Rhododendrons are usually pretty to look at. But here you can find out what to do about yellow leaves, black buds and signs of damage.

Fungal infestation of the flowers is particularly annoying

As easy to care for as they are in every other respect - the genus of rhododendrons (Rhododendron) is susceptible to fungal diseases and animal pathogens are also up to mischief in the evergreen foliage of the rhododendron. Below we list the most important pests and diseases and give tips on how best to deal with them.

Rhododendron: Yellow and brown leaves

The discoloration of the leaves usually has two causes: either the plant is malnourished or suffers from a lack of water or it has been attacked by a fungal disease. Depending on the cause, the yellow or brown lesions can vary. In order to do something about the discoloration, the source of the damage must first be identified. Here you will find a list of common fungal diseases of rhododendrons.

Yellow or brown leaves can have several causes

Frequent fungal diseases in rhododendrons

Mildew

  • Characteristics: Mealy gray coating on leaves and partly on stems
  • Reason: Occurs when there is a lack of ventilation and excessive humidity
  • Countermeasure: Prune affected areas generously
  • Home Remedies: Spray milk diluted with water (in a ratio of 8:1) on the remaining shoots and leaves; rinse after 20 minutes and repeat 2-3 more times; the lecithin in the milk kills fungal spores

Leaf spot disease

  • Characteristics: Combines several clinical pictures; generally there is loss of leaves, stunted growth and spots on the leaves
  • Reason: Leaf spots can be caused by various fungi (over 20 species known), damp weather conditions promote infestation
  • Preventive measures: Choose favorable locations; avoidance of overly shady locations; preventive use of fungicides

Rust

  • Features: Appears on yellow-orange spore bedsidentified on the underside of the leaf, is often confused with leaf spot diseases
  • reason: fungal pathogen; occurs very rarely
  • Preventive measures: Optimal site selection and soil conditions; need-based fertilization; Keeping the leaves dry (when watering, for example) Use of pesticides is possible, but not necessary

Pests of Rhododendrons

But not only fungal diseases can cause problems for your rhododendron. Animal pests can also cause your rhododendron to become ill.

The vine weevil can do significant damage

Ocean weevil (Otiorhynchus sulcatus)

  • Characteristics: Adults feed on leaves; Larvae partially damage roots; the feeding marks are visually unattractive, but no danger for the plant
  • reason: infestation by adults and larvae; widespread in German gardens
  • Control: Pesticides are usually not effective because animals hide; collect; reliable control by parasitic nematodes like our Plantura HB nematodes between April and May; Lightly incorporate neempress cake into soil to repel larvae

Rhododendron bug (Stephanitis rhododendri)

  • Characteristics: Pale yellow mottling on leaves, progressively becoming brown-black necrotic spots; the pests are easily recognizable on the underside of the leaf
  • Reason: Infestation by the bugs is favored by too sunny a location and warm weather
  • Preventive measure: Water occasionally with nettle manure; use soft water; loose and mulched soil
  • The use of a pesticide is usually not necessary

Rhododendron leafhopper (Graphocephala fennahi)

  • Features: Sucks buds; sucking damage only slight; the actual damage comes from a fungus (Pycnostysanus azaleae); The females cut buds to lay eggs, the fungal spores penetrate through the wound, the buds turn black and die off
  • Reason: infestation by adult animals and subsequent fungal disease; one of the most common pests on rhododendrons
  • Control: Break out infected buds and dispose of them (not on compost, but in household waste); Post yellow boards to identify infestation, use insecticides against adult cicada
Rhododendron leafhoppers are vectors for fungal and viral diseases

White Fly

  • Features: feeding on leaves; speckle-like symptoms on leaf surface; plant loses vitality; leaves die off; leaves sugary honeydew on leaves, providing breeding ground for fungi; then mostly black discoloration of the leaves
  • Reason: Infestation by adults and larvae
  • Countermeasure: Post yellow boards; Combat larvae with systemic pesticides, parasitic wasps

Yellow leaves due to improper care

If there is no pest infestation or disease, the rhododendron was cared for incorrectly. If the leaves of the evergreen shrub become increasingly chlorotic, this is usually due to a pH value that is too high or too low. Because if the pH value of the soil is not in the optimal range, the nutrients contained in the soil cannot be properly absorbed either.

Chlorocalcification

  • Characteristics: First, the young leaves turn from yellow to brown, the leaf veins remain green; if no countermeasures are taken, growth disorders will occur
  • Reason: The soil pH is too high (over 5), so the iron in the soil cannot be absorbed, iron deficiency occurs, later manganese and Magnesium deficiency
  • Countermeasures: transplant into lime-free soil; watering with soft rainwater; Fertilize with mulch, compost, or an iron-heavy fertilizer

Nitrogen Deficiency

  • Features: The leaf yellows flat; the plant sprout only sparsely and only forms small leaves that fall off quickly
  • Reason: The supply of nitrogen is insufficient, too little or incorrect fertilization was applied
  • Countermeasures: Fertilize with mineral nitrogen fertilizer, this is available to the plant particularly quickly

Incorrect care not only affects the color of the leaves, but also the formation of flowers. So that your rhododendron can adorn your garden vitally, green and full of flowers, we have compiled everything for you here for the right care of rhododendrons.

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