It is expected to take several days to establish whether or not the deadly fungus has been stopped from spreading into the wider environment and native woodlands.
Forestry Commission (FC) Wales said the affected trees were small and "measures have been put in place to minimise the risk of the disease spreading to the wider environment".
Peel said the closure had been "a purely precautionary measure to prevent passing vessels spreading the oil into a wider area than that already affected".