Resting spores can remain in the ground for several years, so care should be taken when removing dead plants and the associated soil to ensure that this is all disposed of, and fresh soil brought in. To reduce the incidence of honey fungus, it is worth considering stump grinding and removal of any felled or dying trees, especially on damp heavy soils. For further advice, please refer to up to date recommendations from the RHS: Some varieties are prone to azalea gall, which is why we no longer grow Azalea Rosebud! See 3.2 watering for further details. Be aware of new notching on the leaves in your garden, and treat using nematodes if the soil temperature is above 10°C. Provide adequate soil moisture by irrigation where dry soil is a problem. This is where the leaves turn yellow but the veins remain green. There are no chemical cures, so care should be taken to make sure that drainage is improved to prevent the spread of this disease. In all cases, the damage is purely cosmetic, and it will not harm the plant. Frost damage shows as brown markings on large sections of the leaf. The powdery white fungal spores can sometimes be seen on the leaf underside. Avoid irrigation on bright hot days – it is much more effective in the early morning or evening. This could be due to too much fertilizer or too strong a compost such as horse manure. When the tip of the leaf is burnt brown, this usually indicates that too much salt (too much feed) has been applied. These fungal diseases have received a lot of publicity in recent years, and in USA are known as Sudden Oak Death. Reduce humidity by avoiding evening watering which can leave plants wet all night. To control bud blast it is best to pick off and destroy as many infected buds as possible, clear out dead branches, and prune back other trees and shrubs to allow free air movement in and around the plant. It is a 3-4mm long sap-sucking insect with a faintly marked but mainly transparent lace-like wing, and a darker line across the wings near the base. See 3.3 Feeding for more information. of water per week. Nowadays, the most prone varieties are not grown, and plants seem to have developed greater resistance so the disease is not the problem it once was. Galls tend to form in early spring and picking them off before they turn white and infectious is recommended to prevent spreading. This is likely to be a problem after wet winters, or when planting in damp locations, especially when planting too low in the soil. Recognition is difficult and can be confused with other diseases, and even normal winter maturity and change of stem colour. Preventative clearing of all R. ponticum, and any hybrids which are closely related, is certainly recommended to prevent this devastating disease. The disease can also be prevented by ensuring good air movement, better spacing, removing overhanging branches, and not planting in still, humid corners of the garden. Excess water during the growing season can also distort the cells as they develop. Powdery mildew fungi produce microscopic air-borne dispersal spores which have a high water content, enabling them to infect under drier conditions. Phytophthora spreads by tiny spores that swim in water and wet ground. There is little doubt that ramorum dieback is mainly associated with R. ponticum (the mauve ‘wild’ rhododendron), especially where it has become overgrown. Infection is spread in damp wet conditions and develops in warm humid conditions. They are rare in private gardens, but worse in areas that have warm and humid summer conditions such as the Western regions of the British Isles. Treatment is best carried out in early summer when the newly hatched nymphs are present, but further treatment may be required. Shoots become darkly discoloured, and the blackening extends into the leaf stalk, and through the leaf. See 4.3.1 Bud blast. For instance the Loderi varieties can take up to six years to flower, and many of the rare big leaved species may need to be about 2 metres high before they show their first bloom. Alternatively use a chemical vine-weevil killer drenched into the MOIST root-ball to kill grubs in the soil from September to May. However, caterpillar notchings are more gentle and not as jagged, and sometimes even in the middle of the leaf. Some feeds are high in Nitrogen which gives plenty of growth, but no flower. This will disturb the brightly coloured leafhoppers which will hop onto a nearby leaf, a bit like a grasshopper. This is usually terminal and the plant collapses, often quite quickly in the spring. The foliage may look dull and downward pointing, and the plant may lack energy. Using just the naked eye, it is not possible to tell if a leaf spot has a fungal or a bacterial origin, but under magnification, tiny dot-like bodies associated with the lesions would indicate fungal spore-bearing structures. To prevent winter rhododendron stress burn, take good care of the bushes during the growing season. For example, a number of gardens in Sussex and Kent had too much shade until the great storms of 1987 and 1991 which took out many tall trees, but this did enable rhododendrons to flower properly for the first time. The insertion of a vertical barrier of heavy grade polythene to the depth of 45cm can contain any further spreading. This is a fungal disease which disfigures the leaves of evergreen azaleas and is spread by airborne spores. Take special care in May-June and August-September when average temperatures are about 15°C and humidity is high. How to Reduce Bee Poisoning from Pesticides, Alfalfa Leafcutting Bee (Megachile rotunda) Pests, Horticultural, Landscape, and Ornamental Crops, Field Characteristics of Fruit-Tree-Attacking Spider Mites in the Pacific Northwest, Pests of Cabbage and Mustard Grown for Seed, Pests of Rutabaga and Turnip Grown for Seed, Biology and Control of the Garden Symphylan, Bean, Dry Cultivar Resistance to Bean Common Mosaic, Grape (Vitis spp.) This can account for what seems like ‘random’ failures at good times of the year. Millais Nurseries. Copyright 2020. For further details and an up to date list of chemical controls, please refer to the RHS website:, Leaf-hoppers are easily found on affected plants from July to August, by brushing against rhododendron foliage. By ensuring that your plants are moist in late June and early July, you are more likely to initiate flower buds rather than growth buds. Insects develop from the eggs and excrete onto the upper surface of leaves below, creating sooty mould, a fungus which develops over the winter months especially under greenhouse conditions.

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