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RHS Announces the Top 10 Garden Pests and Diseases of 2015
by RHS
March 27, 2016

• Slugs and snails knocked off the top spot by invasive hedge-eating caterpillar

• Honey fungus named the most problematic plant disease for the 20th year in a row

• Record temperatures and rainfall lengthened the season of pests and diseases

For the first time in nearly a decade slugs and snails did not top the list of the most troublesome garden pest of the year, based on enquiries received by the Royal Horticultural Society (RHS) Gardening Advice team. The title of top pest of 2015 has gone to a native of East Asia that was first discovered in the UK in 2011, the box tree caterpillar.

A voracious feeder of box (Buxus), the larvae of the box tree moth are 4cm long and feed on the leaves of the host box plant under a blanket of pale fine webbing which can cover infected plants.

First found in 2011 the pest is now prevalent in the home counties and is spreading across south-east England. Although not the number one reported annoyance for gardeners in 2015, snails and slugs came a strong second in the charity’s annual top pest list, with enquiries remaining high.

Slug and snail damage mainly occurs in the spring and autumn but can become a problem during any damp period. The creatures affect a wide range of ornamental plants and vegetables, especially potato tubers, hostas and daffodils, seedlings are particularly susceptible.

For the 20th year running honey fungus was the most commonly reported garden disease, with its presence recorded on 76 plant genera in 2015.

Considered one of the most destructive fungal diseases in UK gardens, honey fungus attacks and kill the roots of many woody and perennial plants. The most characteristic symptom of the presence of honey fungus is white fungal growth between the bark and wood, usually at ground level. Clumps of honey-coloured toadstools sometimes appear briefly on infected stumps in autumn.

The second most reported plant disease, like the box tree caterpillar also affects box plants, leaving them with bare patches and die-back. Box blight, caused by the fungus Cylindrocladium, is the scourge of hedges, parterres and topiary. Box blight poses a serious threat to the UK’s horticultural heritage, as box plants provide the structure of many historic formal English gardens.

High temperatures and rainfall over the spring saw a spike in enquiries to the RHS about box blight, as the environmental factors combined to create the perfect condition for the disease to thrive. Unusually warm wet weather in December saw an unexpected increase in enquiries at a time when the expected cold weather would suppress the disease.

As in previous years, the close relationship between the weather and the presence and longevity of garden pests and disease was evident. During 2015 the UK experienced its hottest-ever day and the warmest and wettest December on record. December 2015 saw temperatures reach those usually associated with April and May.

RHS Head of Plant Health Gerard Clover said: “Dealing with pests and diseases is a reality for gardeners, but by sharing their observations, insights and samples with RHS scientists they are helping us identify those areas where we need to focus our research efforts to better control pests and diseases.

“We are currently undertaking research that we hope will mean that slugs and snails will become less of a problem in the future, as we work to determine the most effective ways of controlling them. “The project will provide gardeners with the robust, evidence based information they’ll need to confidently take on slugs and snails.

“We are also working with the National Trust, whose properties include many historic gardens, to manage the threat of box blight and are looking to identify ways to mitigate the effects of box tree moth”

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