If buds are formed and then turn brown, the cause is usually frost or occasionally bud blast fungus. Protect opening buds with fleece or plant later flowering varieties esp. azaleas. if flower buds do not form (flower buds are plumper than growth buds):
some varieties, especially species, need more years before they flower.
plant becomes shaded, flowers will decrease or it may not flower at all: open up to the sunlight or move plant.
fertiliser applied after late June: this encourages leaves, not flowers.
Vine weevils; symptoms: eaten nearly all the roots or ringed the bark around the base of the stem
Honey fungus; symptoms: roots are full of black bootlaces with white core; don't plant rhododendrons here again.
Bark split; caused by late Spring frost - variety not hardy enough?
Drowned: we find it difficult to breathe under water and so do plants. Problems with drainage/depth of planting: soil too heavy or compacted or rhododendron planted too deeply. Phytophthora is caused by poor drainage; symptoms: dark brown dead roots
Lack of water, dry out once, thats it.
In the early days of propagation of rhododendrons, grafting rhododendrons onto R.ponticum was common practice. It was found to be a bad choice because it sends up numerous suckers. If these are not cut off, the R.ponticum will eventually take over. Loder Plants have never grafted onto ponticum and very few are grafted at all (with the exception of rare species) but we occasionally source plants which we have been disappointed to find are still grafted on to R. ponticum.
Mildew: pale spots on upper leaf surface, brown/grey patches underneath: use fungicide. (Any rose fungicide will do: Rose Clear etc.)
Rust: black spots on upper surface, lower surface with orange patches: fungicide
NB. where symptoms are black spots with no patches on leaf under surface, this is nothing to worry about, and is not a disease. Some varieties (eg Mrs GW Leak) are prone to this
Drainage is poor: lift plant and improve soil structure or move to better drained spot
Plant is starved: apply fertiliser May - late June
Soil is too alkaline: apply sulphate of Ammonia, (liquid feed) and plant with plenty of peat; water plants with rain water, not tap water