Prevention and control methods for common diseases and pests of seedlings in autumn

Autumn brings moderate temperature and humidity, along with good sunlight, creating favorable conditions for the breeding, reproduction, and infection of some pathogens, often causing serious damage to certain flowers and trees. The symptoms and control methods for several common diseases and pests are introduced below:

Brown spot disease

It primarily affects the leaves. In the early stages, brown spots appear on the leaf surface. As the disease progresses, these spots gradually enlarge and merge into large patches, often irregular in shape due to the restriction of leaf veins. A faded halo surrounds the lesions. The edges are reddish-brown, while the center is grayish-white. Later, small black dots (conidiophores) appear within the lesions. In severe cases, the lesions cover the entire leaf. Affected plants easily lose leaves; in severe cases, the entire plant dies, leaving only bare stems, eventually dying from the bottom up. Azaleas, daffodils, jasmine, lilacs, osmanthus, flowering quince, lotus, water lilies, and photinia are among the most severely affected plants.

Control methods: Promptly prune diseased branches and leaves and burn them to reduce pathogens. Before the onset of disease, spray with Bordeaux mixture (1:1:200) every 7-10 days for prevention. In the early stages of disease, spray with 65% zineb wettable powder (500x dilution) or thiophanate-methyl (1000x dilution). Alternatively, drench the roots with 50% carbendazim (200x dilution).

powdery mildew

Powdery mildew is mostly distributed on the upper surface of large-leaved boxwood leaves, but can also grow on the lower surface. Individual lesions are round and white; as they enlarge and merge, they become irregular. When the white powdery mycelium and spore layer are wiped away, the original diseased area appears as a yellow, round spot. In severe cases, up to 100% of new shoots can be infected. Sometimes, diseased leaves become wrinkled, and diseased shoots become twisted, deformed, and withered.

Control methods: ① Proper pruning to improve plant ventilation and avoiding overcrowding during cutting propagation are all important considerations for controlling this disease. ② Chemical control: Spray with 20% carbendazim at a dilution of 2000 times, or 50% thiophanate-methyl or 500 times dilution of carbendazim. Apply immediately when the disease first appears, repeating the spray every 10 days for a total of 4-5 applications.

anthrax

The disease primarily affects the leaves, but can also infect petioles, young branches, stems, and flower buds and petals. Initially, infected leaves show small, round, pale white, water-soaked spots that gradually enlarge, turning grayish-brown with a brownish border and slightly raised. Eventually, the center of the lesion fades to grayish-white to white; this affected area is extremely thin, semi-transparent, and easily perforated. In some cases, the disease starts at the leaf tip and then spreads inwards. Lesions can cover 1/3 to 1/2 of the entire leaf surface, eventually leading to leaf death. Some leaves continuously fall off; severely affected plants may lose all their leaves. When petioles are affected, the affected area turns brown and slightly sunken, spreading towards the leaf veins and eventually the entire leaf. The lesions continue to spread downwards, from the petiole sap to the main petiole, branchlets, and even the stem, causing browning and death. Under humid conditions, a pale red, sticky substance, the spore discs of the pathogen, often appears on the lesions. Affected plants may experience reduced growth and development, losing their ornamental value, or even die completely. Camellias, roses, milan, magnolia, lilies, southern magnolias, nandina, evergreen plants, clivia, cacti, plum blossoms, and orchids are among the flowers most susceptible to this disease.

Prevention and control methods: Strengthen cultivation management, appropriately increase the application of phosphorus and potassium fertilizers, avoid excessive nitrogen fertilizer application, and avoid overwatering and overly wet potting soil. At the same time, improve ventilation and light penetration, and reduce humidity. Immediately prune and burn any diseased leaves and petioles, and promptly remove any dead plants. When the disease occurs, spray with 50% carbendazim wettable powder at a dilution of 500 times, or 70% thiophanate-methyl at a dilution of 1000 times, or 75% chlorothalonil at a dilution of 800 times, once every 10-15 days, for 2-3 consecutive applications. Alternating pesticides can improve the control effect.

Camphor Green Swallowtail

It primarily damages camphor trees, nanmu trees, laurel trees, magnolia trees, Michelia champaca, Cinnamomum camphora, avocado trees, and citrus trees. Newly hatched larvae initially eat the eggshell, then feed on the leaf tissue on the underside of tender leaves. After the third instar, their appetite increases significantly, potentially consuming all the leaves. The damage is most severe in 2-3 year old larvae. It seriously affects plant growth, development, and ornamental value.

