Sunday, 11 July 2021

Pink Spots on rose petals - what are they?

 I came across this phenomenon the other day - a white rose with pink spots on it.


Pink spots?

On a white rose?

At first sight, I wondered if it was something to do with water: perhaps it was drops of water from recent rains, somehow concentrating the rays of the sun and, err, bringing the latent pink colour of the rose petals to the surface?

The white rose petals...?

My Trainee looked at me, unconvinced.

I wasn't convinced either, so I came home and did some research.

Apparently it's one of the fungal diseases - Botrytis cinerea, to be exact - and the pink spots are the rose's response to the infection. 

So, all roses get pink spots, they are just rather more noticeable on a pale-coloured rose.

The cause, as with so many of the Botrytis infections, is to do with water - in this case, we've had a run of rainy days so the flowers and leaves are physically wet: the ground is wet, therefore the air is generally humid: and the rose is trained against a wall, so it does not have much in the way of air circulation.

This is the perfect recipe, it would seem, for  Botrytis in general, and cinerea in particular.

So, what are the dangers? Will it spread? 

Well, yes: all the Botrytis - er, plural of Botrytis, anyone? - anyway, all of them are fungal, that means they spread, so it's important to remove infected flowers as soon as you see them.

If you don't remove them, the mould will spread to the stem of the flower, and if you are very unlucky, it will continue to spread downwards. 

Another reason for immediate dead-heading is the word "fungal" which has a sub-text of "spores" which therefore comes with a side-order of "invisible but will spread like a mad thing". This is a serious hint, that anything with a fungal infection should be carefully removed from the plant, as soon as it is seen.

Incidentally, don't put this material onto your compost heaps - either burn it, which is best: or put it in the council Green Waste bin, where it will be processed at a much higher temperature than it would achieve in your domestic compost arrangements, and which should therefore destroy all the spores.

So, what will happen to this white rose with pink spots, if I were to leave it?

Answer: the mould would spread to other parts of the plant: flowers would develop pink spots like this, and then they would go brown and die. 

Unopened buds would be infected, and would rot before they could open. 

The mould would spread from those dying buds, back along the stem of the rose.

And so on, and so on.

So be vigilant! When you see the first sign of this infection, dead-head like a mad thing: not only does it remove all the infected blooms, but by removing some of the plant, you are allowing air to circulate more freely in the rest of the plant, which makes the conditions less favourable for the Botrytis.

And there you have it: alas, pink spots on rose petals are not a charming colouration: they are a warning sign that something horrible is approaching.

You have been warned!



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