Violet rust is orange

There's a place near my back door where the violets grow. They've seeded themselves there and I am happy to encourage them in this shady strip where nothing much else grows.

Recently I've been reading an old book on 'Rust, Smut, Mildew & Mould' by Mordecai Cubitt Cooke (5th ed., 1886 — you can find it in the Biodiversity Heritage Library, here) and was particularly struck by a description of one species of rust ...

"... which attacks the violet. The sweetest of flowers as well as the earliest, in despite both of its odour and its humility, becomes a victim to one or more of the ubiquitous race of fungi. Thickened spots at first appear on the leaves; the petioles, or the flower stem, or even the calyx, become swollen and distorted; and at length the cluster-cup breaks through."

So I've been keeping an eye on the violets, wondering if I'd catch sight of these so-called 'cluster-cups'. This morning, I found what I was looking for.

Glancing down from above, it was hardly noticeable. But looking closely at each plant, I found the rust was present on lots of them: on the leaves, the petioles and even the calyx — just as Cooke had described.

Correctly identifying the host plant is a critical step in identifying rusts and often the trickiest part for me. I'm no botanist, but when I was a girl my mum used to 'keep things interesting' on a long walk with a promise of two pence for every plant I could name (twenty pence in winter) — so I can get as far as... "dog violet?"

Happily, Sussex Botanical Recording Society has shared a brief guide to Sussex violets, here. And I think the pale upturned 'spur', notched at the end, suggests Common Dog Violet Viola riviniana.

Scientific knowledge and taxonomy of rusts has moved on somewhat since 1886, so — armed with a hopefully-accurate ID on the plant — I consulted a more modern tome to identify the rust. I've just recently acquired a .pdf of 'Dutch Rust Fungi' (from here) and it includes helpful keys as well as descriptions and illustrations, in English as well as Dutch. I conclude that this 'cluster-cup' — the 'aecial' stage in the rust's life cycle, also known as stage I — is Puccinia violae.

Here is a close-up of those beautiful little 'cluster-cups' on the underside of a leaf. 

The book on 'British Rust Fungi' by Wilson & Henderson (1996) includes a more detailed description of the 'cluster-cups', or aecia [singular = aecium], of P. violae. The outer wall of the 'cup' which you can see here is known as the peridium and is described as having a "white torn revolute [meaning 'curled back'] margin", which you can see here in a focus-stacked image of my collection.

The orange mass inside, and the tiny orange specks scattered about on the leaf's surface, are the aeciospores. Poised to infect their next victim... — if you frame it in Cooke's terms. Or just being aeciospores... — if you're inclined to be less judgemental about this thing called life. 

I don't think there is a 'Spotters Guide to Rusts' but, if there was, this species would surely be in it as it's apparently a common one to find. Ingram & Robertson mention it in their book on 'Plant Disease' (1999) and it's an example of a rust fungus which lives out its whole rather elaborate life cycle in association with one species of plant, in this case Viola riviniana, so it's known as 'autoecious'. There are photos of the rust's other life stages on the Plant Parasites of Europe website, here. So naturally I'll be looking out for those! Although they're rather less showy.


(I've got another rust in the garden which spends part of its life on my Pendulous Sedge Carex pendula and part of its life on something else — possibly the nettle patch or my fruit bushes — but I'm still trying to get to the bottom of what's going on there, so will have to save my 'heteroecious' rust fungus for a future blog...)

For the record
Date: 25/04/2020
Location: My garden, West Sussex