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 recent article of Peter's summarizes what we know (and don't know) about RAD/SAD.
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 tissue above.
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 southwest injury.
- 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 prevent infection.
- 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 rot pathogen.
Stay tuned - we will be hearing a lot more on this as the northeast works together to find some answers.
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|Author:||Kristy Grigg-McGuffin - Horticulture IPM Specialist/OMAFRA; Michael Celetti - Plant Pathologist- Horticulture Crops/OMAFRA|
|Creation Date:||21 December 2016|
|Last Reviewed:||21 December 2016|