Bacteria canker in plums
 

I bought two healthy minarette trees in November 2006; one plum and
one cherry. Both are planted in very large pots and are in a sheltered
position on my patio. Both grew well and the cherry flowered but did
not set fruit last year. Unfortunately I recently discovered that the
plum has got severe bacterial canker (brown depressed areas especially
round the stem joins and brown sticky resin). The plum appears to be
completely dead and the buds died almost immediately after forming.
The cherry however is in leaf with a number of fruits developing and
seems very healthy.

The two trees are next to each other with a 18 inch gap between the
foliage / stems. I want to discard the plum and replace it with a
dwarf apple or pear. Could someone help me with the following:

Is the cherry at risk of catching bacterial canker and should I
sterilise the pot once I have removed the dead plum to eliminate the
risk of spread from the plums old pot to the cherry tree or is the
risk minimal ?

Feral damsons grow in the field behind my house (approximately 100
meters away). Could they be a source of the canker or is the
Pseudomonas syringae generally present anyway?

Although apples and pears get fungal canker am I correct in assuming
the bacterial canker will not harm them and it is therefore safe to
use the plum's old pot to plant an apple or pear tree in ?

Thanks for your help.

Kitti