Time for a Tree Inspection

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When leaves have mostly fallen from your trees, it’s time to give them a good going over. Source: A melbournechapter.net & svgsilh.com, montage: laidbackgardener.blog

The disappearance of leaves from deciduous trees in the fall is often seen by home gardeners as the end of the horticultural season, but it’s also a golden opportunity to study your trees’ condition. Bare of foliage, their entire structure is visible, allowing you to see any flaws. So, a quick annual tree inspection tour, just to make sure everything is fine, is always a good thing.

Structural Problems

The most obvious problems usually have to do with weak, damaged or dead branches. Some branches may have snapped or bent and need removing. Others rub on neighboring branches, weakening and, eventually, killing both. Ideally, one or the other really ought to be removed.

Sometimes, you see clusters of weak branches all growing from about the same point, a probably as the result of an earlier poor choice in pruning. You’ll have to prune off all but one branch (probably the healthiest, straightest one) to give it a chance.

You can also remove any suckers (upright branches with no secondary stems) if you see any. Or there may have a very vigorous upright branch that is threatening to become a second trunk and that usually leads to a weak fork that can tear off and seriously damage the tree. So remove that as well.

Corrective Pruning

In all these cases, the solution is to do a little bit of pruning.

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If you can reach it, you can prune it! http://www.danskhusoghaveservice.dk

You can do the pruning yourself if the branches are near enough to the ground for you to be able to reach them, at least with a pole pruner. (Obviously, any time you remove a branch higher than your head, you should be wearing a safety helmet.) For out-of-reach or oversized branches, it’s better to have a certified arborist do the job. Avoid fly-by-night tree trimmers who often damage trees rather than helping them.

Also, over time, the lower branches of a tree sometimes start get in your way. Since you don’t want anyone to bump their head when walking under the tree, it is often better to “limb it up” (also called “raising the crown”) by removing the problematic lower branches and that’s something you can do yourself.

When to Prune?

To be completely honest, the time of year when you prune really doesn’t make a lot of difference. Yes, many experts say late winter or very early spring is the best time, while late fall/early winter is the second-best choice. In fact, the easiest thing to do is simply to prune when you first see the problem, before it gets any worse.

Two Typical Pruning Scenarios

When you simply want to shorten a branch for whatever reason, just cut it off slightly beyond a secondary branch pointing in the desired direction. The latter will take over and replace the original one, thus preserving the tree’s natural symmetry.

Removing a branch entirely, right back to the trunk, is a different situation. Doubly so if it’s a major branch. If you simply saw it off from above, the branch will likely tear loose as you near the end, pulling off a strip of bark as it falls, thus damaging the tree’s structure. Fortunately, there’s an easy way around this.

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Source: Claire Tourigny, from the book Les 1500 trucs du jardinier paresseux

First cut a notch under the branch, about 1 foot (30 cm) from the trunk and about ¼ to ⅓ of the way through. Then change position. Now saw from the top about 2 to 3 inches (5 to 7 cm) beyond the first cut until the branch breaks right off. It will now do so away from the trunk, thus avoiding any damage to it.

The final cut is the most important one, as you have to remove the stub. Cut as close to the trunk as possible, but without damaging the branch collar, the swollen bump seen at the base of the branch, because it’s from the collar that the new cells designed to cover the wound with fresh bark will form.

Finally, no, there is no need to cover the wound with pruning paint. It’s actually bad for the tree. Read Garden Myth: A Tree Wound Requires a Coat of Paint to understand why.

Bare Tree Leads to Other Discoveries

You may have other surprises when the leaves fall.

Often, you’ll find a bird’s nest, sometimes only a short distance from your front door, you never noticed in the summer, but that is visible now that the leaves have fallen. It will be empty in the fall and winter.

Or maybe you’ll discover a squirrel’s nest, which usually looks like a ball of dead leaves high up in the tree. In this case, the squirrel is likely still inside, as it’s a winter nest. You’ll discover that squirrels don’t actually hibernate. Although they spend much of the winter sleeping, they still come out from time to time to look for the nuts they buried in various hiding places earlier in the fall.

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Wasp nests are empty in the fall; the first frosts will have killed off their inhabitants. Source: USFWS Midwest Region, flickr.com

The other nest you’re likely to notice, looking like a greyish round ball in the branches, is a wasp nest. No need to panic: by mid-fall, the original queen and her workers will already be dead. The only survivor of the whole colony is the new queen and she’ll have left the nest, burying herself in the ground somewhere. She’ll wake up again in the spring and start building a new nest in another location. So, since the old nest is empty, if you want to recover it as a decoration, go for it! Otherwise, the nest will simply disintegrate over time.

Signs of Disease and Insects

So much for pleasant discoveries. Others are not so encouraging.

