Common diseases and pests of azalea and their control methods are as follows:

The harm and prevention of brown spot disease

Also known as leaf spot disease, it mainly occurs on the leaves of azalea. The infected leaves initially show small reddish-brown spots, which then gradually expand into nearly circular or polygonal spots and are dark brown. Later, the center of the spots turns yellowish-white or grayish-white, and the edges are dark brown, but there are differences between varieties. Many black or grayish-brown small particles and mold threads grow on the surface of the spots. In severe cases, the spots are connected to each other, causing the leaves to turn yellow and fall early. This disease is caused by infection by Cercospora ericacea, and the pathogen overwinters on diseased leaves or diseased residues.

The temperature rises in the spring of the following year and the disease spreads with the wind. Generally, the disease starts in April and May and stops spreading in December. Azaleas in greenhouse pots can be infected all year round. High temperature and high humidity conditions are conducive to the occurrence of the disease. Azaleas growing in slightly alkaline and poorly drained clay soil are prone to the disease. Western azaleas are more susceptible to this disease than spring azaleas.

Prevention and control methods : Always keep the soil moist and loose to prevent water accumulation; apply ferrous sulfate frequently to maintain soil acidity; promptly remove diseased leaves and fallen leaves and burn them in a concentrated manner; maintain ventilation and light transmission between plants, and reduce humidity to eliminate the conditions for disease; when the plants are unfolding their leaves, spray 1:1:120 Bordeaux mixture once every half a month, and spray 3 to 4 times in a row to see results; in the early stage of the disease, spray 1000 times of 70% methyl thiophanate or 500 to 800 times of 50% carbendazim wettable powder once a week for 2 to 3 times in a row to effectively kill the disease.

The harm and prevention of leaf swell

Also known as cake disease, it is one of the common diseases of azalea, mainly harming the tender leaves, new shoots, young buds and flowers of azalea. In the early stage of infection, light green, sunken, nearly circular spots appear on the diseased leaves, the back of the leaves is slightly light red, and the leaves are thick and heavy. Then, spherical raised galls are formed, which are yellow-brown to reddish-brown with white powder on them, and finally cause the diseased leaves to wither and fall off.

When young shoots are infected, they become spherical galls. When flowers are infected, petals become thick and fleshy, waxy, and irregularly shaped galls. When new shoots are infected, galls or clusters of thick leaves are formed at the top, often causing dead branches. There are three types of pathogens: Exobasidiomyces ericaceus, Exobasidiomyces hemispherica, and Exobasidiomyces japonicus. The pathogen overwinters in the diseased tissues of the plant and spreads with the help of air currents in spring. It likes low temperatures and high humidity, and generally occurs and spreads when the average monthly temperature is 15~20℃ and the relative humidity is above 80%. Therefore, this disease is prone to occur when it is rainy, there is insufficient sunlight, the plants are densely planted, and ventilation is poor. The disease is severe from March to May every year.

Prevention and control methods : promptly remove and destroy diseased leaves and branches to prevent the spread and spread of the disease; after moving azalea out of the greenhouse in spring, it should be placed in a ventilated and light-transmitted place for maintenance, and should not be placed too densely; spray 1% Baume lime sulfur mixture before germination to kill overwintering pathogens; during leaf expansion, spray 2% Bordeaux mixture, or 0.5% Baume lime sulfur mixture 2 to 3 times, and spray once every 7 to 10 days.

The harm and prevention of leaflet disease

It is a common physiological disease that is very harmful to azaleas. The diseased plants sprout late in early spring, and the new leaves grow in clusters at the top of the branches, and the leaves are obviously smaller. In mild cases, new buds grow at the bottom of the diseased plants, and the leaf color is uneven and deformed. In severe cases, the leaves are only about 1/5 of the normal leaves, the leaf surface is yellow-green or the veins are yellow, the leaves are hard and brittle, the leaves do not spread, and the leaf edges curl up. The roots of the diseased plants rot, the growth is weak, it is difficult to form a crown, the leaf buds and flower buds are significantly reduced, the flowers are small or even do not bloom, and sometimes the whole plant withers and dies.

There are three main causes of azalea leaf disease: one is lack of fertilizer, especially nitrogen or zinc; the second is heavy soil and poor drainage, which causes root rot; the third is improper management, especially under high temperature, stuffy and poorly ventilated conditions, the disease is prone to occur.

Prevention and control methods : focus on improving soil structure and careful management. First, the pot soil should not be waterlogged, especially in winter, and watering should be alternating between dry and wet. Deep pots, large pots, and porcelain pots should not be used for potted plants. Suitable unglazed pots should be selected.

After 2 to 3 years of planting, the pot should be changed once, and the time for changing pots is preferably from August to September. Secondly, careful management should be carried out to create the best growth environment to prevent the plants from growing in a hot and humid environment. Maintain 11 to 25 degrees Celsius during the vegetative growth period and 15 to 25 degrees Celsius during the reproductive growth period. Fertilization should be thin and frequent, with nitrogen fertilizer as the main fertilizer in spring and phosphorus and potassium fertilizer as the main fertilizer in autumn. Thirdly, before budding in spring, spray 40% long-acting phosphorus 2000 times liquid once every half a month for 3 to 4 times in a row, which can not only prevent and control small leaf disease, but also effectively prevent and control pests. After autumn, 0.05% zinc sulfate 1.5 grams per square meter of soil can be sprayed.

Harm and prevention of Phytophthora root rot

The root system and rhizomes of azalea are infected. In the early stage of the disease, the nutrient roots first necrotize, the aboveground parts grow poorly, the leaves expand later than normal plants, the leaves become smaller, dull, and yellow, and the old leaves fall off prematurely. The number of shoots is small, the new shoots are thin and short, and are obviously thinner than healthy plants. The main roots and rhizomes are infected and turn brown and rotten, the epidermis often peels off, the aboveground leaves wilt and droop, and even all die. The pathogen is camphor phytophthora.

