expand into large dark brown patches, and many small black or grayish-brown dots are scattered on the lesions. Affected leaves turn yellow and fall off very easily, which not only affects flowering in the current year, but also has a great impact on flower bud development the following year. The disease is more severe during the plum rain season. After the disease occurs, diseased leaves should be removed promptly, and a 50% Topsin 800-fold solution should be sprayed for control.
② Rhododendron leaf gall disease: It mainly occurs on young leaves, young buds and young stems. After young leaves are affected, the diseased part swells noticeably, is concave on the underside and raised on the upper side, forming a hemispherical shape. In the early stage, the lesions are pale red, then turn yellowish-brown, and a white powdery substance appears on the surface. In the later stage, the lesions turn brown to black. The lesions merge with each other, the whole leaf swells, or becomes deformed and curled, and finally withers and falls off. It is a fungal disease. There are 2 occurrences per year, one in late spring and early summer, and the other in late autumn and early winter, with the spring occurrence being the most severe. Control method: remove and destroy diseased leaves before the white powdery substance is produced on the diseased parts, and spray a 65% zineb 600-fold solution for control.
In addition, there are also flower rot, leaf blight, sooty mold, etc.
(2) Pests
Stephanitis nashi: The adult is small and flat, 3.5-4.0 mm long, blackish-brown. When the forewings are folded, they form an "X" pattern, and the wing surface is densely covered with reticulate markings. The nymph is dark brown, with obvious spines on its body. Adults and nymphs gather near the main and lateral veins on the underside of host leaves to suck and cause damage. Yellowish-white spots appear on the damaged leaf surface, and at the same time many dark brown insect droppings and (shed) skins can be seen on the underside of the leaves. A 50% carbaryl wettable powder 500-700-fold solution; or a 50% phoxim or 50% fenitrothion emulsion 1000-1500-fold solution can be sprayed for control.
② Short-whiskered mite: It mainly pierces and sucks sap near the main vein on the underside of leaves. July to September is the peak occurrence period, often causing large numbers of rhododendron leaves to fall, seriously affecting growth and ornamental value.
In addition, there are also sawflies, scale insects and bagworms, etc.
(3) Physiological diseases
Rhododendron chlorosis is a physiological disease. The main causes are: alkaline soil; rhododendrons prefer acidic soil, and if planted in alkaline soil, the leaves will turn yellow and may even die. Iron deficiency; when the iron element in the growing medium is insufficient, chlorophyll synthesis is affected, the leaves become smaller and appear yellowish-white, with the most obvious symptoms on young leaves. The two have a related relationship, and alkalinity often leads to iron deficiency. Improper care; excessive or insufficient light, overwatering, excessive or insufficient fertilization, heavy clay soil, etc., can all cause physiological disorders, resulting in poor rhododendron growth and symptoms such as leaf yellowing and falling off. For chlorosis, targeted treatment should be applied according to the above causes. If chlorosis is caused by iron deficiency, a 0.2% ferrous sulfate aqueous solution can be sprayed or watered on, once every 7 days, for 3-4 consecutive times, and the leaves will then turn from yellow to green; for chlorosis caused by soil alkalinity, the soil quality should be acidified, and an acidic growing medium based mainly on leaf mold can be used instead; if chlorosis is caused by improper care, comprehensive care measures are needed.
Prevention and Control of Main Pests and Diseases of Rhododendrons
1. Rhododendron diseases
① Brown spot: It is the main disease of rhododendrons. In the early stage of the disease, small brown spots appear on the leaves, then gradually expand into large dark brown patches, and many small black or grayish-brown dots are scattered on the lesions. Affected leaves turn yellow and fall off very easily, which not only affects flowering in the current year, but also has a great impact on flower bud development the following year. The disease is more severe during the plum rain season. After the disease occurs, diseased leaves should be removed promptly, and a 50% Topsin 800-fold solution should be sprayed for control.