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Black knot disease is revealed when the leaves fall. Source: http://www.edmontonlandscapingoutdoorspace.com

Probably the most disagreeable discovery you make when a tree is free of leaves is black knot disease (Dibotryon morbosum, syn. Apiosporina morbosa). This disease of plums and cherries (and occasionally other stone fruits) attacks the tree’s branches: you’ll discover an elongated black mass covering part of a branch. Black knot acts much like a cancer, cutting off the flow of sap to the affected limb and eventually causing its death even as it spreads to other branches. Sadly, once you discover black knot in a tree, it’s probably doomed: at most you can extend the tree’s life by removing the nodules. Read The Cancer of Plums and Cherries to learn how pruning can be of some help.

Whether you decided to prune or not, I suggest you plant a replacement tree (and not a cherry or a plum!) in the spring. When your tree finally does die, its replacement will have already reached a decent size.

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These innocent-looking bumps are scale insects. Source: growingsmallfarms.ces.ncsu.edu

You may see other odd growths on the trunk and branches. Sometimes they’re harmless, like lichens and moss, sometimes they are serious diseases, such as crown gall disease (Agrobacterium tumefaciens), and sometimes the growths are caused by insects. Tiny bumps on the bark, for example, may well be scale insects.

Whatever causes the growths, you’ll have to identify the problem before appropriate treatment can be found.

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The perfect day for a tree inspection! Source: http://www.dpreview.com

Take your inspection tour on a beautiful late fall/early winter day, when you feel like taking a bit of a stroll outdoors anyway. And remember, this annual checkup is not only free, but often helps prevent very serious problems in years to come.

Are Fruits From Diseased Plants Still Edible?

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20170910A Olivier Bacquet, Flickr

Diseased plum trees can still produce edible fruits. Photo: Olivier Bacquet, Flickr

Question: I have a plum tree severely affected by black knot disease, but it still produces a lot of plums. Can I still eat them?

Lise Douville

Answer: Yes, you can eat them.

In general, when you see healthy fruits on a plant suffering from a disease, they still remain perfectly edible and safe to eat. This is not only the case for plums or cherries on black knot-ridden trees, but also tomatoes or squashes from plants suffering from powdery mildew and other leaf diseases or cherries or currants on plants with leaf spot, to give only a few examples. If the plant still produces fruits in perfect condition, don’t hesitate to harvest them and consume them.

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Apple scab may be unsightly, but you can cut it off and still enjoy the fruit underneath. Photo: omafra.gov.on.ca

Fruits superficially marked by disease are also edible. Apples with small amounts of scab or tomatoes slightly affected by late blight remain perfectly delicious and safe to eat: just remove the lesions with a knife.

Obviously, to what degree you tolerate these kinds of imperfection depends largely on you. There comes a point where so much of the fruit is covered with scabs or other lesions—or the lesions are so deep—that there’s not much left to eat!

Also, your tolerance will likely depend on the circumstances. Personally, I don’t hesitate in the slightest to consume imperfect fruits from my own garden, but I still seek impeccable ones when I’m at the grocery store. And when I harvest fruit to make juice or preserves, I’m much less picky than when I plan to display them in a basket on the table where everyone can see them.

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When a fruit is heavily affected by a lesion or rot, just toss it into the compost bin. Photo: omafra.gov.on.ca

That said, there are diseases, such as botrytis rot, that affect both foliage and fruits or fruits alone and really go too far. They alter the taste or texture of the fruit, giving dry, soggy or rotten fruits that are very unappetizing. Generally, they are of such inferior quality that the problem—and the solution—is obvious!

Not to say that even fruits severely affected by disease are harmful per se (there are few plant diseases that can affect humans, ergot [a disease of rye] being the main exception), but there is no reason to consume a fruit that is in such bad shape. When a fruit is thoroughly rotten, simply dump it in the compost bin.

So, sick plants can often produce delicious, healthy fruits … and fruits moderately affected by superficial diseases are also edible. Don’t hesitate to consume them!20170910A Olivier Bacquet, Flickr

Does ‘Schubert’ Chokecherry Come Already Contaminated with Black Knot Disease?

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A typical ‘Schubert’ chokecherry

One of the best-selling small trees in Northern North America is the ‘Schubert’ chokecherry (Prunus virginiana ‘Schubert’)… and I have to wonder why.

Not that it’s not an attractive tree: with green foliage in the spring that quickly becomes dark purple, it really stands out from the crowd. In addition, the plant as usually sold has a strong, straight trunk and an attractive rounded crown.

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‘Schubert’ chokecherry leaves and berries.

Its clusters of small white flowers in spring are pretty, but not spectacular (they suffer from comparison with the much more colorful crabapples and ornamental cherries), but the berries, green at first, then shiny dark purple, almost black, are not without charm, plus they attract birds. And ‘Schubert’ is incredibly hardy too: absolutely thriving as far north as in zone 2.

