Helleborus Problems, Pests, Diseases and Disorders

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Helleborus Problems, Pests, Diseases and Disorders

 

By Howard Drury

 

 

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Botrytis on Hellebore

 


Hellebores are generally easy to grow tough plants that suffer from few pests and diseases or other problems. Most issues arise from trying to grow Hellebores in unsuitable conditions such as ground that is too wet or where they get too hot in the summer, often down to insufficient shade. Paying attention to good hygiene and solving problems at an early stage are important.

 

Black Spotting of the foliage


Symptoms


Large irregular brown or black spots appear on leaves and stems. These often coalesce resulting in yellowing and eventual death of infected leaves. Spots may also occur on flowers and lower sections of stems. These may wilt above the point of attack so that flower buds fail to open, Tiny black fruiting bodies may sometimes be visible within the leaf spots, particularly on the underside of foliage.


Cause


Hellebore leaf spot is caused by the fungus Microsphaeropsis hellebori (syn. Coniothyrium hellebori) A common and damaging fungus that particularly affects Helleborus niger (Christmas Rose). Tougher leaves species and cultivars such as H. argutifolius are less likely to be attacked. It is most often seen from early spring until late summer.


Biology


The fungus causing hellebore leaf spot produces minute spores from small black fruiting bodies that form in the dead, affected tissues. The spores are spread in water and wind-blown rain and thus wet conditions are required to initiate disease. The fungus perpetuates on the plant over the summer and autumn and a new round of infections is initiated at times when new plant growth is occurring.


Non-Chemical Control


Remove all infected leaves and flowers as soon as possible and either burn or bin them. Remove all old foliage in early spring to prevent the fungus spreading.


Chemical Control


There are no specific fungicides recommended for the control of this disease available to home gardeners. However there are broad spectrum fungicides licensed for use on ornamentals The fungicide tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus, Toprose Fungus Control & Protect) has a recommendation for the control of leaf spots on ornamental plants. Additonally, the fungicides tebuconazole (Provanto Fungus Fighter Concentrate) and triticonazole (Scotts Fungus Clear Ultra and Scotts Fungus Clear Ultra Gun) are labelled for the control of other diseases on ornamental plants, and could therefore be used legally on hellebore (at the owner’s risk) to try and control the leaf spotting.

 

The following products contain a combination of both insecticide and fungicide, enabling the control of both insect pests and disease: myclobutanil containing cypermethrin (Westland Resolva Rose 3 in 1, Doff Rose Shield Bug & Fungus Killer, Scotts Roseclear Ultra Gun 2, Vitax Rosegarde) and triticonazole containing acetamiprid (Scotts Roseclear Ultra and Scotts Roseclear Ultra Gun).When a proprietary product contains an insecticide as well as a fungicide it would be preferable to use an alternative product if pests are not a problem on the plants treated.

 

Inclusion of a product does not indicate a recommendation or endorsement by Howard Drury It is a list of products currently available to the home gardener as indicated by the RHS.

 

Hellebore Black Death


Hellebore black death is a serious disease of hellebores, probably caused by the virus Helleborus net necrosis virus (HeNNV), where plants become stunted, deformed and marked by black streaks and netting patterns.

 

What is Hellebore Black Death?


The disease known colloquially among hellebore growers as ‘black death’ causes stunting, distortion and black streaking and netting patterns on the leaves. It is probably caused by a virus called Helleborus net necrosis virus (HeNNV).


In the UK, the most seriously affected hellebore is Helleborus × hybridus (syn. H. orientalis) but similar symptoms have been seen in other species. New damage can be expected from mid-spring. It must be stressed this is a very rare problems compared with the fungal infections affecting Hellebores.


Symptoms


You may see the following symptoms:
Plants show stunting and distortion of the emerging new growth, the damage becoming progressively more pronounced as the season progresses
Patterns of black streaks develop on the leaves, often following the veins, sometimes as rings
Black streaks may also develop on stems and flowers.


The symptoms should not to be confused with the more commonly seen general blackening caused by Botrytis cinerea or leaf spotting by Microsphaeropsis hellebori.

