I read an article the other day that claimed that lavender is immune to disease, a quality linked to its healing properties

I almost choked on my caviar and crisps at such dangerous misinformation. As the most famous musical plagiarist of my youth said:

In the everyday running of our nursery, we are always on the lookout for signs of disease on our plants. They are packed in pretty tight and one rotten apple, so to speak, could easily cause us to lose a lot of stock, which means losing money, and there is nothing worse than that.

Now, Lavender is a very disease resistant plant, and I am not trying to warn the world of impending lavender catastrophe. Whether on a nursery or in a garden, I have seen these problems afflict Lavender only two or three times over a career spanning several human lifetimes.

Nevertheless, you may come across these invaders in your travels and prevent disaster by raising the alarm.
Here are the diseases that we check on to be sure that our plants are healthy. I say "we" loosely here: it's mostly Rodney. OK, it's all Rodney.
Rodney, if you ever read this, write "Bob Dylan is my favourite singer, I just love his voice" in the comments, and I'll give you a raise.

Alfa Mosaic Virus, AMV:

This common virus afflicts loads of plants, and is famously a bore for tomato, potato, and pepper growers.

Alfa Mosaic Virus rarely affects Lavender, and is quite easy to spot: the leaves will turn yellow in patches and rings, then curl up into contorted, crinkly shapes.
If it's allowed to progress, you will also see stunted growth of leaves, stems, and flowers, followed by necrosis.

Alfa Mosaic Virus Treatment: it is mainly spread by aphids, but can be spread by your hands and generally waving infected plants around.
So, it's recommended to pop a plastic bag or bin liner over an afflicted plant, cut the whole thing out at the base, then as with any diseased plant material, burn it or chuck it away, do not compost it. Then wash your hands.

Lavender Shab Disease: Phomopsis lavandula

This fungus kills the stems of the lavender. It's pretty obvious when it strikes: all the shoots wilt suddenly, even though there has been no drought.

Close inspection of the affected plants (you may need a magnifying glass) will reveal very small black shapes called pycnidia emerging from the bark.
Under a powerful microscope, you would see that they are cup shaped and full of spores.
As the disease progresses, stems will turn brown and manky with black patches, but you really want to have removed the plants before you see that.

There was a lavender shab epidemic in the 1970's that killed vast amounts of lavender grown commercially in fields and plant nurseries, where the wind could easily spread fungal spores along the rows.
Although it seems to have disappeared, anyone who grows, sells or maintains large amounts of lavender is on the lookout for this microscopic killer.

Lavender Shab Treatment is the same as for Alfa Mosaic Virus above: bag the whole plant, then burn it, or bin it.

Green Capsid Bug and Froghopper Nymphs (cuckoo spit)

These insects will have a nibble of Lavender, but they are harmless. I have never seen them damage a Lavender plant enough for it to be visible from a metre away, even with my specs on.

Capsid Bugs are sapsuckers that can be a real pest on soft fruit plants, potatoes, and dahlias, but it's not worth using pesticides to kill them on your lavender.

Root Rot Caused by Wet Soil: The Lavender Slayer

This isn't a disease, but it kills approximately a gorillian times more Lavender plants in the UK than all of the above combined.

This is what we check every lavender plant for before it gets sent to its new home.

If the soil around lavender's roots is too wet, especially over winter, rot sets in and the bark begins to die. It will be easy to pull the bark away from the base of the main stem, and the stem itself may simply twist off from the roots with very little force.
Larger plants can struggle on for a little while like this, but they are dead men walking.

The chances of coming across Shab or Alfa Mosaic Virus are pretty slim, my bet is that you will never have a problem with them.
But if you have any lavender woes, let us know and we'd be happy to give our opinion.

Start with healthy specimens, like, for example, oh I don't know, our UK Grown lavender plants, ideally planted with a pinch of rootgrow in well drained soil in a sunny location.
The best time to plant is in late April or early May when the soil has warmed up, and they have all growing season to settle in.

Relax, enjoy and watch your garden grow!