Holmoak and diseases

No holm oak diseases, just organisms

Organisms occur naturally on holm oaks. Under normal conditions, they go unnoticed and pose no problem. However, when they appear in large numbers, this is generally due to two causes:

Structural: a change around the planting site, for example an elevation, the creation of a terrace, increased competition...
Conjunctural: for example, the pruning period coincides with certain weather conditions, making the leaves more susceptible.

Gall mites, gall midges and gall wasps are mainly found on pedunculate oak (Quercus robur) and sessile oak (Quercus petrea). On holm oaks, they are less common and  appear if parasite pressure around the tree is high.

Brown spots, dots, or deposits on the leaves of the holm oak?

Here are the most common manifestations on the holm oak and their causes:

Brown spots under the leaves of the holm oak
Under certain conditions, brown spots may appear under the leaves of the holm oak. They are caused by gall mites. The most common species on Quercus ilex are Aceria ilicis and Aceria quercina.

The gall mite is a tiny, microscopic spider, barely visible to the naked eye (0.1 to 0.2 mm). It lays its eggs under the leaf.

The reaction of the leaf around the eggs can sometimes produce pretty effects, like small red balls of strange shapes. Sometimes, but not always, rust spots appear, making the underside of the leaf less aesthetic.

On the holm oak, both red balls and rust spots can be observed.

Small dry dots on the leaves of the holm oak
In addition to gall mites, there are also small gall wasps and midges. Their aesthetic impact is less, and in many cases, they are useful for controlling aphids in the garden.

The species Aroldiola Quercus and Arnoldiola libera (small dry dots on the leaves of the holm oak) are gall midges.

Dry tips and small pink galls

These phenomena mainly appear on the Turkey oak (Quercus cerris), Hungarian oak (Quercus frainetto), downy oak (Quercus pubescens), English oak (Quercus robur), and sessile oak (Quercus petrea).

On the holm oak, this infestation is rare, as is Contarinia quercina (oak gall midge), which also forms pretty galls.

Brown galleries and blisters in the leaf

If you observe brown galleries or blisters in the leaves of the holm oak, it is the work of nocturnal butterflies.

It is usually the small holm oak miner (Ectoedemia heringella), a tiny butterfly that digs galleries between the layers of the leaf.

The native butterfly Phyllonorycter messaniella is also present on the holm oak and causes slightly more visible damage to the leaves.

Aphids on the holm oak

Like almost all deciduous trees, the holm oak is not immune to aphids. However, Quercus ilex only suffers during a specific period, mainly when the tree produces new shoots (May – June) and this coincides with ideal weather conditions.

Black deposit on the leaves

A black deposit on the leaves of the holm oak is usually a secondary infection caused by a fungus that grows on the sweet secretions of aphids.

It is best to prevent this by avoiding an aphid infestation in the spring. Exceptionally humid conditions can also cause this deposit on old leaves, but it is not serious.

The tree will eventually shed these leaves and produce new ones.

Ants on the trunk and in the crown of the holm oak

Ants on the holm oak do not cause any damage. They simply indicate that there are probably aphids in the crown. You should mainly look at the tips and under the young leaves.

What to do about organisms on holm oak?

If you find organisms on your holm oaks, there's usually no need to worry. Like the pedunculate oak, the holm oak is barely affected. Typically, they don't cause much trouble for people, except perhaps visually.

What might help?

Do nothing If the organisms don’t bother you and the tree shows no signs of distress, simply observe and give it time. Chances are the natural balance will be restored by the following season.

Selective pruning If there are too many organisms or you’d rather not see them, you can prune the holm oak selectively. As long as the time of year permits, a light pruning to curb local infestation is fine anytime. Just cut away the affected shoots.

Control While most of these organisms are easy to manage, we don’t immediately recommend fighting them—especially not with aggressive pesticides. The holm oak has a very rich subterranean soil life, and its ability to support above-ground activity depends on keeping that underground balance intact.

 
Natural control tips

Aphids: These can be easily sprayed off with water. Best done early in the spring, when they start becoming active.
Mites: Water won't help with mites. You’re better off treating them with a biological acaricide, especially when temperatures reach around 20°C. 

Brown leaves and leaf loss in holm oak

Is your holm oak suffering from brown leaves and/or leaf drop? It’s not a disease specific to holm oak, but something is definitely happening. Start by checking the trunk: Make sure it hasn’t suffered significant damage. If not, take a close look at the leaf color.

Gray-brown leaves appearing quickly If your holm oak suddenly develops many gray-brown leaves over just a few weeks, it was likely impacted by drought or toxicity (e.g. excess fertilizer, leached cement, or pesticides). Most commonly, it’s a drought issue, especially soon after planting when roots haven’t built up sufficient reserves.

Solution: Sadly, there’s little you can do retroactively—just be more vigilant moving forward. During severe drought, all plants need water. Not sure? Send in a photo for expert advice.

 
Golden-brown leaves If golden-brown leaves appear before falling, the tree is restoring its internal balance. This is not a disease but actually a sign of good health. When trees face stress, they often shed some leaves as a natural coping mechanism. Once spotted, no intervention is needed.

Note: Don’t overwater—this can disrupt the recovery process. Let nature take its course.

 
Yellow leaves in spring If your holm oak shows yellow rather than brown leaves in spring before shedding, it likely went through a dark winter with little light. In spring, it naturally sheds these older leaves and produces new ones. Again, no cause for concern—it’s a healthy renewal process.

 
Reddish-brown shoots You might notice reddish-brown shoots on your holm oak. No worries—this is perfectly normal and harmless. Young leaves are still small and sensitive. Their green pigment (chlorophyll) is less visible, while anthocyanins—which protect against sun—give them a reddish tint.

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