What is disappointing is its health. It seems to me that every ‘Schubert’ chokecherry I see is infected with black knot (Dibotryon morbosum, syn Apiosporina morbosa), a nasty fungal disease.

What is Black Knot?

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Black knot disease.

Black knot is most visible as black galls that form along branches. They are puffy, hard, cylindrical growths, charcoal black in color and of variable length. They not very visible in summer: you have to look very closely to see them, because the foliage hides them from view. It’s in fall and winter, when the tree is leafless, that they are very clearly visible.

The affected branches grow normally at first, but eventually the gall, one-sided at first, girdles the branch and cuts off its flow of sap. Thus all growth beyond the gall dies.

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Over time, black knot moves inwardly, affecting larger and larger branches.

At first galls mostly appear on the tree’s outer branches, but the disease then seems to evolve inwardly, reaching increasingly larger branches over time. Eventually, the trunk itself is reached and the tree eventually dies, but that may take 7 to 12 years.

Black knot produces spores that are mostly carried from tree to tree by wind, although once the tree is infected, it is also readily spread by rain and infected pruning tools.

Not Much You Can Do

If you turn to the Internet for help, you’ll get some pretty useless advice. Something like “prune off infested branches, cutting about 4 inches (10 cm) below the base of the gall and sterilizing your pruning shears between each cut with rubbing alcohol so as not to spread it.” Sounds great in theory, but have any of the experts actually tried it? Like a cancer that metastasizes, new galls appear on other branches even after a thorough pruning job has been done. I know of no one who has ever managed to cure a ‘Schubert’ chokecherry of black knot by pruning.

You may also be told you can spray the tree with some sort of fungicide, like lime sulphur or Captan, usually in early spring. Have fun with that one too: yet another exercise in frustration!

From what I can see, once the tree shows the first galls (and there is rarely just one, even the first year), it is doomed. It’s just a question of time before it either dies outright or looks so ugly after repeated prunings that you simply remove it.

Here’s another good one: to prevent black knot, you’re told to eliminate any wild cherries or plums and any infested tree growing within 600 feet (180 m) of any ‘Schubert” chokecherry, because they can be hosts of the disease. Good luck with that, because it means getting the cooperation of the entire neighborhood: 600 feet is basically 2 city blocks and I’m not sure that all homeowners of the sector will feel they have to get involved. I mean, when you tell one tree owner he has to remove his tree to protect yours, how likely do you think it is that he’ll agree?

Are ‘Schubert’ Chokecherries Already Infested When You Buy Them?

Now that you know more about black knot, my question is: is it possible that the ‘Schubert’ chokecherries we buy were already infected with the disease before purchase? That the disease was spread in the nursery? I became even more suspicious when I learned that genetic studies have shown that the strain of black knot that affects ‘Schubert’ is genetically distinct from those affecting wild plums and that even the wild chokecherries in most regions usually suffer from different strains of the disease. So, the infection is not coming from “wild plums and cherries” as is usually claimed, but rather from other ‘Schubert’ chokecherries!

I have no proof of what I advance here. It is possible that the disease spreads from ‘Schubert’ chokecherry to ‘Schubert’ chokecherry strictly by spores carried by the wind (you have to admit that this tree is very widely planted in many areas, so few ‘Schubert’ chokecherries are truly growing in isolation) and that every nurseryman producing ‘Schubert’ chokecherries grows them with utmost care, making absolutely sure all specimens sold in nurseries are completely free of the disease and that all infected plants are burned as soon as the first symptom is noted. But I remain skeptical. Before buying a ‘Schubert’ chokecherry, or recommend it to any other gardener, I’d like a confirmation that it is not already contaminated.

What to Do?

20150817GGiven the current situation, where almost all ‘Schubert’ chokecherries seem to suffer from black knot, I suggest not planting this tree until more is known about the source of the disease. If you already own one and you prune it annually to remove the galls that appear, I suggest not waiting too long before planting another tree (and certainly not a ‘Schubert’!) as a replacement. Thus, when you do need to remove yours, there will already be a substitute in place that is actively growing and there won’t be a gaping hole in your landscape.

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Amur maple: just one example of a small tree that would make a good replacement species for ‘Schubert’ chokecherry.

There is no lack of small trees similar size to the ‘Schubert’ chokecherry that you can can then use as substitutes: crabapples, Japanese lilacs, Amur maples, hornbeams, hawthorns, smaller magnolias, etc. There are even a few plums and cherries that are considered resistant to black knot, including Amur chokecherry (Prunus mackii), but personally I’d be a little afraid of tempting fate by planting even a supposedly resistant Prunus species near an infested ‘Schubert’ chokecherry.

I’m curious to know if this blog will generate any kind of response from commercial nurseries growing ‘Schubert’ chokecherry. If I get one, I’ll keep you posted.