 

Biology


This disease has been recognised for about two decades in the UK, but is becoming progressively more serious. It is also known from mainland Europe and North America. A virus called Helleborus net necrosis virus (HeNNV) is associated with the disease and believed to be the cause. The virus is thought to be transmitted by the hellebore aphid, Macrosiphum hellebori. However, conclusive evidence for both suspicions is still lacking.
Control


Non-Chemical Control


All infected plants should be dug up promptly and destroyed


Many viruses are not transmitted through seed, so raising new plants from seed is a possible way for gardeners to ensure disease-free plants, as demonstrated by leading breeders such as Ashwood Nurseries.


Chemical control


There are no chemical controls for plant virus infections. Control of aphid vectors is not feasible with the products available to amateur gardeners because these are non-persistent and would need to be applied at unrealistically short intervals to give any protection against the arrival of winged aphids.


Control of aphid vectors is not feasible with the products available to amateur gardeners because these are non-persistent and would need to be applied at unrealistically short intervals to give any protection against the arrival of winged aphids.
Aphids transmit viruses by feeding on the sap of plants with virus infection, and thus contaminating their mouthparts with virus particles. When they fly to healthy plants and begin to feed, they then infect the plant with virus. Seriously infected plants are so stunted they are unlikely to be attractive to aphids and the most dangerous plants are probably those that are still lightly infected and suitable for aphid feeding.

 

Hellebore Aphid


The sap feeding hellebore aphid is a greenfly that can cause a lack of vigour and sooty mould on the leaves and flowers of hellebores. Perhaps more importantly, these aphids can spread viruses some of which can be lethal to Hellebores especially H. x hybridus selections. It is good practice to control aphids at the very first signs as this aphid Macrosiphum hellebore can form dense colonies in late spring and early summer. The honey dew aphids can easily become a source of food for black sooty moulds which in turn debilitate the health of the plant.


What is Hellebore Aphid?


A sap-sucking aphid or greenfly that can be found on the flowers and foliage of hellebores in spring.


Symptoms


Whitish-green aphids, 2-4mm (about 1/8in) long, sometimes form dense colonies on the underside of hellebore leaves and flowers in early spring
The sugary honeydew they excrete can also lead to the growth of sooty moulds on the foliage
This species of aphid is only found on hellebores.


Control


Non-Pestcide Control


Where possible tolerate infestations of aphids, as control is not easy. Early intervention is to be recommended. Squashing the aphids by hand can reduce infestations but may need repeating as it rarely removes all aphids Unfortunately aphid predators are not usually sufficiently numerous in early spring to reduce hellebore aphid infestations.


Pesticide Control


Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Ecofective Bug Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Organic Pest & Disease Control, Bug Clear for Fruit and Veg) can give good control of aphids. These pesticides have a very short persistence and so may require reapplication to keep aphid numbers in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults
More persistent insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Pest Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer) and cypermethrin (e.g. Py Bug Killer) The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available Follow label instructions when using pesticides Plants in flower should not be sprayed due the danger to pollinating insects


Inclusion of a pesticide product does not indicate a recommendation or endorsement by Howard Drury. It is a list of products currently available to the home gardener published by the RHS.

 

Hellebore Leaf Miner


A leaf mining fly Phytomyza hellebori  that became established in Britain in the 1990s, it can cause considerable disfigurement to the foliage of stinking hellebore, Helleborus foetidus along with Helleborus x hybridus selections.

 

What is Hellebore Leaf Miner?


Hellebore leaf miner is a small fly with larvae that tunnel inside the leaves of Helleborus foetidus with the botanical name of Phytomyza hellebore.


Symptoms


The RHS claim only Helleborus foetidus is affected by this insect; however I personally have seen it on Helleborus x hybridus selections. The mines in the leaves are initially brownish-black blotches, but later become sinuous, whitish-brown tunnels. In heavy attacks, most of the foliage can be disfigured by spring. Damage develops during late summer to early spring


Biology


This hellebore leaf mining fly was first found in Britain in 1999 and is now widespread in England and occurs in parts of Wales. Eggs are laid on the foliage and the larvae begin feeding inside the leaves during August. The mines are not fully developed until the new year. Pupation takes place within the leaf mines.


Control


Although the mines can be unsightly, affected plants are not greatly harmed and so the damage can be tolerated.


Non-Pesticide Control


Heavily mined leaves can be cut off and destroyed during the winter before the adult flies emerge.


Pesticide Control


Pesticides are unlikely to control of this insect as it spends all its early stages within the leaf meaning contact insecticides


Yellow foliage


This can be caused by a number of factors. A low Ph can lead to nutrients such as nitrogen being locked up even after feeding plants, the solution would be to raise the Ph by liming using ground lime or propriety branded products to lift the Ph in return releasing the nutrients to plants.


