A Detailed Guide to Common Pests and Diseases of Platycladus orientalis (Chinese Arborvitae) Leaves and Their Control Methods
Platycladus orientalis (Chinese Arborvitae) leavesmainly suffer from the following diseases:Platycladus orientalis leaf blight,Platycladus orientalis leaf scorch; and the pests includethe Chinese arborvitae caterpillar (larva of the Chinese arborvitae tussock moth),the Chinese arborvitae giant aphid, andthe double-striped cedar longhorn beetle.The following describes the symptoms of these pests and diseases and their control methods.
Diseases
1、Platycladus orientalis leaf blight: Platycladus orientalis leaf blight is one of the major diseases of Platycladus orientalis. When damage is severe, it can cause large areas of Platycladus orientalis foliage to wither and die, appearing as if scorched by fire. The trees become severely weakened, making them susceptible to secondary pests such as the cedar double-striped longhorn beetle and bark beetles (Scolytidae), which accelerates tree death.
Disease pattern: The fungus overwinters on diseased leaves, begins activity in April of the following year, and in May starts infecting leaves from the lower part of the crown first. Infected leaves initially develop yellow spots, which gradually expand, causing the entire branch's foliage to turn yellow and eventually yellow-brown. In June, black dots appear on diseased leaves; these are the reproductive organs of the pathogen, which release spores for reinfection. If the forest is dense, humidity within the stand is high, and temperatures are suitable, the disease spreads and infects extensively, causing a major epidemic, with the most severe incidence in June-July. In dense forests, there is often a disease center that gradually spreads outward into patches; in severe cases it appears as if scorched by fire, with large numbers of diseased leaves falling off, and clusters of new leaves often sprouting from the trunk and branches, which people call “tree beard”.
The disease is generally more severe in gullies than on hilltops, more severe on shady slopes than on sunny slopes, more severe in dense forests than in sparse forests, and heavier in rainy, damp weather.
Control methods(1) In autumn and winter, sweep up diseased leaves under the trees and burn them to eliminate the source of infection. Destroy overwintering pathogens to reduce primary infection. (2) From May to August, spray 1:1:100 Bordeaux mixture once every two weeks as a preventive measure, with particular attention to strictly controlling primary infection; when primary infection centers are found, block them off to prevent spread. (3) Overly dense Platycladus orientalis stands should be appropriately thinned to improve ventilation and light penetration within the stand and reduce conditions favoring disease development.
2. Platycladus orientalis leaf scorch: This disease infects newly grown leaves of the current year, young slender twigs, and scale leaves. Infected plants show no symptoms in the current year, but the leaves wither rapidly in March of the following year. Around mid-June, black granular bodies, which are the pathogen's apothecia, appear on dead scale leaves and slender twigs. The apothecia absorb water and swell into a cup shape when wet, olive-colored. After Platycladus orientalis is affected, the crown withers as if scorched by fire, large numbers of diseased leaves fall off, and branches die. Clusters of small branches and leaves sprout from the main trunk or branches, the so-called “tree beard”. Being affected for several consecutive years causes the entire plant to die.
Disease pattern: The disease is severe in the inner and lower parts of the crown; autumn shoots of the current year are basically not affected. High temperatures and heavy rainfall in June, and cold, dry winters, lead to severe disease the following year, and vice versa. The pathogen is a fungus with weak parasitic ability, and plants with weak growth are easily infected; all factors that affect the growth of Platycladus orientalis favor infection and development of the pathogen.
Control methods: (1) Prune moderately to improve the growing environment of Platycladus orientalis and reduce sources of infection. (2) Apply more fertilizer to promote growth. (3) During the peak period of ascospore release, spray 40% Miebingwei, or 40% carbendazim, or 40% chlorothalonil at 500 times dilution for control. (4) Select disease-resistant varieties.
Pests
1. Chinese arborvitae caterpillar (larva of the Chinese arborvitae tussock moth): The larva of the Chinese arborvitae tussock moth is the main leaf-eating pest of Platycladus orientalis shelterbelts. This insect has 2 generations per year, overwintering as 3rd-5th instar larvae in the leaf litter layer or under stones. In mid-March of the following year, they climb up the trees to cause damage, and by mid-April they mature and pupate. In early June, the 1st generation larvae hatch, and by early to mid-July they mature and pupate. In mid to late August, the 2nd generation larvae hatch, causing damage until early to mid-October when they descend from the trees to overwinter.
Key control points: (1) Agricultural control. Strengthen forest management, prune and fell in a timely manner, ring-bark trees during the overwintering period of this pest and burn the bark in a centralized manner, and manually collect the larvae when they break out in large numbers. (2) Physical control. Based on the adult's phototaxis, set up black-light lamps to trap and kill adults. (3) Biological control. This pest has many natural enemies; when the outbreak is not large, try not to use chemical control, and take care to protect natural enemies, such as releasing parasitic wasps. (4) Chemical control. When larval density is high, treat early; use aircraft for low-volume spraying of 25% chlorbenzuron No. 3 to control larvae, with 23-45 g of pesticide per mu, a spray volume of 10.5 kg, and a control effect of up to 98%. Aerial control should be carried out in clear weather with wind force below level 3; manual spraying of 25% chlorbenzuron No. 3 at 1:1500 dilution can control larvae with an effect of over 90%. Generally, control should be carried out at the 2nd-4th instar stage, and a small amount of pyrethroid pesticides may be added as needed.
2. Chinese arborvitae giant aphid: Also known as the cypress giant aphid. It belongs to Homoptera, Aphididae (Lachnidae). It is widely distributed. It damages Platycladus orientalis, weeping cypress, thousand cypress, dragon juniper, pencil cedar, golden cypress, and Thuja occidentalis, among others. It is one of the important pests on cypress trees, and is especially harmful to Platycladus orientalis hedges and seedlings. The bodies of the insects densely cover young twigs in layers, excreting large amounts of honeydew, causing sooty mold. In mild cases it affects tree growth; in severe cases young trees dry up and die.
Control methods
(1) Protect and utilize natural enemies such as the seven-spotted ladybird, the harlequin ladybird, the sunbeam wasp, aphidid wasps, the large grey hoverfly, green lacewings, and aphid hoverflies. When the spring outbreak is not severe, try not to spray chemicals; spraying clean water to wash off the insects can protect the reproduction and development of natural enemies later on.
(2) Chemical control. When the outbreak is severe, spray 25% thiamethoxam water-dispersible granules or 20% imidacloprid soluble concentrate at 5000 times dilution. About 20 generations occur each year; the body is relatively large, and it causes damage year-round. For control, spray 1.2% bitter tobacco EC at 1000-1500 times dilution, or spray 10% imidacloprid wettable powder at 2000 times dilution.
3. Double-striped cedar longhorn beetle:Also known as the wood-boring worm, it belongs to Coleoptera, Cerambycidae, and is a major pest of fir and cypress. The larvae feed between the bark and wood, cutting off the transport of water and nutrients, causing needle yellowing and declining growth; in severe cases it causes wind breakage and snow breakage, and in serious cases quickly causes the death of entire trees or entire branches. It directly affects the rapid, high-yield growth of fir and cypress and the formation of high-quality timber.