The pathogen overwinters in diseased remains or soil as thick-walled spores and oospores, and is spread by water flow, diseased soil and diseased seedlings. The disease can occur at soil temperatures ranging from 15 to 28°C, with 22°C being the most suitable. Soil temperature is a key factor in the severity of the disease. Poor soil drainage or severe waterlogging can easily aggravate the disease.

Prevention and control methods : timely remove diseased plants during the growing season to reduce the source of infection, thoroughly remove diseased remains, remove contaminated soil, and disinfect; improve cultivation conditions and choose fertile soil with good drainage to cultivate azaleas. Bark compost can be used as a substrate for soilless cultivation.

The mixed matrix of bark and river sand has a significantly lower incidence rate than the mixed matrix of peat and sand; the soil and matrix can be disinfected by sun exposure, hot steam, chemicals, etc. Commonly used chemicals are dichlorodiphenyltrichloroethane, chlorpyrifos, etc., which can be applied by irrigation or soil mixing; before the disease occurs, spray 600 times of 27% copper suspension or 700 times of 47% carrenon wettable powder; in the early stage of the disease, spray 500 times of 70% phosphite·manganese zinc wettable powder, or 800 times of 69% Anke·manganese zinc wettable powder.

Harm and prevention of iron deficiency chlorosis

It is a common physiological disease in azalea cultivation, mostly occurring on young branches and new shoots. In the early stage of the disease, the mesophyll of new leaves turns green and yellow, while the veins remain green, forming green net-like leaves. As the disease worsens, the entire leaf turns yellow-white except for the larger veins. In severe cases, the leaf tip and leaf edge become scorched inward, causing the leaves to fall off early. The main cause of the disease is the lack of essential iron in the soil, which prevents the synthesis of chlorophyll, resulting in yellowing of the plants.

Prevention and control methods : Regularly supplement iron elements. You can use 0.2%~0.3% ferrous sulfate for foliar spraying, or apply alum fertilizer water once every 20 days. The configuration method is 2.5~3 kg of ferrous sulfate, 5~7.5 kg of bean cake, 10~15 kg of manure, and 200~250 kg of water. It can be used after about 1 month of decomposition, and then diluted 1 time when used; it can also be made by mixing 1 part of ferrous sulfate, 5 parts of cake fertilizer or hoof slices, and 100 parts of water; plant azaleas in loose and fertile acidic soil.

The harm and prevention of leaf blight

It mainly occurs on old leaves of rhododendron, but can also harm young leaves. The disease often occurs at the tip or edge of the leaf, forming an irregular leaf edge and causing the tip of the leaf to dry up, with severe cases reaching 1/2 to 1/3 of the leaf.

The lesions are yellow-brown, with a clear boundary between the diseased and healthy parts, slightly darker edges, and small black spots on them. The pathogen mainly invades the leaves through wounds. Plants that are weak due to insect damage or iron deficiency have more serious diseases. Most of the leaves of seriously infected plants fall off, resulting in weak growth of the plants, poor development of flower buds, and even affecting the number of flower buds in the next year.

Prevention and control methods : Remove diseased leaves and destroy them in a centralized manner, while maintaining ventilation and light transmission to avoid excessive humidity; during the disease period, spray 500~600 times diluted 70% mancozeb or 2000~3000 times diluted 25% dichlorvos.

The harm and prevention of military insects

Also known as the crown web bug. Its nymphs and adults gather on the back of the leaves of azalea to suck sap, causing small white spots on the front of the leaves. In severe cases, white spots appear or the entire leaf turns white, and the back of the leaf turns rusty red, which seriously affects the photosynthesis of the leaves, slows the growth of the plant, and affects the ornamental value. The military bug has about 10 generations a year, and the generations overlap. It can be seen almost all year round, and the most serious damage is from July to September.

Prevention and control methods : In the early stage, when there are a small number of insect pests, they can be killed manually, or the insect-infested leaves can be picked off and destroyed, and fallen leaves or weeds can be removed in time to eliminate the overwintering insect source; choose a cool and ventilated environment to cultivate rhododendrons; when a large number of pests occur, use 80% DDT emulsifiable concentrate or 50% zinc thiophos emulsifiable concentrate, diluted 1000~1500 times, or use about 5 grams of 3% furadan granules per pot, and apply it into 5 cm potting soil in 3 holes; you can also use 2000 times diluted 40% dimethoate emulsifiable concentrate, 2000 times diluted 50% carbaryl wettable powder, 500~700 times diluted 40% long-lasting phosphorus emulsifiable concentrate, and 2000~4000 times diluted 10% imidacloprid wettable powder for spraying, once a week, and it will be effective after 2~3 consecutive times.

The harm and control of Sawfly

It is the main pest of azalea. The larvae start feeding 3 to 5 hours after hatching, first feeding from the leaf edge near the base of the petiole, and gradually eating the entire leaf, leaving only the main vein or part of the leaf tip. The larvae like to eat well-grown young leaves. The larvae of old trees burrow into the soil to make cocoons, and some also make cocoons and pupate in the leaf litter on the surface. This attack occurs more often in azaleas in a humid environment, poorly managed, and overgrown with weeds, but rarely in potted azaleas.

Prevention and control methods : Clear weeds around azaleas in time to reduce the possibility of insect pests; spray 5000 times diluted 50% cypermethrin emulsifiable concentrate, 1000 times diluted 80% dichlorvos, and 5000 times diluted 2.5% cypermethrin during the larval stage.