Sickly cotinus leaves

My 10 year old cotinus suddenly developed some sort of leaf spot on the leaves on half of the tree towards the end of last summer. The healthy leaves have dropped but the sickly ones are still on the tree. My question is: should I leave them to drop, collect them and burn them? Or should I prune off the branches with the sickly leaves before they get a chance to drop? I did a drastic prune like this on it a few years ago and it seemed to do it good. Is it at all possible that frost would kill off the disease if I just left it? Here are a couple of photos to help with id.

 

I prune mine very hard each spring anyway. However, if the tree is a bit weak it may not respond well. Depends whether the problem is something systemic or just a temporary blight on the leaves only in which case it may be fine next year. There's been no frost fit to kill anything yet in Central England, let alone N Spain. Which in most ways is good, very good.

 

I'm not sure what sort of age Cotinus lives to. Could it be it's past it's prime and is becoming susceptible to disease?

 

Thanks for those replies. Madahhlia we're at nearly 600 metres above sea level, so frost is very common in winter and we usually get around 4 or 5 copious snow falls, very often at the beginning of April. Funnily enough Sheal, it is the older part of the tree which is affected, but the branches look fine, it's just the leaves which look dodgy.

 

Hi Lorea,
It's a sooty mould...a fungal disease, so best to get rid of as many infected leaves as possible. Trouble is, if you prune now, it's leaving the shrub open to worse things, like coral-spot. I'd strip the old leaves off, burn them then spray the cotinus with a systemic fungicide. You could use Bordeaux Mixture ( are they rabid 'spray everything blue as often as possible' fans round your way, too?) but it's not really very effective.
Then prune and feed in Spring. It should bounce back OK.

 

Thanks noisette. I try to be as ecological as possible, so I'm not really into spraying things with artificial chemicals and up to now I haven't needed to (attitudes have changed a lot here over the last few years - there are more and more council owned organic allotments and people are becoming more concerned about the environment. There is an important bee population in the mountains near our house which was almost completely killed off a few years ago when they sprayed the local golf course with an insecticide.) I think I'll follow yours and John's advice - strip off as many leaves as I can and then prune in spring.