Rhododendron – the most common diseases, fungi and pests

Diseases and fungi

Iron deficiency (chlorosis)

A low pH value of the soil is the linchpin in the care of a rhododendron. If the value does not fluctuate between 4.5 and 5.5, this fact has fatal consequences for the ornamental shrub. Deficiency symptoms primarily occur because the shallow root can no longer access enough nutrients. In particular, too high a lime content in the soil sets the urgently needed iron, which causes chlorosis within a short period of time.

Damage
It begins at the shoot tips of the young leaves. They turn yellow. After a while, the loss of the deep green color spreads over the entire foliage, with the still green leaf veins typically protruding conspicuously.

Prevention and control

• Always plant rhododendrons in substrate with a pH value of 4.5 to 5.5
• Basically water with collected rainwater or decalcified tap water
• Dissolve 150 grams of Epsom salt in 10 liters of water and pour it into the soil to lower the pH value
• Incorporate generous amounts of compost, peat, peat substitutes, bark humus or leaf compost
• Apply the soil additive Rhodovital from Substral and then add an iron fertilizer
• Plant preferably lime-tolerant Inkarho rhododendrons at a pH of 6.0 to 6.5

Bud deaths due to fungal infection (Pycnostysanus azaleae)

There are various causes that cause bud deaths on rhododendrons. As a rule, a fungal disease is behind it, triggered by the pathogen Pycnostysanus azaleae. This uses the smallest of injuries to the plant tissue to penetrate the ornamental shrub. The rhododendron cicada is believed to be the main carrier of this disease. Well-founded scientific evidence has not yet been provided.

Damage pattern
The buds created in the previous year turn brown and wither away in the course of winter. There is no longer any possibility of sprouting next spring. Mostly evergreen rhododendron varieties are affected.

Prevention and control

• In March, cut out any dead buds as deeply as possible
• Strengthen the immune system with algae products or liverwort extract
• Take targeted action against the rhododendron cicada
• If necessary, use an approved fungicide with a systemic function

Leaf spot disease caused by tubular fungus (Septoria)

If unsightly spots appear on the deep green, decorative leaves, there is either a care error or a tubular fungal infection is spreading. If the hobby gardener does not intervene, the leaves will dry up and eventually fall off. At the same time, there is a high risk of infection for all plant neighbors in the garden.

Damage pattern
Yellow, brown, black or reddish spots develop on the rhododendron leaves and spread steadily. If sunburn, frost damage or care errors can be ruled out, a fungal infection can be assumed.

Prevention and control

• Carefully check the site conditions and rectify any deficiencies immediately
• If it is waterlogged, dig up the ornamental shrub and replace the soil
• Eliminate drought by extensive watering with rainwater
• For the time being, do not give any nitrogen-rich fertilizer
• Apply plant strengtheners, such as Neudovital or moss extract
• Remove and burn all infected leaves

Powdery mildew (Erysiphaceae)

One of the most common fungal diseases on plants in the kitchen and ornamental garden does not spare the rhododendron either. The pathogens overwinter in tiny spore housings, in the ground or on weeds. If the temperatures rise in spring, the spores can be carried by the wind, through the gardener’s clothes or by insects to their innumerable host plants, including rhododendrons. For this reason, powdery mildew is also called fair-weather mushroom. In order to germinate on their victims, however, the pathogens need moisture, so that powdery mildew is widespread in the home garden during a rainy summer. The spores swim around on a rain-soaked leaf until they discover a tiny damage to the tissue and use it as an access to the interior of the shrub.

Damage
From initially small, white foci of infection, a floury-white patina develops. Fatally, the spread usually begins on the underside of the leaves and only expands later on to the upper side of the foliage, which is visible from afar. Within a short time the leaf surface is completely covered, so that photosynthesis comes to a standstill and the leaf dies.

Control and Prevention

• Plant the rhododendrons airy and at an appropriate distance
• Weed continuously, but do not rake too deep
• Immediately cut off infested leaves and dispose of them in household waste
• Immediately stop nitrogen-stressed nutrient supply
• Instead, fertilize with potash, comfrey manure or lime-free rock flour
• Spray regularly with a mix of milk and water
• Dissolve heaping tablespoons of baking soda in 4 liters of water, each with 15 ml curd soap and vegetable oil
• Place basil, thyme, garlic or marigolds as neighbors
• Lure ladybugs and sawfly as predators
• As a last resort, inject an approved sulfur supplement

Triebsterben – Verticillium-Welke (Verticillium dahliae und Verticillium albo-atrum)

One of the most common diseases of rhododendrons appears to strike overnight. The leaves suddenly hang limply and soon the shoots as a whole. What at first glance suggests growth depression is in fact the dreaded fungal infection called Verticillium wilt. Infestation from the ground is particularly treacherous, as the spores clog all ducts. In the long run, the water and nutrient supply comes to a standstill and the ornamental wood dies. While powerful fungicides are not available, you are not completely helpless in the face of disaster.

Damage
At an early stage, the older leaves wither, while the young leaves continue to thrive. The more supply lines in the rhododendron clog, the more sparsely new shoots develop. Finally, the wilt appears visibly on the youngest leaves. The more vital the rhododendron, the longer this process takes. The pathogens do not immediately colonize all of the conduction pathways, so that partial supply can take place for a number of years. Only in the course of a longer dry period does the entire ornamental shrub die. For this reason, many hobby gardeners initially assume that their rhododendron has died of thirst and water what it takes. However, this measure can no longer save the plant.

