Harm and prevention of brown spot disease

Also known as leaf spot disease, this mainly occurs on the leaves of azaleas. Infected leaves initially show small reddish-brown spots, which gradually expand into nearly circular or polygonal lesions, turning dark brown. Later, the center of the lesions turns yellowish-white or grayish-white, with dark brown edges, though this varies between varieties. Numerous small black or grayish-brown dots and mold filaments appear on the surface of the lesions. In severe cases, the lesions merge, causing the leaves to wither and fall prematurely. This disease is caused by *Cercospora azalea*, which overwinters on diseased leaves or plant debris.

The disease spreads with the wind as temperatures rise the following spring. It typically begins to appear in April or May and continues until December when its spread ceases. Potted azaleas in greenhouses can be infected year-round. High temperature and humidity conditions favor disease development. Azaleas grown in slightly alkaline, poorly drained clay soils are more susceptible. Western azaleas are more prone to this disease than spring azaleas.

Prevention and control methods : Keep the soil moist and loose to prevent waterlogging; apply ferrous sulfate regularly to maintain soil acidity; promptly remove and burn diseased and fallen leaves; maintain ventilation and light penetration between plants to reduce humidity and eliminate conditions conducive to disease development; when the plants are unfolding leaves, spray with Bordeaux mixture (1:1:120) once every half month for 3-4 consecutive times to see results; in the early stages of the disease, spray with 70% thiophanate-methyl at 1000 times dilution or 50% carbendazim wettable powder at 500-800 times dilution once a week for 2-3 consecutive times to effectively kill the disease.

Damage and prevention of leaf swelling disease

Also known as cake disease, it is one of the common diseases of azaleas, mainly affecting the tender leaves, new shoots, young buds and flowers of azaleas. In the early stage of infection, the diseased leaves show light green, sunken, nearly circular lesions, with a slightly light reddish underside, and become thick and heavy. Subsequently, spherical galls are formed, which are yellowish-brown to reddish-brown, with white powdery substances on the surface, eventually causing the diseased leaves to wither and fall off.

When young buds are infected, they develop spherical galls. Infected flowers develop thickened, fleshy, waxy, irregularly shaped galls. Infected shoots develop galls or clusters of thickened leaves at the tips, often leading to branch dieback. Three pathogens are involved: *Exobasidiomyces rhododendron*, *Exobasidiomyces hemisphericum*, and *Exobasidiomyces japonicum*. The pathogens overwinter in diseased plant tissues and spread in spring via air currents. They thrive in low temperatures and high humidity, generally occurring and spreading when the average monthly temperature is 15-20°C and the relative humidity is above 80%. Therefore, this disease is more likely to occur in rainy, sunny, densely planted, and poorly ventilated conditions, with severe outbreaks occurring from March to May each year.

Prevention and control methods : Promptly remove and destroy diseased leaves and branches to prevent the spread of the disease; after azaleas are moved out of the greenhouse in spring, they should be placed in a well-ventilated and well-lit place and should not be placed too densely; before sprouting, spray with 1% Baume lime sulfur solution to kill overwintering pathogens; during the leaf expansion period, spray with 2% Bordeaux mixture or 0.5% Baume lime sulfur solution 2-3 times, once every 7-10 days.

Harm and prevention of little leaf disease

This is a common and relatively harmful physiological disease of azaleas. Affected plants sprout late in early spring, with new leaves clustered at the branch tips and noticeably smaller. In mild cases, new buds sprout from the lower part of the plant, with uneven leaf color and deformation. In severe cases, leaves are only about 1/5 the size of normal leaves, with a yellowish-green surface or yellow interveinal areas, a hard and brittle texture, and fail to unfold, with the leaf margins curling back. Root rot occurs in affected plants, growth is weak, it is difficult to form a crown, leaf and flower buds are significantly reduced, flowers are small or even fail to bloom, and sometimes the entire plant withers and dies.

There are three main causes of azalea small leaf disease: first, nutrient deficiency, especially nitrogen or zinc deficiency; second, heavy, poorly drained soil, which causes root rot; and third, improper management, especially in hot, humid, and poorly ventilated conditions, which makes the disease more likely to occur.

Prevention and control methods : The key is to improve soil structure and manage carefully. First, the potting soil should not be waterlogged, especially in winter; watering should alternate between allowing the soil to dry slightly and then allowing it to become wet. Deep pots, large pots, and ceramic pots are not suitable for potted plants; suitable unglazed pots are preferable.

Repotting is necessary every 2-3 years, ideally in August or September. Secondly, careful management is crucial to create the optimal growing environment, avoiding hot and humid conditions. Maintain a temperature of 11-25℃ during the vegetative growth period and 15-25℃ during the reproductive growth period. Fertilize frequently but in light amounts, primarily with nitrogen fertilizer in spring and phosphorus and potassium fertilizer in autumn. Thirdly, before spring budding, spray with a 2000-fold dilution of 40% phosmet every two weeks for 3-4 consecutive times. This will prevent small leaf disease and effectively control pests. In autumn, spray 1.5 grams of 0.05% zinc sulfate per square meter of soil.

Damage and control of Phytophthora root rot

This pathogen infects the roots and rhizomes of azaleas. In the early stages, the nutrient roots die first, leading to poor growth of the above-ground parts. Leaf expansion is delayed compared to healthy plants, and the leaves become smaller, dull, and yellow, with older leaves prematurely fading and falling off. There is a reduction in the number of shoots, and new shoots are thin and short, significantly smaller than healthy plants. The main root and rhizome turn brown and rot, with the epidermis often peeling off. The leaves wilt and droop, eventually leading to the complete death of the plant. The pathogen is *Phytophthora camphorata*.

