Rhododendron problems: pests, diseases & yellow leaves

Rhododendrons are usually a beautiful sight. Find out what to do about rhododendron problems such as yellow leaves, black buds or feeding marks.

Even though rhododendrons are very low-maintenance, these evergreen shrubs are prone to fungal diseases and pests. Read on to find out about some of the most common rhododendron problems, from pests to diseases, as well as tips on how to deal with them.

Rhododendron problems: yellow and brown leaves

There are two reasons why a rhododendron’s leaves might change colour. Either the plant is malnourished and dehydrated, or it is infested with fungal disease – you should be able to tell which by looking at the damage. First, let’s look at the most common rhododendron diseases:

Common fungal diseases

Mildew

• Signs: grey coating with a floury texture on the leaves and sometimes on the stems
• Cause: lack of fresh air and excess of moisture
• Countermeasure: generously cut back the infested areas 
• Home remedy: spray diluted milk (with 8:1 ratio of water to milk) onto the remaining shoots and leaves, rinse after 20 minutes and repeat two or three more times; the lecithin contained in the milk kills fungal spores

Grey leaf spot

• Signs: loss of leaves; impaired growth; spots on the leaves
• Cause: various fungi (more than 20 species known); damp weather conditions can make the infestation worse
• Preventive measures: appropriate location – avoid excessively shady areas; preventative fungicides

Rust

• Signs: yellow to orange spores on the bottom of the leaves; often confused with leaf spot disease
• Cause: very rare fungal pathogens
• Preventive measures: optimal location; good soil; appropriate fertilisation; keeping rhododendron leaves dry (even when watering); use pesticides, though this is unnecessary

Common rhododendron pests

Not only fungal diseases can cause rhododendron problems. Rhododendron plants can be also plagued by various pests.

Black vine weevil (Otiorhynchus sulcatus)

• Pattern of damage: adults feed on leaves; larvae can damage roots; although unappealing, the marks are not dangerous for the plant
• Cause: infestation by adults and larvae; widespread in gardens across Europe
• Countermeasures: remove the animals from the plants; use parasitic nematodes; work neem cake (a by-product of neem oil production) into the soil to deter larvae; pesticides are usually ineffective

Rhododendron lace bug (Stephanitis rhododendri)

• Pattern of damage: pale, yellow speckles on the leaves, which turn to brown-black necrotic spots; pests are easy to spot on the bottom of the leaves
• Cause: excess sunlight and warmth
• Preventive measure: occasionally water the plant with liquid fertiliser made from stinging nettle; use soft water; use loose and mulched soil; pesticides are usually unnecessary

Rhododendron leafhopper (Graphocephala fennahi)

• Pattern of damage: sucking marks on buds, causing minor damage; evidence of fungus (Pycnostysanus azaleae), causing the buds to turn black and die (female leafhoppers scratch rhododendron buds before laying their eggs; the fungal spores enter the plant through these wounds)
• Cause: infestation by adults and subsequent fungal disease; one of the most common rhododendron pests
• Countermeasures: break off and dispose of infested buds (in household waste; do not add them to your compost); put up adhesive traps

Whitefly

• Pattern of damage: feeding marks on the leaves; speckles on top of the leaves; loss of vitality; leaves die off; sugary honeydew residue on the leaves, providing a breeding ground for fungi; later, rhododendron leaves might turn black
• Cause: infestation by adult insects and their larvae
• Countermeasures: put up adhesive traps; use systemic pesticides; introduce ichneumon wasps

Yellow leaves caused by incorrect care

If neither pests nor diseases are the cause of your rhododendron’s leaf problems, chances are, it has not been maintained correctly. The leaves of the evergreen shrub become increasingly chlorotic if its pH levels are too high or too low. In this case, the plant will not properly absorb the nutrients within the soil.

Lime-induced chlorosis

• Signs: initially, young leaves turn from yellow to brown while leaf veins remain green; without countermeasures, the plant will have impaired grow
• Cause: soil is too alkaline (pH above 5); iron within the soil is not absorbed, so the plant is iron deficient; later, it will suffer from manganese and magnesium deficiency
• Countermeasures: re-plant your rhododendron in lime-free soil; use soft rainwater; fertilise with mulch, compost or a fertiliser rich in iron

Nitrogen deficiency

• Signs: large areas of the rhododendron leaves turn yellow; sparse sprouting, only small leaves develop, which quickly fall off
• Cause: not enough nitrogen; incorrect application of, or not enough, fertiliser
• Countermeasures: use a mineral fertiliser

Incorrect care not only affects the leaf colour, but also the flowering. To ensure that your rhododendron is vibrant, green and full of flowers, have a read of our article. It has everything you need to know about how to care for rhododendrons.