Prevention and control

• In the run-up to planting, prepare a soil analysis with regard to a stock of permanent spores
• When buying young plants, examine them carefully for symptoms of disease
• Always plant rhododendrons with drainage and a snorkel for good root ventilation
• Strengthen with injections of manure from field horsetail, comfrey, nettles, tansy and wormwood
• Biological soil fumigation using seed mixtures from mustard species, oil radish and Sudan gas (biofumigation)
• Never place rhododendrons in heavy, cold, waterlogged soil
• Targeted fertilization with compost, horn meal and potash
• Thoroughly thin out and cut back every year with disinfected tools
• Treat cuts immediately with charcoal ash
• If the Verticillium wilt progresses unhindered, the plant is cleared over a large area

Earlobe disease (Exobasidium japonicum)

The smut fungus species lives as a parasite preferably on small rhododendrons or room azaleas. At first unnoticed, the spores grow into the storage tissue of the leaves in order to access the nutrients there. The infection is only noticed when they break through cracks on the leaf surfaces in order to multiply.

Damage
Red spots appear on the tops of the foliage. Turn the sheet over and you can see galls up to 3 cm in size. In the further course the white mycelium grows and covers the entire surface, whereby the galls are also overgrown. Stems and flower buds thicken, become fattened and deformed.

Prevention and control

• Avoid susceptible varieties such as ‘Brilliant’ or the azalea ‘Mother’s Day’
• Cut off all infected parts of the plant down to the healthy wood
• Dispose of in household waste or burn; definitely not on the compost
• In the worst case, treat with an approved fungicide

Pests

Not only diseases due to deficiency symptoms or fungal infections afflict a rhododendron, but also the following pests.

Rhododendronzikade (Graphocephala coccinea)

As the name suggests, this pest specializes in rhododendrons. Apart from that, the larvae in particular have it on the sap. The brightly colored cicadas are 8 to 10 mm long and prefer to cavort on the undersides of the foliage. When the sun is shining, the cicadas sit in hordes on their tops to relax after their shameful hustle and bustle. From the end of April until June, the larvae suck the sap of the ornamental shrub out in order to pupate afterwards and produce the next generation. The rhododendron cicada is also considered to be the main suspect for the transmission of the fungal disease Pycnostysanus azaleae.

Damage picture
In May and June, small, yellowish-green larvae colonize the undersides of the foliage. They are often mistaken for aphids. As a result of their sucking activity, spots appear which enlarge. In summer, hordes of flyable cicadas rise when the rhododendron is touched.

Prevention and control

• In summer move the branches and catch the cicadas with the net
• Put up yellow boards to which the insects will stick
• Cut off diseased branches and leaves and burn them
• Treat with Spruzid pest-free
• Spread the seeds as a sprinkling powder

Rhododendron net bug (Stephanitis rhododendri)

The adult bedbugs act as parasites in the same way as their larvae. From May onwards, they suckle the leaves together with their proboscis. The adult specimens lay their eggs on the underside of the leaves between August and October in the previous year. Fortunately, since the pests are not particularly mobile, the degree of their spread is within a controllable range. They owe their name to the network-like structure of their physique, which, due to their tiny length of 2-3 mm, can only be seen with a magnifying glass.

Damage
The foliage is covered with spots on the underside. If the attack pressure is high, young leaves roll up and wilt. If the weather is warm and dry, the net bugs tend to multiply on a large scale. The larvae secrete a wax that makes their bodies appear dusted with flour. A cursory glance at the ornamental shrub could lead to an initial confusion with powdery mildew. From August onwards, the females sink the eggs of the next generation into the leaf veins. This creates a scab-like coating.

Prevention and control

• Rhododendron varieties with hairy leaves are rarely attacked
• Remove affected parts of the plant as soon as possible
• Use natural sprays based on rapeseed oil
• Alternatively, use Spruzid pest-free outdoors

Gefurchter Dickmaulrüssler (Otiorhynchus sulcatus)

It is 10 mm long, grained black and aims at rhododendrons. The nocturnal beetle sets out at dusk to gnaw at the leaves. As if that weren’t enough, the females lay their eggs at the roots. The hatched larvae feed on the delicate, sensitive fiber roots and hinder the supply of water and nutrients to the ornamental shrub. The damaging effect of the brood is therefore to be rated higher than that of the adult black weevil.

Damage
pattern The semicircular feeding pattern immediately catches the eye. The black weevils gnaw the leaves in various places from the edge. The larvae mostly cavort in the ground. Sometimes they eat the root neck of a young rhododendron. The real debacle shows itself in a weakening of the ornamental shrub as a whole, with wilting, because the supply no longer takes place in full.

Prevention and control

• Nematodes are considered to be the most effective natural control agent
• Put pots filled with wood wool upside down as a beetle trap
• Spread the seeds of the neem press cake around the bush
• Attract predators such as birds, hedgehogs, spiders and common toads
• Collect the beetles with the flashlight in the dark

Conclusion

Rhododendron is not one of the most popular ornamental shrubs in the garden, but a sought-after target for pathogens and pests. However, no hobby gardener should be deterred by this downer from cultivating the enchanting flowering wood. Against the most common diseases, fungi and pests, preventive and control measures are definitely available that have been proven to work.