The pathogen overwinters as chlamydospores and oospores in diseased plant debris or soil, and is spread by water flow, diseased soil, and infected 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 exacerbates the disease.

Prevention and control methods : During the growing season, promptly remove diseased plants to reduce the source of infection, thoroughly remove diseased plant debris, and clean up contaminated soil, then disinfect it; improve cultivation conditions, and choose well-drained, fertile soil for azalea cultivation. For hydroponics, bark compost can be used as a substrate.

The disease incidence rate was significantly lower in a mixture of bark and river sand than in a mixture of peat and sand. Soil and substrate can be disinfected by sun exposure, steam, or chemical agents. Commonly used agents include dichlorvos and chlorpyrifos, which can be applied by irrigation or soil mixing. Before the onset of disease, spray with a 600-fold dilution of 27% copper hydroxide suspension or a 700-fold dilution of 47% carbendazim wettable powder. In the early stages of disease, spray with a 500-fold dilution of 70% fosetyl-aluminum wettable powder or an 800-fold dilution of 69% mancozeb wettable powder.

Harm and prevention of iron deficiency chlorosis

This is a common physiological disease in azalea cultivation, mostly occurring on tender shoots and new branches. In the early stages, the leaf tissue of new leaves turns yellow while the veins remain green, forming a green reticulated pattern. As the disease progresses, except for the larger veins, the entire leaf turns yellowish-white. In severe cases, scorching occurs from the leaf tip and margin inwards, leading to premature leaf drop. The main cause of this disease is a lack of essential iron in the soil, preventing chlorophyll synthesis and resulting in yellowing of the plant.

Prevention and control methods : Regularly supplement iron. You can spray the leaves with 0.2%~0.3% ferrous sulfate, or apply alum fertilizer solution once every 20 days. The preparation method is to use 2.5~3 kg of ferrous sulfate, 5~7.5 kg of soybean cake, 10~15 kg of manure, and 200~250 kg of water. It can be used after about 1 month of composting. Dilute it by 1 time before use. Alternatively, you can mix 1 part ferrous sulfate, 5 parts cake fertilizer or hoof meal, and 100 parts water. Plant azaleas in loose, fertile, acidic soil.

Damage and control of leaf blight

It mainly occurs on the older leaves of azaleas, but can also damage the younger leaves. The disease often occurs at the leaf tip or edge, forming irregular leaf edges and causing the leaf tip to dry out, sometimes affecting 1/2 to 1/3 of the leaf.

The lesions are yellowish-brown, with a clear boundary between the diseased and healthy parts. The edges are slightly darker and have small black dots on them. The pathogen mainly enters the leaves through wounds. Plants weakened by insect pests or iron deficiency are more severely affected. Severely infected plants lose most of their leaves, leading to weak growth, poor bud development, and even affecting the number of buds for the following year.

Control methods : Remove diseased leaves and destroy them in a concentrated manner. At the same time, maintain ventilation and light, and avoid excessive humidity. During the disease period, spray with 70% mancozeb at a dilution of 500-600 times, or 25% difenoconazole at a dilution of 2000-3000 times.

Harm and control of lace bugs

Also known as the lace bug. Its nymphs and adults congregate on the undersides of azalea leaves, sucking sap, causing small white spots on the upper surface of the leaves. In severe cases, white spots appear or the entire leaf turns white, while the underside turns rust-red. This severely affects photosynthesis, slows plant growth, and diminishes its ornamental value. The lace bug has approximately 10 generations per year, with overlapping generations, and can be seen almost year-round, with the most severe damage occurring from July to September.

Control methods : In the early stages with a small number of pests, manual killing can be carried out, or the infested leaves can be removed and destroyed. Fallen leaves or weeds should be removed in a timely manner to eliminate overwintering sources of pests. Azaleas should be cultivated in a cool and well-ventilated environment. In the case of a large-scale infestation, 80% dichlorvos EC or 50% zinc phosphide EC can be diluted 1000-1500 times, or about 5 grams of 3% carbofuran granules can be applied to each pot in 3 holes in 5 cm of potting soil. Alternatively, 40% dimethoate EC at 2000 times dilution, 50% fenitrothion at 2000 times dilution, 50% carbaryl WP at 500-700 times dilution, 40% phosmet EC at 2000 times dilution, or 10% imidacloprid WP at 2000-4000 times dilution can be used for spraying once a week for 2-3 consecutive times to see results.

Damage and Control of Three-segmented Sawflies

This is a major pest of azaleas. The larvae begin feeding 3-5 hours after hatching, starting from the leaf margins near the base of the petiole and gradually consuming the entire leaf, leaving only the midrib or part of the leaf tip. The larvae prefer to eat healthy, young leaves. Larvae on older trees burrow into the soil to spin cocoons, while others pupate in the leaf litter layer. This pest is more prevalent in clump-forming azaleas in humid, poorly managed environments with abundant weeds, but rarely occurs in potted azaleas.

Control methods : Remove weeds around the azalea in time to reduce the possibility of pest infestation; spray with 50% fenitrothion EC at 5000 times dilution, 80% dichlorvos at 1000 times dilution, or 2.5% deltamethrin at 5000 times dilution during the larval stage.