② Rhododendron leaf gall disease: It mainly occurs on young leaves, young buds and young stems. After young leaves are affected, the diseased part swells noticeably, is concave on the underside and raised on the upper side, forming a hemispherical shape. In the early stage, the lesions are pale red, then turn yellowish-brown, and a white powdery substance appears on the surface. In the later stage, the lesions turn brown to black. The lesions merge with each other, the whole leaf swells, or becomes deformed and curled, and finally withers and falls off. It is a fungal disease. There are 2 occurrences per year, one in late spring and early summer, and the other in late autumn and early winter, with the Spring Festival occurrence being the most severe. Control method: remove and destroy diseased leaves before the white powdery substance is produced on the diseased parts, and spray a 65% zineb 600-fold solution for control.
In addition, there are also flower rot, leaf blight, sooty mold, etc.
2. Pests
① Stephanitis nashi: The adult is small and flat, 3.5-4.0 mm long, blackish-brown. When the forewings are folded, they form an x-shaped pattern, and the wing surface is densely covered with reticulate markings. The nymph is dark brown, with obvious spines on its body. Adults and nymphs all gather near the main and lateral veins on the underside of host leaves to suck and cause damage. Yellowish-white spots appear on the damaged leaf surface, and at the same time many dark brown insect droppings and (shed) skins can be seen on the underside of the leaves. A 50% carbaryl wettable powder 500-700-fold solution; or a 50% phoxim or 50% fenitrothion emulsion 1000-1500-fold solution can be sprayed for control.
② Short-whiskered mite: It mainly pierces and sucks sap near the main vein on the underside of leaves. July to September is the peak occurrence period, often causing large numbers of rhododendron leaves to fall, seriously affecting growth and ornamental value.
In addition, there are also sawflies, scale insects and bagworms.
3. Physiological diseases
Rhododendron chlorosis is a physiological disease. The main causes are: alkaline soil; rhododendrons prefer acidic soil, and if planted in alkaline soil, the leaves will turn yellow and may even die. Iron deficiency: when the iron element in the growing medium is insufficient, chlorophyll synthesis is affected, the leaves become smaller and appear yellowish-white, with the most obvious symptoms on young leaves. The two have a related relationship, and alkalinity often leads to iron deficiency. Improper care; excessive or insufficient light, overwatering, excessive or insufficient fertilization, heavy clay soil, etc., can all cause physiological disorders, resulting in poor rhododendron growth and symptoms such as leaf yellowing and falling off. For chlorosis, targeted treatment should be applied according to the above causes. If chlorosis is caused by iron deficiency, a 0.2% ferrous sulfate solution can be sprayed or watered on, once every 7 days, for 3-4 consecutive times, and the leaves will then turn from yellow to green; for chlorosis caused by soil alkalinity, the soil quality should be acidified, and an acidic growing medium based mainly on leaf mold can be used instead; if chlorosis is caused by improper care, comprehensive care measures are needed.
Methods for Preventing and Controlling the Main Pests and Diseases of Rhododendrons
The main diseases of rhododendrons:
(1) Brown spot:It is the main disease of rhododendrons. In the early stage of the disease, small brown spots appear on the leaf surface, then gradually expand into large dark brown patches, and many small black or grayish-brown dots are scattered on the lesions. Affected leaves turn yellow and fall off very easily, which not only affects flowering in the current year, but also has a great impact on flower bud development the following year. The disease is more severe during the plum rain season. After the disease occurs, diseased leaves should be removed promptly, and a 50% Topsin 800-fold solution should be sprayed for control.
(2) Rhododendron leaf gall disease:It mainly occurs on young leaves, young buds and young stems. After young leaves are affected, the diseased part swells noticeably, is concave on the underside and raised on the upper side, forming a hemispherical shape. In the early stage, the lesions are pale red, then turn yellowish-brown, and a white powdery substance appears on the surface. In the later stage, the lesions turn brown to black. The lesions merge with each other, the whole leaf swells, or becomes deformed and curled, and finally withers and falls off. It is a fungal disease. There are two occurrences per year, one in late spring and early summer, and the other in late autumn and early winter, with the spring occurrence being the most severe. The other is in late autumn, and the spring occurrence is the most severe.
Control method:Remove and destroy diseased leaves before the white powdery substance is produced on the diseased parts. Spray a 65% zineb 60-fold solution for control. In addition, there are also flower rot, leaf blight, sooty mold, etc.