What are the common disease control measures for rhododendrons?
The common diseases of rhododendrons and their control measures can be grouped into the following categories:
1. Rhododendron gray mold
Symptoms
Gray mold occurs on the leaves and flowers of rhododendrons. Freeze damage is often the trigger for leaf infection. In spring, the flowers of rhododendrons grown indoors and outdoors are often infected with gray mold. In the early stage of the disease, necrotic spots appear on the petals, spread quickly, and connect with each other to form large lesions. Under conditions of high humidity, the diseased parts produce a large amount of gray conidial layers, which is how gray mold got its name.
Pathogen
Botrytis cinerea [BotrytiscinereaPers.exFr.] belongs to the subphylum Deuteromycotina. The fruiting bodies of the pathogen arise from mycelium or sclerotia. The conidiophores are clustered, gray, later turning brown, and the conidia are oval. Its sexual stage is Botryotinia fuckeliana [Botryotinia****eliana(deBary)Whetze], which belongs to the subphylum Ascomycotina.
2. Rhododendron brown spot
Symptoms
Rhododendron brown spot is also called leaf spot. At first, small pale yellow round spots appear on the leaves, gradually expanding into irregular shapes; the lesions turn light reddish brown, with a dark brown center. Diseased leaves drop prematurely, and in severe cases plant growth is stunted. Upon examining diseased leaves, small black grains can be found on the lesions; these are the pathogen's pycnidia. Rhododendrons grown in greenhouses are severely affected, and the disease is worse under high humidity.
Pathogen
Septoria azaleae [SeptoriaazaleaeVogl.] belongs to the subphylum Deuteromycotina. The fungus overwinters as mycelium and pycnidia in the tissues of the host plant or on fallen leaves, and the following year a large number of conidia are produced on diseased leaves and spread to cause damage.
Control methods
In winter and spring, promptly sweep up and burn fallen leaves. After the plants leaf out, spray Bordeaux mixture (1:1:100) once every half month, for 2-3 consecutive sprays, to prevent the disease. In the early stage of the disease, spray 50% thiophanate-methyl wettable powder at 1000 times dilution 1-2 times to inhibit the development of the disease.
3. Rhododendron iron deficiency chlorosis
Rhododendron iron deficiency chlorosis, also called yellow leaf disease or chlorosis, is a common disease of potted rhododendrons in various regions.
Symptoms
This disease mostly occurs on the young leaves of new shoots. In the early stage, the mesophyll between the veins loses its green color and luster, then gradually turns yellow-white, but the veins remain green, making the green on the leaves appear net-veined. Subsequently the degree of yellowing gradually intensifies; except for the larger veins, the whole leaf turns yellow or yellow-white, and in severe cases, the tissue along the leaf and leaf margin scorches inward.
Causes
Rhododendron iron deficiency chlorosis is a physiological disease, mainly caused by a lack of iron in the soil or the inability of iron to be absorbed and utilized, thereby affecting the synthesis of chlorophyll and turning the leaves yellow-green. In general, there are the following situations: in calcareous alkaline soil, the available soluble ferrous iron is converted into insoluble ferric salts and precipitates, so the roots cannot absorb it; frequent watering of potted flowers causes excessive leaching and loss of soluble iron in the soil; in areas with heavy clay soil, poor drainage or an excessively high water table, the development of the plant's root system is affected, the normal physiological activities of the roots cannot proceed, and the roots' ability to absorb iron is reduced.
Control methods
Rhododendrons prefer acid and fear alkalinity, so avoid planting them in alkaline soil or soil containing a large amount of calcium; when planting in open garden beds, do not place them near cement, brick walls or places where lime has been used. Potted rhododendrons should use acidic soil; if the soil is on the alkaline side, add or replace it with acidic soil. For rhododendrons planted in nursery beds, apply compost, green manure or other organic fertilizers; the organic acids produced by these fertilizers can dissolve the insoluble iron in the soil, making it easier for the plants to absorb. Alternatively, mix ferrous sulfate into the fertilizer and apply it; the preparation is: ferrous sulfate 2.5-3 jin, oil cake 5-7.5 jin, manure 10-15 kg, water 200-250 jin; after mixing, wait until it decomposes and turns black before use. This method gives good results. In slightly alkaline soil, water with a 0.1-0.2% potassium dihydrogen phosphate solution; this solution has a pH of 4.7 and can turn alkaline soil into acidic soil, which can treat the yellowing of young leaves; at the same time, potassium dihydrogen phosphate also contains phosphorus and potassium, which help flowering and increase flower quantity.
Frequently watering with ferrous sulfate solution will cause excessive sulfur and available iron in the soil and poison the plants. To avoid this, dissolve 0.14 g of disodium edetate and 0.1 g of ferrous sulfate (chemically pure) in 500 ml of tap water and spray it on the foliage, covering both the front and back of the leaves, once every 3 days for several consecutive times, which can achieve good results.