Plants growing on sandy soils are more likely to show yellowing of the foliage as nitrogen is easily leached from sandy soils. Feeding is to be recommended during the growing season. Yellowing can also be caused by lack of summer shading. While Hellebores benefit from good light in late winter and spring, most prefer some shade against the heat of the mid-day sun, taller hardy geraniums make perfect shade, and their weak stems often propped up by the stiffer Hellebore foliage.


Ashwood Nurseries have produced a selection known as the Evolution strain and this is characteristic of the strain with many plants displaying pale yellow foliage, often more pronounced at certain times of the year.


Yellowing foliage can also be caused by water-logging, especially at certain times of the year.


Grey Foliage


This can be caused by water-logging or frost damage, especially on the less hardy species (H. lividus) and the hybrids derived from less hardy species used in a breeding programme.

 

Botrytis

 

Botrytis cinerea or grey mold is a fungal disease that infects most ornamental plants including Helleborus. The fungus causes a decay of plant tissues and will grow fuzzy gray-brown mold over the decaying areas, such as the buds, leaves, and flowers. Parts of the plant may shrivel and die after exposure to the mold, particularly the flowers. Typically the fungus will only infect plants through an open wound or when the plant is under stress, but it has also been known to infect plants in humid conditions.

 

Control

 

To treat the infected plant, the first step is to remove infected and dying leaves, buds and flowers immediately along with any other dead plant materials around the hellebore. The next step is to reduce the humidity around the plant by improving the ventilation and ensuring the plants are not overcrowded


Downy Mildew


Specific mildews attack many species of plant. It is thought that one specific mildew does attack Hellebores, particularly if the plant has been subject to stress such as drought. Mildew will sometimes attack once the foliage has become wet again. There are no specific fungicides for mildew on Hellebores however the broad spectrum fungicides licensed for use on ornamentals my proved some control, it is better to improve cultural conditions to prevent mildew occurring.


Mice


For several years growers have reported non flowering of Hellebores. However after more investigation it has been possible to see a pattern emerging. In most cases plants were left unpruned and in grassland such as meadows or wildlife areas where the buds are particularly attractive to mice when food is short in winter and there is good habitat for the mice to over-winter.

 

This is perhaps one good reason for removing foliage of acaulescent types in December / January to remove the cover for the mice. Traps may be employed along with more planned careful management of wildlife areas. Mice will also devour young seedling sin pots and in the garden in spite of them being slightly poisonous.


Lack of Flowering


This can be caused by lack of water the previous season when buds were being formed; hot weather can also reduce flower bud formation. On lighter soils Hellebores may need more feeding to induced better flowering. Older plants may also show signs of fewer flowers especially if in competition with other plants.


Root Problems


Hellebores are also known to be susceptible to root decay and in particular the soil borne pathogen Phytophthora. This fungus is more prelevant in waterlogged soils and to avoid problems it best to improve the drainage and inhibit is growth. There is no soil sterilant that is effective in killing the fungus which may form resistant structures to survive in the soil for long periods.


Hellebore species and cultivars may become infected with a Hellebore cyst eelworm which is very difficult to control other than by practising sanitary removal and starting with fresh stock in another areas as cysts can remain active in the soil for many years even in the absence of host Hellebore plants.

Mildew on Helleborus

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Black Death of Hellebore

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Aphids on Hellebores

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Howard Drury, Horticultural Broadcaster, Speaker, Lecturer, Writer, Adviser and Consultant

Morningside, 8, Bagnell Road Kings Heath Birmingham B13 0SJ

Telephone 0121-443-3212 Fax 0121 441 2760 Mobile 07710-038-467

Email This email address is being protected from spambots. You need JavaScript enabled to view it.

 

The information given in this Fact Sheet is provided in good faith.  It is however of necessity general information and advice on the topic.  Howard Drury will not be under any liability in respect of the provision of such advice and information and you are strongly advised to seek independent advice on any particular gardening problems or queries you may have, preferably from experts who can (when appropriate) inspect the problem before providing advice.

 

(C) 2020. This material has been produced by Howard Drury and must not be reproduced in part or full without the written consent of Howard Drury, Kings Heath Birmingham, B13 0SJ.

JHD/03/02/2020


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