Officials were slow to assess the risk of ash dieback in the UK because they were dealing with a number of diseases and had few experts available, MPs have been told.
The disease, which causes leaf loss and crown dieback and can lead to tree death in ash trees, was originally thought to be caused by a fungus already found in the UK and banning imports of ash trees would not have stopped its spread.
But concerns have been raised that officials were slow to assess Chalara ash dieback and the need for an import ban to prevent its spread once scientists had found the disease was caused by a fungus that was not endemic in the UK.
Quizzed by the Environment, Food and Rural Affairs Committee, the Forestry Commission said it had started a pest risk assessment in 2011 after the new evidence on the cause of the disease emerged.
Roger Coppock, head of analysts at the Forestry Commission, said: "We did start work on a pest risk analysis in 2011 once that new evidence came out.
"The difficulty was we were already dealing with a number of outbreaks of other pests and disease at that current time, and the record will show the number of pathologists available in Britain to deal with some of these pests and diseases is very small at the present time. Ideally we would have liked to get the pest risk analysis done more rapidly than we did do but were dealing with fires at home."
Mr Coppock said that more people were being put into place to be able to do preliminary work on risk assessments before it was signed off by specialists.
Environment Department (Defra) chief scientist Professor Ian Boyd said that ash dieback had been just one of many diseases threatening trees.
"At that time the tree health expert group had identified up to 28 different pathogens threatening our shores and the amount of evidence required to deliver a pest risk assessment on all of those is immense. There's a huge task there," he said.
Prof Boyd said that, with hindsight, things could have been done differently, but it was hard to see which diseases were likely to become a problem.