Our 24th annual pest and disease ranking reveals new and growing areas of concern for gardeners – as well as underlining the problems posed by perennial pests such as box tree caterpillar.

Very hungry caterpillars

Box tree caterpillar topped the pest ranking again as it becomes more widespread. 2019 was the third year in a row that it’s been at the top of the list, bringing more enquiries than the rest of the top five combined. The continued prevalence of enquiries was matched by the doubling of reports received via the RHS web survey.

However, the alder leaf beetle is in the top ten for only the second time, becoming the fifth most popular pest enquiry in 2019.  Its appearance may be due to range expansion and, while it is unlikely to have a long-term effect on tree health, this blue beetle can cause defoliation of its main host, alder.

Fungi under the spotlight

Honey fungus also remained the most prominent garden disease – as it has done since the ranking’s inception in 1995. However, it dropped from 26% of enquiries in 2019 to 18% in 2019. Increased rainfall in the summer of 2019 compared with 2018 meant that plants were less likely to suffer from drought stress – which can be the final blow to plants that have lost a lot of their root system to honey fungus infection.

Brown rot of fruit, along with apple and pear scab, was of increasing concern to gardeners, taking fifth and seventh spots respectively. The same fungi are responsible for blossom wilt in the spring and then brown rot in summer. The increased prevalence is possibly due the wetter, milder weather. 

An unexpected entrant into the top ten was rose black spot, despite many gardeners knowing the signs of this disease and its management. We are investigating the cause of this upsurge in enquiries.

Hit parade of garden nasties


Top pests 2019


Top diseases 2019


Box tree caterpillar


Honey fungus


Vine weevil


Phytophthora root rot


Slugs and snails


Box blight


Fuchsia gall mite


Pear rust


Alder leaf beetle


Brown rot of fruit


Woolly aphid


Leaf spot and canker of Prunus


Rosy apple aphid


Apple and pear scab


Viburnum beetle


Rose black spot


Glasshouse red spider mite


Blossom wilt of fruit trees


Plum leaf-curling aphid


Powdery mildew of Prunus

Researching greener controls

To provide the very latest evidence-based advice to gardeners, RHS Plant Health researchers work across a range of pests and diseases. On box tree caterpillar, investigations are focusing on biological controls such as the use of nematodes, while work on box blight is engaged in breaking the disease cycle by minimising spore production.   

On vine weevil, work is due to start on biological controls in different peat-free growing media. A wide range of slug and snail research being undertaken as the RHS seeks to understand which species are causing problems in gardens, how slug fauna is changing, and test control methods.

For honey fungus, recently updated advice involves the development of integrated disease management strategies. On phytophthora root rot, diagnostic data is helping to establish a robust ranking of susceptibility or resistance of garden plants as well as investigating factors that contribute to the spread of the disease.

Matthew Cromey, RHS Principal Scientist (Pathology), said: “Pests and diseases are among the main challenges we face as climate change affects our gardens and horticulture more widely. As the UK’s gardening knowledge bank, our research will help increase biosecurity and provide best practice. We want to develop a nation of gardeners equipped and motivated to deal with the challenges of our changing world.”
Read more about our work protecting plant health in gardens.



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The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.