Basically, the Japanese azalea, like all azaleas and rhododendrons, is a fairly robust plant. However, it can also be attacked by various pathogens and pests.
The robust Japanese azalea usually only falls ill when it is weak
deficiency diseases
Deficiency diseases are first seen in leaf discoloration. Only if the cause is not eliminated after the first signs will stunted growth or deformed shoots, leaves and flowers follow - although the latter may also not occur.
iron deficiency chlorosis
The chlorosis is indicated by the yellowing of the leaves. Typically, however, the leaf veins remain bright green. An iron deficiency occurs very often when the pH value of the soil is too high and the roots cannot absorb enough of the mineral as a result - calcareous soil interferes with the absorption and metabolism of iron.
nitrogen deficiency
If there is insufficient fertilization and/or if the soil is too firm and impermeable to air, a strong yellowing of the leaves tends to indicate a lack of nitrogen. This can be eliminated with the gift of blue grain.
fungal or viral infections
Leaf spot diseases on azaleas are also quite common. These appear as yellowish or dark brown, mostly irregular, round spots that spread quickly and can also spread to the shoots. Various fungi or viruses are responsible for this phenomenon, with the mosaic virus being particularly noteworthy. In all cases, only the pruning of affected parts of the plant helps.
Beware of the wilt disease
The dreaded Verticillium wilt occasionally affects azaleas. In this case, quick action can sometimes save the plant. However, it is not enough to simply cut back the affected plant, you must also dig it up and carefully remove the soil clinging to the roots. The pathogen sits in the ground and infects the azalea from there. As a rule, however, only the disposal of the diseased azalea helps. Make sure that you do not plant any more Verticillium-sensitive plants at this point or that you replace the soil generously.
Common Pests
Even pests do not stop at the Japanese azalea.
rhododendron bug
The rhododendron bug (Stephanitis rhododendri and Stephanitis obertii) is initially noticeable through yellowish or greenish leaf spots. Remove affected leaves if infested - especially before winter.
Rhododendron Leafhopper
Cicadas are leaf sap suckers, but basically do not cause too much damage. They are only dangerous as carriers of certain pathogens such as bud rot. Put up yellow boards in June/July to combat them.
tips
Due to the shady and therefore often damp location of the Japanese azalea, the leaves can occasionally be covered with a greasy, green coating. This can be wiped off easily. These are algae. To combat this, you need to provide more light.