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If the rhododendron suddenly shows brown leaves, it is not easy to find the exact cause, because so-called physiological damage are considered as well as various fungal diseases. Here we have listed the possible sources of problems and explain how to get the damage under control.
If the rhododendrons' leaves become partially brown during the summer, at best it is just a sunburn. The large-flowered rhododendron hybrids and most wild species need a location without direct midday sun. If they are in full sun, good water supply must be guaranteed. A sunburn occurs only on the branches that are particularly exposed to sunlight. Since the leaves of the rhododendrons often have no even surface, but are curved slightly downwards in the edge region, usually not the whole leaf dries. Only areas are damaged, on which the sun's rays hit vertically and which are not shaded by other leaves.
Sunburn is relatively easy to control: In the spring, simply move your Rhododendrone to a location that is more favorable to the site or make sure that the plant is better supplied with water. The third option is to replace the plants with the more sun-tolerant Yakushimanum hybrids.
If your rhododendron shows dried leaves or even dead shoot tips in the spring, it is most likely the so-called freeze-drying of the triggers. This is a frost damage, for which a too high solar radiation is responsible. As with sunburn, the leaves turn partially or completely uniformly brown and show no special drawing or pattern. The phenomenon occurs especially in snowy winters in heavy frost. When the soil and the branches are frozen through and the warm winter sun thaws the water in the leaves and thin shoots, the stomata of the leaves open and evaporate water. Due to the frozen channels, however, no water flows from the ground, so that the leaves can not compensate for the loss of moisture and dry. In severe frost, the younger shoots are damaged.
If a cold, clear winter's day is predicted and your rhododendron is too sunny, you should protect it from the sun with a shading net or garden fleece as a precaution. In thaw, you should also water the plants if the soil is too dry. And again, if possible, look for a more favorable, partially shaded location for your rhododendron and plant it in the spring. Frozen shoots are simply cut out with the pruner at the start of the season.
Branch dying (Phytophtora)
This fungal disease is also referred to as instinctual dying or Phytophthora wilt and manifests itself mostly by brown, in the center brightly dried patches or dead end buds and wilting shoots whose leaves are beginning to flabby at the branch ends, later dry brown and hang vertically. The young, green branches turn mostly brown-black. In case of heavy infestation, the wilting also attacks the older branches and continues downwards, so that the whole plant dies. Infection can occur over the leaves and shoot tips or, in worse cases, directly over the roots. Entry ports are usually wounds such as dead fine roots, but also natural openings such as the stomata of the leaves.
Leaf infections with a Phytophthora fungus (left) can be seen on larger spots with often bright, dry tissue in the center. When a root infection (right) usually whole branches begin to wilt
The root infection mainly takes place on unfavorable, too heavy, wet and compacted soils. Careful soil preparation is therefore extremely important when planting rhododendrons, as this is the only way to achieve a balanced water balance and a high volume of important air pores in the soil, if these properties are not natural. Other preventive measures include an airy location, a low pH of the soil and a restrained nitrogen fertilizer.
For root infections, only the disposal of the affected rhododendron remains. Replanting without prior soil replacement is strongly discouraged because the pathogens, which can actively move in the soil, remain infectious for a long time as so-called permanent spores. The shoot tip infection can be stopped by immediately cutting back the affected plant far into the healthy instinct parts.Then disinfect the secateurs with spirit and treat the plant preventively with a suitable fungicide such as "Special Mushroom-Free Aliette".
Leaf spot diseases
The term leaf spot diseases is a collective diagnosis for various leaf fungi such as Glomerella, Pestolotia, Cercospora and Colletorichum. Depending on the species they cause reddish brown to brownish-black, round or irregularly shaped leaf spots, which are bordered with a yellow, rusty red or black border. Under wet conditions, the affected areas are sometimes covered by a mold lawn. Leaf spot diseases are usually easy to identify as the spots are initially relatively small and sometimes grow together as the infection progresses. The fungi are quite common, especially in hot and humid summers, as are particularly susceptible to the yellow-flowering rhododendron hybrids.
Leaf spot diseases usually do not cause much damage and are also easy to combat. You should simply pluck and dispose of severely affected leaves, then treat the plants with a fungicide such as "Ortiva Spezial Mushroom-free".
Leaflet fungi cause relatively small-scale defects
Rhododendron rust is very rare and can easily be confused with leaf spot diseases. He differs from these, however, by the leaf-submerged yellow-orange spore bearing.
Like most rust diseases, the rhododendron rust is not life threatening to the plants and can be well controlled with commercial fungicides. He can be prevented - like all other mentioned fungal diseases - by the right choice of location, optimal soil conditions, a moderate nitrogen fertilization and the absence of an overhead irrigation, so that the foliage is not unnecessarily wet.