What are the common pests and diseases of rhododendrons?

Rhododendron black spot disease, also called brown spot disease, is a common and frequently occurring leaf spot disease of rhododendrons. After infection, black spots form on the leaves, which gradually dry up and fall off until the plant dies. The disease mainly affects rhododendron leaves and is caused by infection with the fungus Cercospora rhododendri. The pathogen first appears as several small brown spots; as these spots continually expand, the lesions join together into patches, producing grayish-brown mold mycelium. The affected leaves begin at the petiole, turn from green to brownish-yellow and drop early, and in severe cases this affects the growth and development and ornamental value of the rhododendron. The pathogen overwinters on leaves or diseased plants, and spores spread with the wind.

The peak period of this disease is generally in hot and humid seasons, and it also occurs in greenhouses in winter. Generally, weaker-growing plants, small seedlings, lower leaves, plants with poor ventilation, and Western azaleas (Rhododendron obtusum hybrids, compared with Rhododendron mucronatum) are more severely affected. According to the pattern of occurrence, the following control measures can be taken:

(1) Strengthen cultivation management. For potted rhododendrons, use leaf mold as the potting soil, keep the pH of irrigation water between 4.5 and 6.5, and water so that the substrate dries and then becomes moist. Regularly apply some well-rotted dilute organic liquid fertilizer to promote vigorous growth and thick leaves and improve resistance. Potted rhododendrons should be placed in a ventilated spot, and in summer they need shading, moisture protection, and prevention of waterlogging. Diseased leaves should be removed promptly and burned, and attention should be paid to selecting disease-resistant varieties while also strengthening plant pest and disease quarantine work.

(2) Chemical control: after flowering, promptly spray 70% thiophanate-methyl wettable powder at 800 times dilution or 50% carbendazim at 300 times dilution. In addition, pesticides can be sprayed regularly for prevention. Protective fungicides and fungicides can be mixed and applied; for example, alternating Polyram with M-Dithane 45 and Kocide can achieve good results, or use Bordeaux mixture at 1:1:150 once a week for 2 consecutive sprays. Once the rainy season begins, spray Bordeaux mixture at 1:1:100 once every 10 days for 3 to 4 consecutive sprays.

Rhododendron chlorosis

Rhododendron chlorosis is a non-infectious disease and is also a common disease in rhododendron cultivation. After the plant develops the disorder, the leaves become thinner, lose chlorophyll, and turn pale, so this disease is also called chlorosis. The lack of green is obvious in new leaves. When the color of new shoot leaves becomes lighter and the leaf mesophyll turns yellow while the veins remain green, this is generally diagnosed as iron deficiency. This phenomenon mostly occurs in alkaline soil and in areas where slightly alkaline water is used to water flowers. After symptoms appear on the leaves, photosynthesis can no longer proceed normally. In mild cases this affects the plant's growth and development and ornamental value, and in severe cases it can lead to plant death. Control measures: strengthen cultivation management, and along with irrigation, apply ferrous sulfate solution; a 0.5% ferrous sulfate solution can be sprayed on the foliage, which can gradually restore the green color of the plant's leaves.

Rhododendron red spider mite

The female adult is about 0.3 mm long, dark red, nearly oval, with a raised back and 4 pairs of dark red legs; the male adult is 0.25 mm long, wedge-shaped, flat and bright red. It occurs in multiple generations a year. Adults overwinter at the base of the plant and mainly damage the underside of leaves near the main vein close to the petiole. In June to July, hot, rain-sparse, and dry weather leads to rapid reproduction and a relatively large population. When damage is severe, the affected leaves scorch and fall off.

Control methods: manual control. After April, inspect frequently; if red spider mites are found on individual leaves, promptly remove the infested leaves and burn them. Where conditions permit, natural enemies such as ladybugs, green lacewings, and thrips can be released artificially for biological control. If red spider mites are found on many leaves, chemical control can be used, such as spraying 40% dicofol at 800 to 1000 times dilution, which has strong killing power against adults, nymphs, larvae, and eggs; as long as it is sprayed evenly, the effect is good.

Rhododendron lace bug

The rhododendron lace bug, also called the rhododendron army-match bug or little stink bug. The adult is about 3.5 mm long, dark brown, with wings covered in a net-like pattern; the forewings are slightly rectangular, and at rest the dark brown markings on the wings form an "X" shape. Nymphs resemble adults but are tender and small and wingless, about 2 mm long. Eggs are 0.6 mm long, oval, milky white, laid at the midrib and larger veins; when first laid they are pale green, later becoming pale yellow.

This insect occurs in 3 to 4 generations a year. Adults overwinter in mid-to-late October in bark and cracks in the soil, and begin activity in April to May of the following year, feeding and causing damage on the underside of leaves. Damage is heaviest from July to August. Adults and nymphs like to gather in groups near the main vein on the underside of leaves and suck the leaf sap. Their secretions form relatively large yellowish-brown rust spots, and in severe cases the leaves turn pale, causing early fall and affecting the growth and flowering of rhododendrons.

Control methods: (1) After winter begins, clear away fallen leaves and weeds near the flowers and trees, bury them deeply or burn them, and eliminate overwintering adults. (2) Chemical control: in May, after overwintering adults appear and during the first-generation nymph occurrence period, spray 80% dichlorvos emulsifiable concentrate or 50% fenitrothion emulsion at 1000 times dilution once every 7 to 10 days for 2 to 3 consecutive sprays. You can also use 3% carbofuran, burying about 5 grams per pot, which also gives very good results.

Rhododendron @#@254@#@, slugs mainly damage the roots. Sometimes slugs also damage the leaves. The growing medium used should be sterilized at high temperature to kill insect eggs. If pests are found, carbofuran or aldicarb can be applied in the pot for control.