Westland Resolva Bug Killer). Stop feeding in June. Roots need to breathe. You may need to wait till plants are 10 years old for large leaved species for example. Phythopthora cinnamomii is most active at a soil temperature of 29-22C (68-72F). Most commercial plants are protected by compost insecticide. Scale Insect on rhododendrons and camellias, This is a serious problem in the UK now and getting worse. This unsightly fungal disease is characterised by green, pink or red swellings on the leaves and shoots. Move sunburned varieties into a more shady place if the problem persists. (NOT the same as Sudden Oak Death.). Avoid watering with tap water if it is chalky. The time to spray is June-July/August to stop the insects moving fronm the old growth to the new young growth. 2. If scales are dead, they will be hollow and the coverings will flake off easily. Check hardiness rating and for bark split. Nothing to worry about, not a disease. You might need to spray twice in the season. Rhododendron is planted too deep? Very little can be done about this. The time to spray is June-July/August to stop the young newly hatched crawler insects moving fronm the old growth to the new young growth. 7 in neutral. A serious problem in some areas. Lift the plant. If you bury the roots under soil the plant will die. We use dolomitic limestone on these to bring the pH to around 6 which they prefer. Rhododendron ponticum is by far the worst host for it. Root rot susceptible rhododendron varieties: Glendoick Mail Order 2020-21 October-March, Kenneth Cox's book Rhododendrons and Azaleas a Colour Guide. But try to avoid deep shade. These rapidly enlarge turning the petals into a slimy grey mass with sinks limply onto the leaves below. These munch random shapes into leaves and sometimes holes in the leaves too. Deciduous azaleas are the most wind resistant. Pest and Disease information adapted from Kenneth Cox's book Rhododendrons and Azaleas a Colour Guide. Often they have gone by the time you notice. If it has been in for a long time you may see new roots forming near the soil surface. The effect is most severe on plants grown in full sun and in shade it may not be a problem, where there are more lacebug predators to keep them under control. The smaller the leaf, generally the more wind resistant. There is little doubt that stressed plants (by water logging, drought etc) are more susceptible to the fungus. Often rhododendrons are one of the few evergreen plants which the deer and rabbits can easily reach and so considerable damage such as the loss of leaf tips, branch tips or more seriously, the stripping of the bark, may occur. Lichen tends to grow on weak plants rather than causing the weakness. Replant at the proper level. Roseclear, etc. This is mainly cosmetic but if it keeps happening you may need to move a plant to a more sheltered site or cover it up in frosty weather. 2. Freshly composted bark has been shown to have some root-rot resistant properties. As the insects are on the underside of the leaf, it needs a particular approach,: You need a very high pressure sprayer which makes tiny droplets of spray and add a wetting agent. It can often take 6-9 months before you notice a sudden collapse. Many of the fungicides used to control mildew and rust also seem to have an effect in reducing this disease. Most members of subsections Taliensia (e.g. The disease has been found on a wide variety of plants including Rhododendron, Camellia, Viburnum, Pieris, Kalmia, Taxus, and the symptoms are similar to those of Botryosphaeria die back. It takes 2-3 days for the flowers to be completely ruined and a whole bush or group of plants can quickly be affected. Can be severe in Germany and other countries. Rhododendrons which are not happy tend to show this by their leaves turning yellow. According to the American Rhododendron Society, if the plant has older leaves that have turned yellow between green veins, you likely have a Magnesium deficiency. Some tricky species are particularly susceptible to honey fungus. Reasons include: because they are too wet, too dry, planted too deep, starved (needing feeding), soil too alkaline, or soil is compacted: This is a general sign of unhappiness and can have many causes: 1. To avoid frosted buds, protect opening buds with fleece or plant later flowering varieties such as deciduous azaleas. Often a last gasp attempt to flower heavily will be made. The larger the leaf the more shelter from wind is required. If it is very bad, consider destroying the worst affected plants. Lace bug damage on leaves causes a greyish, or whitish leaf upper surface with a discolouring on the lower surface. The disease is caused by inadequate drainage and warm or hot soil temperatures. Soil Too Acid Some rhododendrons dislike very acidic soil. Unsighly but not usually harmful though if the plants defoliate early for a few years they can be weakened. Flowers caught by frosty nights in Springtime: you can protect with fleece or move plants to a more sheltered site. Black spots with no patches on leaf undersurface. 3. In really severe infestations, it may be necessary to poison rabbit warrens using sodium cyanide or similar gasses. Keeping the foliage and especially the flowers as dry as possible is the best way to avoid the disease. Ensure that the soil where the rhododendrons are growing is not allowed to become compacted by people or animals walking over it. Both these animals can cause great destruction in the rhododendron garden. At this point they are most vulnerable to insecticide. Beware of putting deep mulch on top of rootballs. Dark brown dead roots= Phytophthora caused by poor drainage. It is found on wild populations of some species and also found in gardens on some species and hybrids, and especially on evergreen/Japanese azaleas and R. ferrugineum and its hybrids. Holes in the leaves (internal) and irregular chunks of eaf removed (as opposed to notches around the edge) are the symptoms of catterpillars. Evergreen and deciduous azaleas seem particularly prone. In areas with heavy clay soils, the best practice is often to plant above soil level, either in raised beds or by mounding up the soil around the root of the plant. So why has my rhododendron got yellow leaves? Ideally fencing is the perfect option but this is expensive, especially where deer are concerned. Old scales may remain on the plant for some time after the death of the insect. Sometimes sending up conspicuous fruiting bodies, the main part of the fungus lives underground, sending out vigorous rhizomorphs (commonly known as bootlaces), which colonise both dead and living tree and plant roots.
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