Kousa dogwood problems – diagnose (and solve) them easily

Kousa dogwood is a great choice of tree if you ask me. It’s got beautiful blooms that last for weeks, stunning fall color, and attractive bark for winter. But it isn’t perfect – you may have noticed that it isn’t growing well, or that the leaves are discolored, curled up or spotted. Or there may be growths or marks on the bark. What’s going on?

Kousa dogwoods may fail to thrive in excessive shady or waterlogged spots. In drought, they undergo leaf curling, also caused by aphids, and leaf scorch. Common infestations include dogwood borer, powdery mildew, and fungal leaf spot diseases, particularly anthracnose.

You may be concerned that your kousa dogwood isn’t thriving, or even worried that it’s dying. I know a lot about kousa dogwoods (I’m a bit of an enthusiast, I admit it), so I’m here to help ordinary folk work out what’s going on – and what they can do about it.

Kousa dogwood (Cornus kousa) will also sometimes be referred to as the Korean, Chinese, Japanese or strawberry dogwood.

Why isn’t my kousa dogwood tree thriving?

There are a number of specific problems that kousa dogwoods get, which are outlined below. But maybe you’re concerned that your kousa dogwood just isn’t growing as it should, or generally doesn’t look healthy to you.

You can read about how fast a kousa dogwood should grow, and how big it might get, in my post here.

Generally, a kousa dogwood doesn’t grow well if:

  1. the soil is waterlogged
  2. it doesn’t get enough sun – it will grow in full sun or partial shade
  3. if it’s too cold – generally the coolest region of the US it will grow in is USDA hardiness zone 4 or 5 (click here for a US zone map, and here for a Canada zone map (Canada also uses the USDA scale too)
  4. if it’s too hot – USDA zone 9 or above – see links from no. 3 above
  5. It’s underwatered – which might show itself in leaf-curling – see below.

Regarding numbers 3 and 4: f you’re reading in the UK, kousa dogwood usually is happy in all regions.

Why are my kousa dogwood’s leaves curling up?

cornus kousa dogwood leaf curl

The most common issue with kousa dogwoods. It usually happens in high summer – which is a clue to the reason.

Kousa dogwoods dry out easily because they have a shallow root system. Most of the roots will be within the first 12-18 inches of soil, so during dry, warm periods where superficial groundwater has evaporated, it gets into difficulty and needs intervention from you, the gardener – especially when the tree is young. Read more in about how to water your kousa dogwood tree in this article.

A sign of this is the leaves curling around their central vein. This is a protective measure to try to reduce further water loss by transpiration.

Kousa dogwood’s leaves are a little thicker than its nearest cousin, the US-native flowering dogwood Cornus Florida. Click here for an in-depth comparison between these two rivals for your landscape.

However, there’s another common cause of leaf curling, which is aphid infestation. Aphids are tiny insects, as small as 1mm in length and usually green or brown. Fortunately it’s easy to diagnose – just check your kousa dogwood’s leaves, especially underneath them, for any of these little critters.

Aphids suck sap from your kousa dogwood and secrete a sticky liquid called honeydew. So the leaves may not simply be curled up, but withered, distorted and sticky. Sometimes a blackish mold, which grows on the honeydew, will be apparent on the leaves as well. Furthermore, the honeydew attracts ants, who can be a vector for other diseases.

You can control aphid infestation fairly easily if your tree’s small enough to get at. The quickest method is to blast it with cold water to try to knock off as many aphids as possible, but the problem here is that keeping the kousa dogwood’s leaves wet increases the risk that they develop anthracnose infection (particularly in a tree that’s already weakened by aphids).

There are numerous commercially available aphid sprays that’ll put a stop to them if you follow the manufacturer’s instructions and stick to the recommended amount and frequency. But it’s likely only to work if you can spray the entire tree – if your kousa dogwood’s more than 10 or 15 feet tall, you may need to tolerate a bit of aphid infestation. You may use some leaves, but the tree will still survive.

Why are my kousa dogwood’s leaves brown at the ends?

Cornus kousa dogwood leaf scorch

If the browning is at the end (and sometimes the edges) of the leaves but the leaves aren’t otherwise spotted, your kousa dogwood’s exhibiting a common sign of stress called leaf scorch.

Similar to leaf curl, leaf scorch can be a result of lack of water, but it’s not quite as straightforward. Even a well-watered plant can get leaf scorch, though it’s less likely.

Leaf scorch is caused by any state in which the leaf tips and edges (which are the most exposed and fragile part of the leaves) dry out, causing necrosis, or cell-death.

Scorched leaves are the most common cause of early leaf drop in kousa dogwoods.

What causes leaf scorch in kousa dogwoods?

Leaf scorch in kousa dogwoods can result from:

  1. Lack of water
  2. Leaf-drying from wind exposure
  3. Overwatering, or soil that is always waterlogged (preventing strong root development)
  4. Root limitation – where there is too little soil area for them to adequately grow outward
  5. Over-zealous application of fertilizer

Kousa dogwoods don’t tolerate heat (looking at the southern US states here!) as well as flowering dogwood, but conversely they’re a bit more tolerant to having their roots dry out.

What to do about kousa dogwood leaf scorch

File:Cornus kousa 4499.jpg

Once it’s happened, the leaf tips will wither and die – you can’t stop it. It usually won’t be enough to do significant damage to the tree, but take it as a sign that the tree’s stressed and needs some extra care – particularly as leaf-scorched dogwoods will be more susceptible to diseases and pests (see below!).

It’s all about identifying the cause.

