Warm and wet weather this year has led to box blight and slugs attracting more queries than usual, pushing them up the RHS list of top pests and diseases for March-May 2014.
RHS chief horticultural advisor Guy Barter said: "Slugs had a high survival rate over the winter and have had good conditions for the last six weeks so numbers are pretty high."
On box blight, Barter said: "Infections are persisting and increasing after wet weather last autumn. People are reporting it after they have bought new box.
"We tell them to put it in quarantine for six weeks in a glasshouses to make sure it's free of disease.
Barter said designers at Chelsea Flower Show this year used yew instead of box and awareness of box blight is high at the moment. He said: "We have identified hotspots for box caterpillar too. It's widespread in Germany but CABI suggests because of cooler British summers it might be restricted to southern regions here.
"But we're seeing more cases, and in some areas in the same place year after year so it's getting established. Clearly box has not got a great future, which is really sad."
Box caterpillar, first seen in the UK in 2005, is established in Woodford and Loughton in east London and in Stoke Poges, near Slough, with eight sightings this spring in all.
On import bans, he said: "History is not encouraging here. Sooner or later most things that get across the Channel."
Barter added that horse chestnut leaf miner and ash dieback meant those trees were not worth planting "for the long-term", while pests on Spanish chestnut and plane, which have led to import bans, are "not encouraging". He did not recommend "spending time and money on these trees until the situation is clearer". On oak, he said oak processionary moth meant, if it gets established "people will have big problems with young oaks defoliating. It will be difficult to get new ones growing".
He added that stress on plants was attracting queries after a cold and wet snap in May damaged soft growth.