General manager Chris Mills said: "The UK is always at threat from new pest and disease species and with more and more importing happening every year, it's vital that we step up the biosecurity.
"Ash dieback has been well documented in the media, but there is more on the horizon posing a major threat to trees.
"Southern Europe is suffering with Ceratocystis platani, also known as canker stain of plane, an invasive fungal pathogen which will have a devastating effect on the urban tree population.
"There's also Agrilus planipennis, a green beetle more commonly referred to as emerald ash borer, which is spreading from Moscow at a rate of 25 miles per year.
"It's believed that this insect, which is native to Eastern Asia, has spread through wooden packaging such as crates and pallets."
Mills warned that while research into treating trees was ongoing, responsibility also lay with tree planters to ensure they sourced good-quality stock.
"There is always research taking place in the background. However, in most cases when a pest or disease has entered the UK it is already too late.
"When planting new trees it's vital they are selected from a well-established nursery that has tight biosecurity measures in place.
"Where a tree is already infected, we need to act fast to minimise destruction as far as possible. Anyone worried about an existing tree should contact a local tree consultant.
"There is also useful information available from the Forestry Commission website. The spread of disease in trees has a devastating effect on the environment. It will also have significant financial ramifications for the forestry industry and the UK economy as a whole."
Mills flagged up a report by MP Zac Goldsmith in partnership with the Countryside Restoration Trust, which put the total annual cost of invasive species to the British economy at around £1.7 billion.