RHS top 10 pest & disease enquiries of 2006
The top 10 pest & disease enquiries received by the RHS Advisory Service has been announced.
New at number one in the pest charts is an insect that has never made the finals before - horse chestnut leaf miner. Ants were another new entry. The usual suspects (slugs/snails, vine weevil and lily beetle) still remain favourite pest enquiries.
In the disease chart the top three of 2005 were still top last year, with honey fungus remaining at number one by a long way.
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RHS top 10 pest enquiries
1. Horse chestnut leaf miner (Cameraria ohridella) NEW
This is a tiny moth whose caterpillars feed within horse chestnut leaves, causing extensive white or brown blotches. This pest was new to Britain in 2002 when it was found at Wimbledon. It is now widespread in south east England and is rapidly spreading to other parts of Britain. By late summer, trees can be so heavily infested that they appear to be dying, because of the dried-up appearance of the foliage. The trees will survive and develop normal leaves in the following spring but renewed damage will be seen from late June onwards.
2. Slugs and snails (various species) - no 1 in 2005
Slugs and snails are invariably one of the top three garden problems and have been number one in three of the last five years. These pests are most active from early spring to autumn when they feed on a wide range of plants. The most vulnerable are seedlings and the emerging shoots and foliage of herbaceous plants, but some trees and shrubs, such as Choisya, Magnolia and Laburnum can also suffer damage. Slugs are also a problem for some root vegetables, especially potato tubers.
Find out more about slugs and snails.
3. Vine weevil (Otiorhynchus sulcatus) - no 3 in 2005
Vine weevil causes problems on a wide range of plants, both in its adult and larval stages. The adult beetles eat irregular notches in leaf margins, while the more damaging grubs destroy the roots and tubers of many plants, especially those grown in pots or other containers. Vine weevil is usually one of the top three pests.
4. Lily beetle (Lilioceris lilii) - no 2 in 2005
This beetle is a voracious pest of lilies (Lilium species) and fritillaries (Fritillaria species), with both the larvae and adult beetles devouring the foliage during spring and summer. Lily beetle is continuing to spread, although it remains a local problem in Scotland, Wales and Northern Ireland.
5. Viburnum beetle (Pyrrhalta viburni) - joint no 11 in 2005
This has become a more troublesome pest in recent years. The principal host plants are Viburnum opulus, V. lantana and V. tinus but some other Viburnum species may also be attacked. The larvae cause most of the damage, sometimes reducing the foliage to lacework during May and June. The adult beetles also feed on the leaves in late summer but less extensive damage occurs at that time.
6. Ants (mostly Formica and Lasius species) NEW
Ants tend to be most abundant in sunny gardens with well-drained soils. They cause little direct damage to plants, but the soil excavated from their nests can be a nuisance in lawns, on patios and in flower beds where low-growing plants may become partly buried. Although there are insecticides for ant control, it is difficult to eliminate nests from gardens and so the presence of ants has to be tolerated.
7. Woolly aphid (Eriosoma lanigerum) - joint no 11 in 2005
This is mainly a problem on edible and crab apples but this pest also infests Pyracantha and Cotoneaster horizontalis. Infested shoots and branches are coated with a fluffy white waxy substance secreted from the aphids’ bodies, while their feeding induces softy lumpy swellings in the bark. These can split in frosty weather and create entry wounds for canker on apples.
8. Cushion scale (Chloropulvinaria floccifera) - joint no. 9 in 2005
This insect feeds on sap and is mainly found on Camellia but also attacks the foliage of other evergreen shrubs, including holly, Euonymus japonicus, Trachelospermum, Rhododendron and Choisya. The foliage becomes heavily coated over the winter months with a sooty mould that grows on the pest’s sugary excrement.
9. Rosemary beetle (Chrysolina americana) - no 4 in 2005
This originates from southern Europe and it became established in southern England in the mid 1990s. Both the adults and larvae eat the foliage of rosemary, lavender, thyme and sage. It is becoming increasingly widespread but there were fewer enquiries than last year.
10. Glasshouse red spider mite (Tetranychus urticae) - joint no 14 in 2005
This pest sucks sap from the foliage of a very wide range of indoor ornamental plants, fruits and vegetables. Hot dry weather in late summer is ideal for this pest when it becomes a problem on outdoor plants such as raspberry, strawberry, runner beans, roses and many other garden ornamental plants.
RHS top 10 disease enquiries
1. Honey Fungus (Armillaria spp) - no 1 in 2005
In 2006, the species of Armillaria were identified on 154 plant samples using molecular techniques. The species identification of three isolates remains to be confirmed using further molecular tests. The species breakdown is the following:Armillaria mellea; Armillaria gallica; Armillaria borealis/A. ostoyae (Betula and Aesculus); Armillaria cepistipes/A. gallica (Rhododendron).
The most common species isolated was A. mellea which was responsible for the death of a wide range of woody plants. Thirty percent of the cases of A. gallica were rhizomorphs growing in composts, leaves or decayed wood. The remaining cases originated from dead trees sometimes still standing therefore its role as a secondary pathogen remains to be elucidated.
The most frequent hosts for Armillaria were Ligustrum, Viburnum, Malus, Rosa, Prunus, Rhododendron and Betula.
New hosts for the disease recorded were Cordyline (A. mellea), Dahlia (isolation under way), Mimosa (A. gallica), Stachys (failed isolation) and Grevillea (failed isolation). Among them no published records have been found on Cordyline, Dahlia and Stachys.
