Apple tree collapse: What
we know (and don't know)
Grower reports of young apple trees collapsing first started coming
in shortly after bud break this past year. The number of Ontario
orchards affected steadily increased throughout the season to over
20 sites across the province documented by OMAFRA, with many other
growers describing similar issues.
In all cases, seemingly healthy young trees were breaking bud or
growing normally, then suddenly dying in a matter of time that felt
like almost overnight. Samples submitted to the University of Guelph
Pest Diagnostic Laboratory were coming back with various pathogens,
mainly phomopsis, cytospora or black rot canker. However, these
are all weak and opportunistic pathogens that can infect wounds
or injured, stressed and weakened trees. What was causing these
trees to be weak in the first place remained a mystery.
While attending the New England, New York and Canadian Fruit Pest
Management meeting in October of this year, I was surprised to hear
many regions in the northeast were experiencing the same issues.
Following that meeting, Ontario has joined forces with researchers
and extension specialists from many areas including Penn State University
(PSU), Cornell University, North Carolina State University, Virginia
Tech, and University of Massachusetts to attempt to find some answers.
Kari Peter from PSU has referred to this issue as Rapid Apple Decline
(RAD) or Sudden Apple Decline (SAD) due to the sudden collapse of
apple trees from the time first symptoms appear to tree death. A
article of Peter's summarizes what we know (and don't know)
Similar to the symptoms Peter describes in her article, what was
typically seen in Ontario this year included:
- Young (3-6 year old) dwarf trees, typically Gala/M9 rootstock
though some collapse on M26 rootstock was observed later in the
- Other regions in the northeast were seeing symptoms on
2-8 year old trees. Gala/M9 rootstock was mostly affected,
though collapse was also reported on Fuji and Golden Delicious
on M9, too.
- Dead or declining trees were intermixed in a block with healthy
trees (Fig 1).
- Purplish canker present with necrosis, or dying tissue occurred
at the graft union and extended up the trunk (Fig 2).
- Below the graft union, rootstock and root system appeared healthy
and often sent out a large number of root suckers (Fig 3).
- Leaves had a pale yellow hue, and then rapidly turned purple
or reddish just before the tree collapsed.
- Collapse occurred in blocks as early as bud break to trees with
a full fruit set and normal crop load (Fig 4).
Figure 1. Dead (center) or declining (far right)
trees in block with healthy trees (far left).
Figure 2. Dead trees with bark removed. Note necrotic
tissue at graft union and extending up trunk with healthy green
Figure 3. Collapsing tree with large number of root
suckers. Note purple canker at graft union.
Figure 4. Collapsing tree with full crop load. Photo:
K. Peter, PSU
Figure 5. Presence of borer holes in declining tree.
What could be causing this collapse?
- Weather-related injury - As OMAFRA's plant pathologist Michael
Celetti cautioned in his April 2016 ONNL article, Alert: Cankers
in Apple Orchards, cankers can show up in orchards 1-2 years after
a severely cold winter. With the cold winters experienced in Ontario
during 2014 and 2015, canker-causing pathogens that infected through
the cold injury wounds could result in noticeable symptoms by
2016 and even possibly cause tree decline. Trees that are 3-6
years old now would have been newly planted or in their first
years during the 2014/2015 winters and therefore, extremely vulnerable
to cold injury during those severe cold snaps.
There is also a good chance the dry conditions experienced across
the province this year may have helped push already weakened trees
over the edge. It is hard to know whether collapse would have
been as extensive in some blocks had there been adequate moisture.
It is possible that the combination of the winter injury in 2014/2015
and subsequent infection by weak pathogens, followed by the dry
conditions in 2016 exacerbated tree decline.
- Pest-related injury -Typical pathogens such as fire blight and
Phytopthora that often cause tree collapse were ruled out in the
majority of the cases in Ontario. In most orchards with RAD/SAD,
no bacteria were isolated from affected tissue submitted to the
University of Guelph Pest Diagnostic Laboratory. However, there
has been quite a bit of discussion in the research community,
as of late, whether the fire blight pathogen can go undetected
at times and cannot be ruled out as part of the cause in some
orchards. That said, symptoms caused by these pathogens often
occur at ground level just below the graft union in the rootstock
part of the tree, whereas trees with RAD/SAD had symptoms (necrosis)
extending upward only from the graft union.
As mentioned earlier, fungal pathogens were detected in practically
all orchards where samples of affected trees were submitted to
the lab. These fungi included Phomopsis spp., Cytospora
spp. and Botryosphaeria obtusa (black rot). While these
pathogens would have likely contributed to the tree decline, they
were likely a secondary infection entering a pre-existing injury.
In some cases, wood-boring insect infestations were found in affected
trees (Fig 5). However, borer issues were not identified in all
orchards with tree collapse. The most likely scenario is that
the borers came in after the tree was already stressed.
Damage from herbicides on the base of young trees has not been
looked at extensively in Ontario due to the longevity that is
required for such a study. Impacts from grower-applied herbicide
may not manifest for years after treatment, so pin-pointing the
cause in a commercial orchard can be very difficult. However,
herbicide application has the potential of wounding a young tree
and increasing its susceptibility to secondary infection.
- Site or cultivar selection - Many questions have been asked
whether the tree collapse is a site selection or replant issue.
In Ontario, tree collapse was reported across the province in
all growing regions on both virgin and old orchard land. As well,
only certain varieties in a block were typically affected. For
instance, in one orchard approximately 10-15% of Gala/M9 were
dying while the McIntosh/M9 immediately adjacent were showing
no symptoms at all. If the issue was site-specific, one may expect
to see more consistent problems in a block among varieties/rootstock
of similar disease susceptibility and likely restricted to one
region of the province.
The interesting pattern of affected trees being mainly Gala/M9
suggests that there may be a variety/rootstock combination issue.
However, other regions in the northeast have seen issues with
different varieties and/or rootstocks.
Unfortunately, there is no clear-cut answer to this problem at
the present time. With the growing trend towards higher density
plantings and pushing trees into productivity in the 2nd or 3rd
year, there is bound to be an increased number of stressors placed
on young apple trees. In all likelihood, RAD/SAD is a complex of
issues caused by any number of stresses: Drought? Cold injury? Herbicide?
Replant disease? All of the above? At this point, more research
is needed to answer these types of questions.
What can be done?
This is a difficult question to answer without knowing the cause
for tree collapse. The best thing to do is minimize tree stress
and remove potential sources of inoculum where possible.
- Consider irrigation, especially on M9 rootstock.
- Maintain a proper nutrient program throughout the year based
on soil and leaf analysis.
- Paint trunks of young trees white or use tree guards to prevent
- If re-planting on an old orchard, stagger planting rows to avoid
planting directly in old tree sites and be sure to test soil for
nematodes and nutrient levels prior to planting.
- Prune out fire blight infected shoots as soon as possible to
prevent spread to rootstock.
- Orchards planted near woodlots may be more vulnerable to canker-causing
diseases. Protect trees during times of disease development to
- Remove dead or dying trees during the dormant season to reduce
the inoculum sources within the orchard. Burn trees or chop prunings
with a flail mower.
- Remove mummified fruit left in trees which can harbor the black
Stay tuned - we will be hearing a lot more on this as the northeast
works together to find some answers.