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The Nations Worst Plant Pests and Diseases in 2020 Revealed

Published on March 31st 2021

Cherry tree blossom Snail

by Peter Vahlersvik. 1290591602

A pink flower on a plant
Slugs and snails have overtaken box tree caterpillar to top the Royal Horticultural Society’s annual survey of the nation’s worst plant pests, while honey fungus has retained the unenviable title of the most troublesome plant disease – a title it has held for a quarter of a century.

Plant Problems on the Rise as More Take Up Gardening

The RHS’s 25th annual pest and disease rankings provide an insight into the challenges that gardeners faced during the previous growing season while highlighting emerging threats from new pests and diseases.
With up to three million people taking up gardening for the first time during lockdown last year, according to the Horticultural Trades Association, the RHS saw an 88 per cent increase in reports of garden problems during 2020, as the UK dug for victory during the pandemic.
Pest Chart
Hot on the heels of slugs and snails in the top 10 pests of 2020 chart came vine weevil, box tree caterpillar, ants, woolly aphid, glasshouse red spider mite, fuchsia gall mite, glasshouse thrips, rosy apple aphid and capsid bug/glasshouse mealybug.
A close up of a black vine weevil

Black Vine Weevil

Otiorhynchus sulcatus

A close up image of a box tree moth caterpillar on a green plant leaf

Box Tree Moth

Cydalima perspectalis

A myrmica ruba ant on a leaf



Woolly Aphid

Eriosoma lanigerum

Red Spider Mite

Tetranychus urticae

Some Fuschia flowers affected by Aculops fuchsiae Fuschia Gall Mite

Fuchsia Gall Mite

Aculops fuchsiae

A yellow flower in the snow



A close up of 2 rosy apple aphids Dysaphis plantaginea on a plant

Rosy Apple Aphid

Dysaphis plantaginea

A close up of a capsid bug Miridae on a flower

Capsid Bug


A close up of a mealybug from the family Pseudococcidae



Disease Chart
In the disease chart, honey fungus was followed by pear rust, leaf spot/canker of prunus, rose blackspot, bracket fungi, powdery mildew of prunus, blossom wilt of fruit trees, Phytophthora root rots, rose powdery mildew and brown rot of fruit.

Box Topiary to Make a Comeback?

The study provided an optimistic outlook for owners of box topiary and parterres. In recent years, this favourite of formal planting schemes has been dealt a cruel blow by box blight – a fungal disease that causes dieback and bare patches – and box tree caterpillar, which was first witnessed in the UK in 2007 and has defoliated box plants.
In 2019, box blight took the third position in the RHS’s top diseases chart, but it was notably absent from 2020’s study. Although box tree caterpillar took third place in 2020’s chart of the worst plant pests, the RHS pointed out that caterpillar reports had fallen by 40 per cent. Plus, separate research claimed that the pest’s spread had slowed last year following a period of rapid growth in northern regions and Wales.

All Change for Slug Control

Slugs and snails, regarded as one of the biggest gardening irritations, recaptured the title of number one garden pest for the first time since 2017, with gardeners reporting damage on ornamentals such as hosta and clematis, as well as potatoes and beans in kitchen gardens.
Gardeners are embracing new methods of slug control as metaldehyde, the active ingredient in traditional chemical slug pellets, is to be phased out for outdoor use. The sale of metaldehyde-based slug pellets in the UK will end this month , while gardeners must use up or dispose of remaining stocks by 31st March 2022.
However, many retailers have already switched to ferric phosphate-based slug pellets, which are claimed to provide effective control without putting wildlife at risk. The withdrawal of metaldehyde is throwing the spotlight on natural slug deterrents such as biological controls, beer traps, gravel or eggshell barriers and copper tape around pots.
Red alert for future threats While many pests and diseases in the study are well known to gardeners, the RHS expressed caution about emerging threats. RHS principal entomologist Andrew Salisbury said: “It is imperative that we continue to anticipate future threats such as the disease Xylella, which is already present in Europe, and the marmorated stink bug, to protect gardens in future.”
The brown marmorated stink bug, reported at three locations in England, has established as a pest in America. In contrast, outbreaks of Xylella fastidiosa have hit continental Europe, attacking olive, lavender, cherry and rosemary.

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