2. Rusts - no 3 in 2005
The top rust enquiry this year was Gymnosporangium sabinae which causes rust on pears and junipers. This fungus accounted for 55% of the rust enquiries. This is a very significant increase from 2005 where the disease accounted for 13% of the rust enquiries. G. sabinae requires two hosts to complete its lifecycle. In the spring it infects pears and produces horn-like protrusions that erupt from pustules on the under surfaces of the leaves. These spores infect species of juniper. Here they cause slight swellings on stems that erupt in the spring to release another type of spore that can infect pear leaves. The infections on the juniper are perennial, so they can continue to re-infect the pear every year. Therefore, the severity of the disease on pear will be largely dependent on conditions that favour the development and the release of the spores on junipers and the infection on pear leaves in the spring.
For interest we also recorded two cases on ornamental pears, one on Pyrus salicifolia and one on Pyrus elaeagnifolia ‘Silver Sails’
Other interesting rusts included Gymnosporangium clavariiforme and Gymnosporangium confusum on Juniper species. Both fungi have also an alternate host which is Crataegus. In addition, G. confusum can also infect Cydonia and Mespilus.
3. Saprophytic fungi - no 2 in 2005
The number of saprophytic fungi enquiries we received is steady over the years. One main enquiry is about white mycelium growth in soil which seems to alarm gardeners. Fungal growth of this type is not harmful and usually enhances soil fertility. This mycelium growth is usually sterile so no fruiting bodies are formed and species cannot be identified. Among the saprophytic fungi, we received unusual samples of earth star (Geastrum rufescens).
4. Powdery mildews - no 4 in 2005
The top two hosts for powdery mildews were Prunus andRhododendron. We had two cases of the recently described fungus Oidium lauracearum on Bay Laurel (Laurus nobilis). We also received good samples of wisteria powdery mildew which allowed us to make a description of this fungus. Usually sporulation of this species is very sparse and therefore identification of the species was not possible. The symptoms on Wisteria are dark necrotic spots and marks with yellowish background on both surfaces of the leaves. From the work we have done so far it appears that the species infecting Wisteria is closely related to oak powdery mildew (Erysiphe alphitoides).
5. Phytophthora root disease - joint no 7 in 2005
Phytophthora has been over the years the second most important root pathogen (after honey fungus) isolated in gardens. The fungus is also known to cause leaf and twig blight on specific hosts. A systematic identification of Phytophthora species isolated from the samples received in Pathology has been carried out since 2003. As there is no satisfying way to control the disease, information about species along with host range, environmental conditions in which that particular species thrives helps the gardeners to make an informed choice about replacement plants or create an environment less favourable to the development of the disease.
The most frequent hosts for Phytophthora were Taxus baccata and Rubus ideaus. Taxu has been consistently a top host over the years for Phytophthora. The relatively high number of cases identified on Rubus is due to the addition of a technique to our range of Phytophthora assays which already include apple baiting tests and molecular assays.
Potential new hosts for some species are: P. citricola on Buxus and Viburnum; P. citrophthora on Choysia and Dahlia; P. cryptogea on Aconitum, Buddleja and Fagus; P. syringae on Taxus; P. cactorum on Lavandula and P. hibernalis on Rosa.
6. Leaf spots - no 6 in 2005
These include a number of different fungal diseases causing leaf spot. There was no particular pattern this year. However, we recorded a few unusual cases of leaf spots including a Septoria on Stephanotis, Guignardia on Prunus and Ramularia on Viburnum.
= 6. Root and stem rots including damping off NEW
This section includes the soil-borne pathogens Pythium, Rhizoctonia and Fusarium. With increasing tests of root rots using molecular techniques we had a better success in detecting and identifying the species of Pythium. One notable case is the identification Pythium vanterpoolii on the lawn in front of the laboratory. This pathogen is responsible for the melting out of the grass quite noticeable near the pond.
8. Volutella blight (Pseudonectria rousseliana) - joint no 11 in 2005
Volutella blight on Buxus is caused by the fungus Pseudonectria rousseliana. This is a disease that is more prevalent on stressed plants and usually needs a wound to infect. Once the fungus has entered the main stem it can kill the plant. It is regularly seen in association with the fungus Cylindrocladium buxicola. We recorded twice as many cases of Volutella in 2006 compared to 2005. The increase in the number of enquiries on Buxus and therefore its diseases is due to its growing popularity as a hedge plant or topiary as well as publicity on its problems such as Cylindrocladium blight.
9. Viruses - no 5 in 2005
Most samples received with virus symptoms could not be confirmed as being infected with a virus as this requires specialist tests which the laboratory does not have. A quarter of the enquiries are coming from the garden staff working in the Glasshouse Department. We usually carried out a limited number of tests for the staff for the three main viruses occurring in glasshouses: Cucumber Mosaic Virus (CMV), Impatiens Necrotic Spot Virus (INSV) and Tomato Spotted Wilt Virus (TSWV).
10. Wood decay fungi - no 10 in 2005
The wood decay fungi identified in 2006 were Meripilus giganteus, Ganoderma species, Laetiporus sulphureus, Polyporus squamosus, Heterobasidion annosum, Pleurotus ostreatus and Oxyporus populinus. There was no particular pattern regarding their abundance.