Will black knot fungus harbor in the roots of a recently cut down tree and infect another tree planted in its place?

Question

I purchased a house last November with a beautiful tree in the front yard. I don't know what breed it was, but I believe that it might have been a type of plum tree. (see photo at bottom)

Unfortunately, that tree turned out to be completely covered in black knot fungus (looked like poop turds hanging from branches). After consulting with a few people, it was decided that the tree had to come down, as the fungus was on all limbs.

The tree was fairly young (~10 yrs from what the neighbors tell me). I cut the tree down a few weeks ago and am now considering replanting another tree in its place.

Do I need to remove the roots completely, or should grinding the stump down, say 4" down be ok? Is the fungus something that could be still 'living' in the roots of the old tree, and could it possibly infect a new tree that I plant in the same place? If so, what can I do, short of trenching out all the old roots?

Answer 1

Black knot tends to affect the upper, aerial parts of plum trees, sometimes the trunk if left long enough, so boring out the stump should be done to the usual depth (12-18 inches), but the main roots which spread outwards can be left. It probably isn't wise to plant another plum tree though - some investigation round the local area is called for. This disease is airborne - it spreads during rainy spells when the temperature is above 55 deg F, and it does it by spores, which may be blown fair distances, so if you're downwind of an infected tree in the area, it'll infect your new tree all over again, and your own tree may well have already infected other plums nearby. There is one called President which is reputed to be resistant to black knot, so you could try that if you're desperate for another plum, otherwise, select a tree which isn't a Prunus - black knot can also affect cultivated cherries, including ornamental cherries, as well as plums, and to a much lesser extent, both wild plum and cherry. Prunus triloba (flowering almond) though, does not appear to be susceptible to this infection.