RHS pest and disease ranking reveals another bad year for box

Company: The Royal Horticultural Society

The 24th annual pest and disease ranking from the Royal Horticultural Society (RHS) reveals new and growing areas of concern for gardeners - as well as underlining the problems posed by perennial pests such as box tree caterpillar.

The charity’s analysis of thousands of gardener enquiries revealed that the warm, wet summer in 2019 appears to have been a particular problem for tree fruit growers.

Box tree caterpillar topped the pest ranking again as it becomes more widespread. It’s the third year in a row that it’s been at the top of the list - and brings 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 survey1.

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 2018 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 – the final blow to plants that have lost a lot of their root system to the disease.

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.

Brown rot of fruit, along with apple and pear scab, were 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. The RHS is investigating the cause of this upsurge in enquiries.

RHS research

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.
  • On 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.
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