Rhododendron problems: pests, diseases & yellow leaves

Rhododendron problems: pests, diseases & yellow leaves

Rhododendrons are usually a beautiful sight. In this article, you can find out what to do about rhododendron problems such as yellow leaves, black buds or feeding marks.

Even though the evergreen shrubs are very low-maintenance, the genus Rhododendron is prone to fungal diseases and pests. In the following, you can find a list of the most important rhododendron pests and diseases as well as tips on how to get rid of them.

Rhododendron: yellow and brown leaves

There are two possible reasons for why your rhododendron’s leaves are changing colour: either your plant is malnourished and lacks water, or it is infested with a fungal disease. The damaged leaves can look different depending on the cause of the discolouration. Therefore, in order to do something about the rhododendron leaves turning yellow or brown, you need to identify the source of the problem first. These are 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: cut back rhododendron’s infested areas generously
  • Home remedy: spray some milk diluted with water (at a ratio of 8:1) onto the remaining shoots and leaves; rinse off after 20 minutes and repeat two or three more times; the lecithin contained in the milk kills fungal spores

Grey leaf spot

  • Signs: several different disease patterns, such as loss of leaves, impaired growth and spots on the leaves
  • Cause: rhododendron leaf spot can be caused by various fungi (more than 20 species known); damp weather conditions can make the infestation worse
  • Preventive measures: choose appropriate location for rhododendrons and avoid excessively shady areas; preventative use of fungicides

Rust

  • Signs: yellow to orange spores on the bottom of the leaves; often confused with leaf spot disease
  • Cause: fungal pathogens; occurs very rarely
  • Preventive measures: optimal choice of location with good soil conditions; appropriate fertilisation; keeping rhododendron leaves dry (even when watering); using pesticides is possible but not necessary

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: adult animals feed on leaves; larvae sometimes damage the roots; although the feeding marks don’t look good, they are not dangerous for the plant
  • Cause: infestation by adult animals and larvae; widespread in gardens in Europe
  • Countermeasures: collecting the animals from the plants; using parasitic nematodes; working neem cake (a by-product of neem oil production) into the soil in order to deter the larvae; pesticides are usually not effective because the animals hide away

Rhododendron lace bug (Stephanitis rhododendri)

  • Pattern of damage: pale yellow speckles on the leaves, which turn into brown-black necrotic spots; pests are easy to spot on the bottom of the leaves
  • Cause: locations with an excess of sunshine and warmth promote an infestation with the rhododendron bugs
  • Preventive measure: occasionally water the plant with a liquid fertiliser made from stinging nettle; use soft water; loose and mulched soil
  • Use of pesticides is usually not necessary

Whitefly

  • Pattern of damage: feeding marks on the leaves and speckle-like spots on top of the leaves; plant loses its vitality; leaves die off; whiteflies leave sugary honeydew on the leaves, which provides a breeding ground for fungi; later, rhododendron leaves might turn black
  • Cause: infestation by adult insects and their larvae
  • Countermeasures: putting up adhesive traps; controlling the larvae infestation with systemic pesticides; ichneumon wasps

Rhododendron leafhopper (Graphocephala fennahi)

  • Pattern of damage: sucking marks on buds which cause minor damage; actual damage is caused by a fungus (Pycnostysanus azaleae); female leafhoppers scratch rhododendron buds for their oviposition; fungal spores enter the plant through these wounds, which causes the buds to turn black and die
  • Cause: infestation by adult animals and subsequent fungal disease; one of the most common rhododendron pests
  • Countermeasures: breaking off and disposing of infested buds (not in your compost but in household waste); putting up adhesive traps; using insecticides against adult leafhoppers

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 can become increasingly chlorotic if its pH levels are too high or too low. In that case, the nutrients within the soil cannot be properly absorbed by the plant.

Lime-induced chlorosis

  • Signs: initially young leaves turn from yellow to brown while leaf veins remain green; without countermeasures the growth of the plant can be impaired
  • Cause: soil pH is too high (above 5), the iron within the soil cannot be absorbed, resulting in iron deficiency and later in manganese and magnesium deficiency
  • Countermeasures: re-planting your rhododendron into lime-free soil; watering with soft rainwater; fertilising with mulch, compost or a fertiliser that is rich in iron

Nitrogen deficiency

  • Signs: large areas of the rhododendron leaves turning yellow; plant sprouts sparsely, only developing small leaves which fall off quickly
  • Cause: nitrogen supply is too low, or fertiliser has been used incorrectly or in too small of a quantity
  • Countermeasures: fertilising with a mineral nitrogen fertiliser, which is available to the plant very quickly

Incorrect care not only affects the leaf colour, but also the flowering of the plant.



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