There are 2-3 generations per year. The larvae overwinter as pupae hanging on the lower branches and leaves of the host plant. Adults emerge successively from mid-April to late May. The overwintering generation and the first to third generations of larvae have larval stages from mid-May to mid-June, early July to mid-August, and late August to late September, respectively. Adults emerge at night and fly in the forest for 1-2 days after emergence, foraging for nectar to supplement their nutrition. Several days later, they mate and lay eggs. One egg is laid at the tip of a young leaf, occasionally two. Each female lays 18-34 eggs. The larvae have five instars, molting every 4 days. The larvae grow larger with each instar, with the fifth instar larvae having the largest appetite, each larva consuming 1-5 camphor leaves per day. Mature larvae crawl behind hidden branches and leaves, securing their tails with silk, and pupate in 2-3 days. The egg stage generally lasts 4-6 days, the larval stage about 20 days, and the pupal stage of the overwintering generation about 90 days. The optimal temperature for larval development is 20-28℃.

Control methods: ① During the adult egg-laying period, inspect the camphor seedlings in the nursery and promptly remove eggs laid on the tips of the seedlings. ② In the early stage of larval emergence, spray with a 1500-fold dilution of 40% methamidophos emulsifiable concentrate. ③ In conjunction with winter management, remove overwintering pupae.

Crape myrtle scale

It mainly damages crape myrtle, pomegranate, and mulberry trees. Adults and nymphs pierce and suck honeydew from bud axils, leaves, and branches, causing the branches and leaves to turn black, leaves to fall off, affecting growth, and losing ornamental value.

There are 2-3 generations per year. The nymphs overwinter in crevices of branches and trunks. They continue to feed and cause damage the following year. Mature male nymphs pupate and emerge as adults in April, mating with females and dying shortly after mating. Adult females begin laying eggs in early May. The first generation of nymphs hatches from late May to early June. The second generation of adult females lays eggs in large numbers in early to mid-August. Nymphs hatch in large numbers from late August to early September, developing unevenly. There is overlap between generations. The insects are mostly distributed in crevices and depressions of branches and trunks. A few are found on the base of the underside of leaves, and are more abundant on the shady side of branches than the sunny side.

Control methods: ① Use bamboo strips or a brush to scrape off infested branches and trunks. ② During the late hatching stage of nymphs, spray 2-3 times consecutively, using 25% phosmet at a dilution of 600-800 times or 40% chlorpyrifos emulsifiable concentrate at a dilution of 1500-2000 times. ③ Combine pruning with the removal and burning of infested branches.

Stinging moth

There are several species of tussock moths, including the yellow-spotted tussock moth, the brown-edged green tussock moth, and the flat tussock moth. They mainly damage trees such as Chinese tallow tree, poplar, willow, elm, black locust, cherry blossom, red maple, crabapple, wintersweet, crape myrtle, osmanthus, and large-leaved boxwood.

There are two generations per year. The larvae overwinter in cocoons in the shallow soil. Adults are nocturnal and fly during the day, and are attracted to light. Adults mate and lay eggs. Dozens of eggs are usually laid in clusters on the underside of leaves, arranged in a fish-scale pattern. The incubation period is 5-7 days. Newly hatched larvae do not feed. After the second instar, they feed on molted skin and leaf tissue. After the third and fourth instars, they eat through the leaf epidermis. After the sixth instar, they feed from the leaf margin inwards.

Control methods: ① Based on the gregarious nature of newly hatched tussock moth larvae and the easily visible transparent necrotic spots on affected leaves, manpower can be organized to remove infested leaves. ② Use light traps to kill various adult tussock moths and prevent them from laying eggs. ③ Spray pesticides before the tussock moth reaches the 3rd instar larvae. Use 20% diflubenzuron suspension at a dilution of 10,000 times, 50% phoxim EC at a dilution of 2,000-3,000 times, or 50% malathion EC at a dilution of 1,000 times. ④ Combine winter pruning with the removal and burning of pupae and cocoons, and with measures such as loosening the soil around the tree and applying fertilizer, dig out other underground tussock moth cocoons to eliminate them.

Boxwood inchworm

It mainly damages trees such as silk cottonwood, boxwood, euonymus, elm, poplar, and willow. The larvae swarm to feed on the leaves, and after consuming all the leaves, they gnaw on the bark of the tender branches, causing the entire tree to die.

It has 2-3 generations per year, overwintering as pupae. Adults hide in foliage during the day and are active at night, mating and laying eggs. Eggs are laid on the underside of leaves, arranged in double rows or clusters, with dozens to over a hundred eggs per cluster. Eggs hatch mostly between 5:00 and 8:00 AM, with larvae congregating on the undersides of leaves to feed on leaves and the bark of tender branches. Mature larvae spin silk and fall into the soil to pupate. Adults are not strong fliers but are strongly attracted to light.

Control methods: ① During the adult emergence period, use light traps to kill them. ② Take advantage of the adults' weak flying ability and their tendency to concentrate on the upper and lower parts of parasitic plants in the early morning and evening, using simple tools to capture and kill them. ③ During the larval stage, spraying with 50% phosmet emulsion at a dilution of 200 times, 50% fenitrothion emulsion at a dilution of 500 times, or 25% carbaryl wettable powder at a dilution of 300-500 times is effective. ④ During the peak period of adult egg laying, remove egg masses and newly hatched larvae from weeds around the roots. Tilling the soil around the roots in winter can kill overwintering pupae.