Give the tree the right amount of water. Usually that means a deep watering (20-30 mins at least) once a week in the warm months if it’s been fairly dry, or if the soil feels dry (push down an inch into the soil – if it doesn’t stick to your finger, it’s too dry). Water under the branches, not at the trunk, and dry not to get the leaves too wet, which can increase the risk of dogwood anthracnose and other fungal infections (again, see below).

If leaf scorch is severe, give the tree a really deep watering once every 4-6 weeks the following winter as well, as long as the temperature’s above freezing (freezing over the roots will make the problem worse).

Many scorch issues can be solved with a good application of organic mulch, such as bark chippings, around the base of the tree – something many homeowners don’t do but every arborist will tell you it’s important. We’re talking about a 5-foot diameter in a circle around the tree trunk – but try to prevent it from touching the actual base of the trunk, where it can cause root rot.

The mulch helps keep the roots cool and stops them from drying out. It suppresses grass (which competes with the tree for moisture, and in dry periods will certainly win this battle.

If you’re applying fertilizer, make sure you stick closely to the manufacturer’s instructions

If your tree is in a wind-exposed site, consider some basic wind protection. Maybe some fencing’s needed. Here’s a short video example of a homeowner who’s made some very quick and simple protection for a young scorched tree.

There’s a white coating on my kousa dogwood’s leaves

This is a really common fungal infection called powdery mildew. Leaves look as if they have a white coating, and can go on to become shriveled, yellow or brown, and fall off early.

It’s fairly easy to identify. Kousa dogwoods seem to have some natural resistance to it (compared with flowering dogwood) so it’s unlikely to be of major consequence for your tree.

Prune off any infected leaves as soon as you see them. To prevent the infection from spreading:

  1. Be sure to carefully collect any leaves you prune off, and put them in the trash – don’t throw them on the ground, where their spores can spread
  2. Clean your pruners with rubbing alcohol or alcoholic hand sanitizer to sterilize them between cuts.

In the winter, carefully rake up any fallen leaves and throw them in the trash.

Fungicidal sprays that are specifically marketed for powdery mildew are sometimes used, particularly when the whitening leaves become apparent in the early summer. Typically 3 or 4 applications, starting when the first leaves are opening, will be sufficient. Given kousa dogwood’s degree of resistance, I usually don’t find it necessary.

Why are my kousa dogwood’s leaves spotted?

Cornus kousa leaf spot

If your tree’s leaves have spots over them, it’s going to be one of four fungal infections:

  • dogwood anthracnose – caused by Discula destructiva
  • spot anthracnose – caused by Elsinoe corni
  • septoria leaf spot – caused by Septoria cornicola
  • cercospora leaf spot – caused by Cercospora cornicola

Fortunately, kousa dogwoods have natural resistance to dogwood anthracnose (which can be destructive to Cornus florida, the flowering dogwood), while spot anthracnose, septoria leaf spot and cercospora leaf spot are not usually dangerous enough to cause much harm to the tree – they’re more of a cosmetic issue. If lots of leaves are infected, you can get some defoliation (leaf drop).

Accordingly, I don’t think it’s that important to distinguish one fungal pathogen from the other in the kousa dogwood.

Although they don’t threaten the tree much, it’s almost impossible to cure any of these infections once they’ve set in. They’re controlled, rather than cured. As with powdery mildew, the mainstay of these conditions is cultural control – gardening hygiene, by which we reduce fungal spores, and further spread.

Once again this means:

  1. pruning out any spotted leaves and putting them in the trash
  2. cleaning pruners with rubbing alcohol or alcohol hand sanitizer between cuts
  3. carefully raking up fallen leaves and putting them in the trash

Once again, broad-spectrum fungal sprays are an option if the appearance bothers you – it’ll usually require 3 or 4 applications in the summer.

You can also reduce the risk of fungal leaf spot infections by

  • thinning out the canopy yearly, to improve airflow, so leaves dry out adequately after it rains and
  • avoiding getting the foliage wet when you’re watering the tree.

Kousa dogwood bark problems

Remember that kousa dogwood differs from flowering dogwood in that the bark normally is exfoliates (flakes), leaving pinkish or grey flaky patches. This is actually one of its attractive characteristics and makes the kousa dogwood a particularly interesting sight in winter.

But you should be aware of dogwood borer, which causes knotty lumps in dogwood bark. This moth, Synanthedon scitula, lays its eggs in dogwood bark.

Dogwood Borer Synanthedon scitula

Bark damage caused by dogwood borer
Bark damage caused by dogwood borer

Kousa dogwoods again seem to have some resistance to this infestation, but they’re more at risk if the bark becomes damaged. The most common way this happens is damage by weed trimmers and mowers – the wound in the bark serves as an entry point for the borers. Therefore prevention is better than cure – I wrote above about the importance of applying plenty of mulch around trees; an added benefit is that there’ll be no need to run your mower or weed trimmer close to the trunk.

Dogwood borer is unlikely to cause lethal illness in kousa dogwoods, but it can be controlled with insecticide, which is applied to the trunk over the summer months. Pyrethrin is probably the most frequently used.

Kousa dogwood not flowering?

For this one, have a look at this post, which answers this very question!

Kousa dogwood not producing berries?

Check out this post for a very complete answer.

More original posts, all about dogwoods

Image attributions:

Slarson789, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons

Slarson789, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons

Amada44, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons

Christina Butler from Georgia, United States, CC BY 2.0 https://creativecommons.org/licenses/by/2.0, via Wikimedia Commons

John A. Weidhass, CC BY 3.0 https://creativecommons.org/licenses/by/3.0, via Wikimedia Commons

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