I'm not an expert on replant disease, but I'm not certain that the
world really has any experts on this subject because the causes and
responses to treatments are so variable both among and within
specific geographic locations. No one knows for certain what causes
replant disease, but it seems likely that replant disease is a
complex syndrome with "contributing factors" that vary with geography
and soil type. All of the following have been identified as
contributing factors at one time/place or another:
1. High nematode populations, especially Pratylenchus penetrans.
Nematodes are more likely to be a contributing factor in sandy soils,
and therefore may be more important on cherry than on apple.
2. Arsenic toxicity, as already mentioned in earlier posts, is a
problem in the arid production regions of Northwestern US but has
never (so far as I can recall) been identified as a problem in the
Northeastern United States.
3. Herbicide residues: Dr. Ian Merwin at Cornell has shown that
even very small residues from residual herbicides such as simazine
can stunt new trees. Generally, I would suggest that growers
planning to replant old orchards without fallowing the land for
several years should stop all use of residual herbicides at least 2
years before they remove the orchard and put in new nursery stock.
4. Actinomycetes in growing in old orchard soil have been
implicated as a potential component of replant disease, as have other
"root nibbling" fungi.
5. Rhizosphere (root surface) bacteria that suppress growth of
new trees may survive from old root systems and rapidly colonize
newly planted trees.
6. Add anything else here that might you might wish to target as
a research project or list any organisms against which you wish to
establish a personal vendetta :). Actually, there has been pretty
conclusive research demonstrating that a living organism is involved
in at least some cases of replant disease, but we just don't know
which of the multitude of soil organisms is/are the culprit(s).
Suggestions for working around replant disease are as varied as the
list of potential causes.
1. Fumigation is the most widely recommended "cure". However, it
is not practical in many sites, especially if you have large rocks in
your soils that will bounce the injectors out of the soil every few
feet down the row. Also, getting the perfect soil conditions that
will allow a good fumigation job is not as easy as it sounds. Too
dry and the fumigant will come right back up through the air spaces
in the soil before it can do the job. Too wet and the fumigant will
not distribute evenly. Too shallow and the replant organism(s) may
be surviving lower in the soil profile. Using methyl-bromide with
poly tarping to keep the fumigant in place was almost foolproof if
you could afford it, but methyl bromide is on its way out.
2. Fallowing land between planting has merit, but I believe that
early work in Holland showed that effects could still be detected
when orchard land was replanted after 8 to 10 years of alternate
cropping. That may not apply to all geographic locations, but it
suggests that fallowing alone may not be the complete answer.
3. Placing soil in the planting hole can work on a small scale,
but probably not for 10-acre plantings at tree densities of 500
trees/A.
4. In acid soils, adding lime may help to reduce effects of
replant disease. Getting pH adjusted prior to planting may alter the
"balance of power" in soil microbial communities so that replant
disease organisms cannot dominate. I suspect that adding organic
matter can also help, but I'm not certain that I've seen studies
where that has been documented because it is difficult to alter
organic matter content without also altering fertility.
5. Getting newly planted trees watered immediately after planting
and at regular intervals thereafter often promotes rapid
establishment of trees. Again I have no evidence, but I've wondered
if rapid growth during the first 3 months after transplanting allows
trees to "outgrow" replant disease, perhaps by promoting colonization
of growth-promoting soil bacteria instead of the growth-inhibiting
organisms associated with replant disease. In eastern US where we
traditionally depended on rain to do our watering, I suspect that we
too often allowed newly planted trees to go into water stress during
the critical period of root development immediately after planting.
The soil may look and feel wet, but the very limited root surface
areas of new transplants may mean that trees cannot access adequate
water unless soils are close to field capacity for a while after
transplanting. Observation would suggest that adding irrigation in
late June for trees planted in April is of little value. Instead, I
see now see successful growers applying irrigation within 24-72 hr
after planting and then supplementing rainfall regularly thereafter
until trees have several feet of new growth.
6. At the Hudson Valley Lab, we've found that replant disease can
often be circumvented just by planting new trees in what were
formerly row middles of the old planting. In my experience, this
works even if the new trees are only about 2-ft off-set from the
driplines of the trees that were removed. Where we've used this
approach, we have not tilled the entire field. We simply ripped the
new tree rows with a chisel plow (working in lime and fertilizer),
and then rototilled the tree rows to get a good planting bed while
leaving the rest of the land untilled exept as need to level old tree
holes. I'm not certain if tilling (especially if one tilled
crosswise to the old row direction) would distribute the replant
problem into the former row middles and therefore reduce the
effectiveness of this approach. One encounters difficulties,
however, when the new planting involves a different row spacing than
the old planting. In fall of 1994, we removed a 2.5-acre fungicide
test block that was on 25-ft row centers and replaced them with trees
on 18-ft row spacings. I was able to adjust most of the new rows so
as to avoid planting over old rows, but one row ended up squarely on
top of the old row. That row never grew as well. When the planting
was 6 yr old, I decided to topwork every third tree in the row to a
different cultivar. We had a hard winter the year after grafting,
and nearly all of the topworked trees in the "replant" row
winter-killed whereas very few top-worked trees in the other rows
were affected. That incident illustrates that replant disease does
not just slow trees down: it keeps them weaker and/or less productive
throughout their life history (as has been documented in other
published reports).
So to summarize this long message, the following are my "top 4"
recommendations for avoiding apple replant disease in northeastern
United States:
1. Do not use any residual herbicides for at least two years
before planting new trees.
2. Fallow land (or grow other crops, but no herbicides!) for at
least one year (2-3 years is better), OR offset rows in the new
planting compared to the old planting.
3. Adjust soil pH at least six months before planting, and work
lime down at least 12-16 inches if possible.
4. Water trees immediately after planting and keep watering them
for several months after planting unless rains keep soil moisture
near field capacity.
--
**************************************************************
Dave Rosenberger
Professor of Plant Pathology Office: 845-691-7231
Cornell University's Hudson Valley Lab Fax: 845-691-2719
P.O. Box 727, Highland, NY 12528 Cell: 845-594-3060
http://www.nysaes.cornell.edu/pp/faculty/rosenberger/
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