Seven Common Pests and Diseases and Their Control Methods

Brown Spot Disease  Brown spot disease is a worldwide disease that, in severe cases, often causes large numbers of plant leaves to turn yellow and drop off.

Symptoms The pathogen of brown spot disease is a fungus that mainly attacks the tips of leaves, appearing brown with a clear boundary from the healthy part. Later, lesions also appear on the edges and middle of the leaves, and the disease usually starts on the lower leaves and then gradually spreads upward. The early lesions are of varying sizes, round or oval, and the upper and lower ends turn yellow rapidly. When the lesions appear, they cause the leaves to twist. In the later stage, the diseased parts rupture; when moist, the diseased parts are densely covered with dark brown small dots, from which a dirty yellow substance oozes out, later becoming flesh-red masses of conidia. When the disease is severe, the lesions merge into patches, the entire leaf turns yellow rapidly, and it drops off early.

Living Habits The pathogen overwinters in diseased leaves in the form of mycelium or conidiomata, and the following year produces conidia that are carried by the wind to new sites. Old leaves are severely affected. When plants are planted too densely, the leaves collide with each other and easily produce wounds. Brown spot disease generally begins to occur in early summer and is severe during high temperature and rainy periods; lower leaves are more susceptible than upper leaves.

Control Methods Horticultural control: After discovering the disease, promptly prune off diseased leaves and pull out diseased plants to reduce sources of infection. Drain accumulated water promptly after rain, do not apply excessive nitrogen fertilizer, strengthen cultivation management and shaping and pruning, and keep the plants well ventilated and exposed to light.

Chemical control: Spray 25% tebuconazole 3000-fold solution, 40% flusilazole 6000-fold solution, 45% prochloraz 3000-fold solution, or 70% thiophanate-methyl 800-fold solution, etc. Apply the first treatment after spots appear, then once every 20 days, for a total of 3 sprays.

Powdery Mildew  Powdery mildew is a fungal disease, and most plants are important targets of attack. It occurs in Sichuan, Yunnan, Jiangsu, Beijing, Tianjin, Qingdao, Hebei, and Shandong, and is severe in Beijing and Chongqing.

Symptoms  After the disease occurs, it often causes the leaf tips and tender stems of the plant to become infected, twisted, and deformed, covered with a layer of white powder, and later the powdery mildew layer turns gray. These leaves become more purple than normal leaves; infected plants become shorter, with tender leaves twisted, deformed, and withered, and the leaves do not unfold. In severe cases, flowers cannot open normally, and leaves turn yellow and drop off, directly affecting tree vigor and ornamental effect.

Occurrence Pattern  Powdery mildew overwinters as mycelium on diseased buds, diseased leaves, and diseased branches. The disease is closely related to temperature. It can occur at 15°C, begins in March and April, and peaks from May to July. The occurrence differs by variety; smooth-leaved, trailing, and many-flowered varieties are more disease-resistant. It is prone to occur when nitrogen fertilizer is applied excessively and potassium is insufficient. Powdery mildew occurs severely during the rainy season or under conditions of relatively high humidity, and is prone to occur when planting is too dense or ventilation and light transmission are poor.

Control Methods  Horticultural control: First, select disease-resistant varieties. In addition, after pruning all branches in winter, spray 3-degree lime sulfur mixture to kill overwintering pathogens, and burn the diseased branches in a centralized manner to thoroughly eliminate the source of disease. After the disease occurs, ensure ventilation and light transmission, and do not let humidity become too high. Control the planting density well, strengthen management, increase phosphorus and potassium fertilizer, control nitrogen amount, and improve the disease resistance of the plants.

Chemical control  Before budding, spray 3 to 4 degrees lime sulfur mixture. When the disease occurs, spray 80% zineb 500-fold, or 70% thiophanate-methyl 800-fold, or spray 25% tebuconazole 3000-fold solution, 40% flusilazole 6000-fold solution, 45% prochloraz 3000-fold solution, once a week, for 3 consecutive sprays.

Aphids  Aphids belong to Homoptera, Aphidoidea, and are major worldwide pests, distributed throughout the country. They have more than 300 host species and can transmit at least 55 kinds of viruses.

Morphological Characteristics  Winged parthenogenetic adult aphids are 2.1 mm long; in spring and autumn they are dark green, and in summer they are mostly yellow. The body and back have markings, and the cornicles and cauda are black. Wingless parthenogenetic adult aphids are 1.9 mm long, and their bodies range from grass green to yellow, mostly yellow.

