Rhododendron diseases and parasitic pests

Vine weevils are nocturnal parasitic pests that primarily attack young shoots and the leaves of rhododendrons, leaving behind irregularly notched edges and significant aesthetic damage. These insects can also compromise the growth of plants by gnawing at the roots, especially when they are in the larval stage.

To combat vine weevils, the introduction of their natural predators, such as the nematodes that specifically attack the larvae, represents an effective solution. These nematodes are applied to the soil around the affected plants, where they infect and eliminate the larvae. Another method involves trapping the adults using pheromone traps or by placing boards or fabrics around the plants, beneath which the weevils hide during the day and can be collected and disposed of manually.

It is advisable to avoid planting rhododendrons in areas already known to be infested by these pests. Thoroughly inspect every new plant purchased.

→ Read our article on vine weevils.

Perforation on a Rhododendron leaf (©Oregon State University)

The azalea lace bug, Stephanitis rhododendri, is a parasitic insect that feeds on the leaves of these shrubs, causing visible damage in the form of discoloured spots on the upper surface of the leaves. This damage, visible in summer, detracts from the plant’s appearance, but can also reduce its vigour. The adult insect is recognisable by its black and white striate wings.

To control the azalea lace bug naturally, it is recommended to increase moisture around the plants, as these insects prefer dry environments. Regular water sprays on the foliage or regular overhead irrigation can therefore help reduce their presence. White clay sprays (kaolinite) can be effective.

→ Read uses and benefits of clay in the garden

Prevention to avoid azalea lace bug infestation involves regularly inspecting the plants for early signs of attack and introducing natural predators such as ladybirds or lacewings, which can help control the population of these parasites.

Chlorosis, often observed in rhododendrons, manifests as yellowing of the leaves while the veins remain a deeper green. This condition is typically a sign of iron deficiency or a soil pH imbalance that reduces iron availability to the plant, making iron less available to the plant.

foliar spray of iron chelates directly onto soil or onto the leaves can supply the iron needed to correct chlorosis. The nettle manure also has an anti-chlorosis effect.

The underlying problem should then be addressed with preventive measures. Improve drainage, stimulate biological activity with amendments and mulch, as soil that is too wet or too dry and bare soil are aggravating factors. And above all, avoid planting in limy soil.

→ Read the article on iron chlorosis.

Powdery mildew is a fungal disease that manifests as a white or greyish felt on the leaves and sometimes the stems of rhododendrons. On ornamental plants, it mainly affects the plant’s appearance. Powdery mildew thrives in hot, dry air and confined environments.

To combat powdery mildew naturally, several treatments are possible, based on sulphur, sodium bicarbonate or milk. Remove the affected branches and the dead leaves, and burn them.

→ Read our powdery mildew article to learn which solutions to apply.

Powdery mildew prevention relies on sound cultural practices. Choose well-ventilated locations for planting, improve air circulation around the plants by pruning dense branches. Avoid excess nitrogen fertilisers. Mulch to retain moisture in the soil.

Red spider mites, tiny mites, cause yellowing, drying and premature leaf drop by sucking sap from plant cells. These parasites prefer warm, dry conditions.

To treat red spider mite infestations naturally, there are solutions based on rosemary essential oil, garlic or natural predators such as the Phytoseiulus persimilis.

→ Read our article on red spider mites.

As a precaution, avoid excessive nitrogen fertilisers that can encourage red spider mite populations, water the foliage of the plants during dry periods, and keep the soil moist with mulching. Spray in advance with a nettle manure or horsetail decoction.