If growing plants indoors, avoid high humidity to reduce spores in the air. All plants that we produce are treated against vine weevil using natural nematodes at the propagation stage, and all our potting compost is treated to kill any grubs in the pots. Bark split occurs when the stem is full of sap and freezes hard, expanding to crack open the bark, like water in bottle. 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. This is where the leaves turn yellow but the veins remain green. In severe cases, a reddish brown discolouration can be found when cutting into the stem at the base of the plant. Rhododendrons can be prone to a variety of different leaf spots, and plant collectors have noted these even in the wild. 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. Recognition is difficult and can be confused with other diseases, and even normal winter maturity and change of stem colour. Since these leaf parts are the last to be supplied with water from the roots, they are usually the first to be affected. Adults, cast skins, and brown excrement can be found on the undersides of affected leaves. 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. Take special care in May-June and August-September when average temperatures are about 15°C and humidity is high. Use a good organic or chemical insect spray after the flowering season. It is spread by leaf hoppers (see 4.2.5) which are a pale green insect that appears between June and September. Most people recommend Iron Sequestrine, but this is an expensive product and we would recommend Maxicrop with Iron for a quick improvement, followed by Iron Sulphate to give some longevity. Many are cosmetic, and whilst they may be unsightly, they cause little long-term damage. 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. The damage is not likely to destroy the entire rhododendron. These insects do not cause any damage whilst feeding, but the trouble comes in the autumn when they lay their eggs in next year’s flower buds. Powdery mildew is associated with water stress and is characterized by dark brown or black spotting on the upper and lower surfaces of the leaf. All rights reserved. Cause Leaf scorch on rhododendrons is a response to stress. Leaf scorch can be caused by incorrect nutrients, see 4.1.2 Nutrient Imbalances, or by the weather. These diseases are Notifiable to APHA (Animal and Plant Health Agency), and more information can be found on the DEFRA website. However, sometimes the leaves show that they are not as healthy as they should be, and hopefully this section will enable you to make corrections. Frost damage to flower buds can be reduced by covering the plant with fleece which will save 2-3°C. Symptoms Symptoms include browning of tips or margins of leaves, with the damage sometimes spreading to the center of the leaf. This is especially a problem after rainfall or irrigation. The incisions into the bud, allow the fungal disease bud blast (See 4.3.1) to develop, which prevents flowering next spring. Millais Nurseries. Deciduous azaleas with R. occidentale parentage can be prone to powdery mildew in early September. For further details and an up to date list of chemical controls, please refer to the RHS website: https://www.rhs.org.uk/advice/profile?pid=253. Look out for tell-tale notching on the edge of Rhododendron leaves. If this happens regularly, it may be best to select a hardier variety more suited to your locality. Several applications may be needed to control different generations from June to September. This notching is usually very indented like a mini oxbow lake and often about 3-4mm in diameter, though sometimes more extensive. Many rhododendrons originate from monsoon areas of the Himalayas and are used to really moist conditions during June and July. Rings of chicken wire around vulnerable plants or new borders for a few years will normally offer sufficient protection. Spores often spread from infected fallen leaves, which mature in spring, ready to be splashed onto newly emerging leaves. Caterpillars can feed on rhododendrons, sometimes dropping off overhanging trees onto the foliage, where they make notches on leaves which can resemble vine weevil damage. Chlorosis of the leaf can also be caused by poor conditions such as drainage problems, soil pH or drought, so check these first. Some feeds are high in Nitrogen which gives plenty of growth, but no flower. Please refer to the RHS website for the most up to date list of chemicals available. For further details and an up to date list of chemical controls, please refer to the RHS website: https://www.rhs.org.uk/advice/profile?PID=663, Cushion scale is a sap-sucking insect that attacks the leaves of evergreen plants such as rhododendron and camellia. The adult is a hard-shelled black beetle about 9mm long, which lives under leaf litter in hedges and under plants. Our Nursery has four inspections each year and by operating a tidy and hygienic nursery, we have a clean bill of health. These fungal diseases have received a lot of publicity in recent years, and in USA are known as Sudden Oak Death. The best time for preventative treatment is April (before hatching) and early September (before winter). If the flower buds formed in the autumn, but turned brown during the winter, it suggests that you are growing a too tender variety and they may have been frosted. We also treat for adults during the summer months, and we regularly apply millions of nematodes to our crops from spring until autumn. https://www.gov.uk/guidance/disease-control-in-flowers-and-shrubs. We make a point of only growing varieties which are normally resistant to powdery mildew, and this has been very successful in keeping our nursery and gardens clean. 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. Some varieties are prone to azalea gall, which is why we no longer grow Azalea Rosebud! Please refer to the RHS website for more information. Further information can be found on the RHS website: https://www.rhs.org.uk/advice/profile?PID=740. The first cold frost in the autumn after an ‘Indian Summer’ can also cause severe bark split in stems of less hardy varieties.

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