Eggs are orange-yellow when first laid, later becoming glossy pitch-black. Newly born nymphs are pale yellow, later becoming yellow-green, and in spring and autumn they may also be blue-gray.

Living Habits  They can reproduce 30 generations per year, overwintering as eggs in the axils of buds of weeds or Weigela, hibiscus, pomegranate, and other plants. In the season of fastest reproduction, they can complete 1 generation every 5 days, and 3 generations in half a month. A single aphid can produce a maximum of 100 offspring, and in a very short time the total aphid population can surge. After autumn when the temperature drops, they fly back to Weigela to mate with aphids flying in from elsewhere and then overwinter.

Damage Characteristics  Aphids often cause deformation of branches and leaves, and slow or stagnant growth; in severe cases they cause leaf drop and even death. When plants are damaged by aphids, because their saliva contains certain amino acids, after being injected into plant tissues it causes an increase in auxin or a decrease in its decomposition, resulting in spots, leaf curling, wrinkling, galls, tumors, and many other types of damage symptoms. At the same time, their excrement often induces sooty mold. Another major harm of aphids is that they can carry hundreds of plant viral diseases and other diseases, causing very serious indirect damage.

Control Methods  Use natural enemies: Some ladybugs, lacewings, and others can already be mass-reared artificially and released at the appropriate time. In addition, aphid fungi and the like can also be artificially cultured, diluted, and sprayed. These live biological agents are already available as ready-made commercial products in some areas.

Chemical control  Spray 3% pyrethrin, 2.5% derris extract, or 40% nicotine sulfate; all three of the above can be diluted to 800 to 1200-fold solution. Also 20% dichlorophenyl ether or insecticidal ester 3000 to 4000-fold solution, 40% methamidophos 1000-fold solution, 40% omethoate 2000-fold solution. Spray once every 5 weeks, for 3 to 4 consecutive times. Apply 40% omethoate 100-fold solution to the stems. Apply 3% carbofuran granules to the roots, which both controls aphids and protects natural enemies.

Brown Soft Scale  Brown soft scale, also known as wax scale, belongs to Homoptera, Coccidae. It is distributed in North China, Northeast China, Central China, Sichuan, Shaanxi, and other places, and is widely distributed in tropical and subtropical regions, Europe and America, Oceania, and many other areas, and has become an important pest of garden plants.

Morphological Characteristics  Female adults are flat, oval, and closely attached to the plant surface; the back color may be light yellow, brown, yellow, brown, etc. Male adults are about 1 mm long. Eggs are long oval, pale yellow, about 0.20 mm long. Nymphs are oval, pale yellow, and become flat and thin after secreting wax.

Living Habits  It occurs four to five generations a year. On plants damaged by scale insects, adults, eggs, and nymphs are almost always present. The number of generations per year varies by location: 2 generations a year in Beijing, 3 in Shanghai, and 4 in Shandong. Scale insects reproduce quickly in warm and highly humid environments. The overwintering forms of scale insects include fertilized females, second-instar nymphs, or eggs, etc., usually overwintering in cracks in branches and trunks. From early June to mid-July and from mid-to-late August to September each year are the peak periods of nymph hatching, and in late March and early April the first-generation nymphs cause damage.

Damage Characteristics  Nymphs mostly parasitize the stems and the bases of tender leaves. Adults gather on branch tips, leaf backs, leaf axils, and other places, living and reproducing in shaded environments, causing poor plant growth and inducing sooty mold. Their excrement is abundant and sticky, and every drop can breed mold; after accumulating into dirty patches, it is not easy to remove, seriously affecting ornamental value.

Control Methods  Horticultural control: early manual brushing and removal. When the proportion of affected plants is low, insect density is not high, and the area is small, spraying laundry detergent water or manually scraping them off with a brush or hard objects such as bamboo strips can also be effective, as long as the plant's soft bark tissue is not damaged.

Strengthen daily maintenance and management, and do not let branches and leaves become too dense. Prune reasonably; in combination with winter shaping and pruning, remove pest-infested branches and burn them in a centralized manner.

Chemical control  Spray 50% monocrotophos 1000-fold, 40% omethoate 1000-fold, or 50% fenitrothion 800-fold, etc., once every 1 week, for 3-4 consecutive times. Two or three of the three chemicals fenitrothion, omethoate, and monocrotophos can be mixed. Mixing method: 1 part of A plus 1 part of B, then add 2000 parts water; or 1 part each of A, B, and C plus 3000 parts water. Mixed use is more effective than single use and safer for plants.

Biological control  “Using insects to control insects”: in the use of natural enemy insects, protect the red-ringed ladybug to control scale insects.