Replanting and fireblight

I've been dealing with some fireblight the last couple years, trying to stay ahead of it with pruning. I lost two Prairie Spy trees to the nasty bacteria. I know the bacteria is everywhere, but does it particularly stay in the soil? Is it a mistake to plant new trees in the same spot? Thanks!

Pat McBride
Twin Oaks Orchard
Zone 4b in Minnesota

Fireblight infection is only likely if live tissue in the orchard is already infected. So, provided you have removed all infected trees or wood, you are safe. This does mean, however, that root sections that may harbor the bacterium throwing up infected shoots. So be thorough. Fireblight dies out when not on a suitable host, which includes aged prunings. Once live tissue dries out, conceivably even a full tree, it is unlikely to be a vector.

A point worthy of bringing up with any disease is overall management. It is obvious that trying to remove any trace in the orchard is good form, but what is often overlooked is how it got there in the first place. Unless a pathogen hitched a ride in on stock, it is more than likely that a problem disease is giving us trouble because it is indigenous. Fireblight has a lot of hosts besides the pome fruits, and many fungal diseases have an ungodly amount of places to hide. This means that plant and soil management and cultivar choice are often more important than an attempt at clean slate agriculture.

Replanting into the same spot can be a problem and has been dubbed "replant disease" in some older texts on orchard management. There are multiple and often interacting potential causes such as depleted nutrients, nematodes, detrimental fungal and bacterial populations and even just compacted soil. Phytotoxins from the decomposition of the dead trees roots have also been implicated. As Todd points out, In the case of fungi, bacteria and phytotoxins simply waiting a year can take care of the problem. Sometimes going to an alternate, not closely related fruit species (e.g. cherries instead of apples) works.

There is an excellent discussion on replant disease in the 1998 publication, Organic Tree Fruit Management by Linda Edwards of the Certified Organic Associations of British Columbia.

Pommes de Terre Acres
USDA Zone 5 - Dixon, Montana
Intermountain West Region