Dahlias 2013 Season

Sorry to here you are both still having problems with your cuttings.
If your cuttings look healthy on the tuber then it has to be a combination of heat/too wet compost, with whatever container you are trying to root them in.
When I first started taking cuttings I tried many methods, always lost at least half of them, until eventually I ended up with what I now use, shown below. Since then, apart from the ones I knew would blacken and rot because of how the cutting looked before taking it, I rarely lose any.
The compost I use is 2 parts any peat based multipurpose compost/ one part horticultural sand/ one part perlite and when mixed together looks like the top picture. Now I've used the same compost year in year out for at least the last 10 years. I just top it up a little every year .
The next bit is to prepare the "propagator" and to do this i place 20 square 3" pots in a deep seed tray and surround them with peat. early season I'll get 4 cuttings minimum per pot. Later on, up to 7.
Once filled I give it a real good soaking, leave for a few days to settle, then fill up the pots with the rooting compost. This is then lightly watered. The ideal "wetness " is that when you squeeze the compost it sticks together nicely, doesn't drip, or fall apart. That won't need watering again until the cuttings are rooted.
All this is done about a week before the first cutting is taken. The whole thing then goes onto the aluminium heat mat, thermostat set to 60'f. The lid is put on the seed tray
Once the cuttings are taken they are just left in the propagator with the lid on at all times until rooted. There are a couple of vents on the lid that I leave half open.
The heat mat does not stay on all day. The thermostat probe is placed in one of the trays of dahlia tubers, so it comes on when the temperature drops. However, because I've used peat to surround the pots, it retains the heat nicely so the compost is kept warm, never to hot. This may be where you are having problems, the compost may be getting too much base heat.
As you can see from the bottom left picture, there is condensation on the lid, but the cuttings do not damp off. They also do not wilt in hot weather...a few times in the last week temperatures have hit 30'c during the day in the greenhouse, and at night have been close to freezing. So temperature variation doesn't affect them using this method.
The last picture bottom right shows the cuttings with the lid off.
Those have been in there for 7-21 days. Some will be already rooted, but being in 3" deep pots means you can leave them quite a while once rooted before potting them on.
Everyone has their own method that works for them but what I like best about mine is that I don't need to mess around with any spraying or covering and could quite easily leave them for days on end knowing they'll be OK no matter how hot or cold it gets in the greenhouse.

Forgot to say I got the tray and cover from Wilkinson's.
What I have also found with my method is that early cuttings seem to grow before they have rooted, so it can be a bit deceptive. A few times I've taken cuttings out thinking they were done only to find no roots at all, yet they had definitely increased in size. I just dip them in a bit more rooting powder and stick them back in.
With your coir, I tried it when it first came out, not for rooting but potting on and ended up losing so many plants...overwatering.
Never used it again.

I am using a multi purpose/sharp sand mix this year instead of jiffy sevens (getting to expensive) and at the moment I am having no problems.I have given up on oakwood goldcrest tubers producing any cuttings and have asked a mate to keep me some.

Went through all cuttings taken so far. Tipped out of compost and checked for roots, rots etc. Re-inserted those that had not rooted enough for potting on, binned those that had started to blacken/rot and potted up the rest.

'Trelyn Kiwi' has been the most successful with 8 potted on, 3 needing another week and only 1 rotten.

I can't say the same for either 'Willo's Violet' - thirty cuttings taken and all rotted - or 'Rossendale Peach' which also had a 100% rot rate!

The rest were a mixed bag of rooting and rotting. Those successfully potted were: 'Ruskin Andrea' x 2, 'Hillcrest Candy' x 1, 'Ryecroft Laura' x 1, 'Cherwell Skylark' x 2 and the 'Trelyn Kiwi' x 8.

I have taken another 15 'Willo's Violet' cuttings. These have all been dipped in Clonex rooting hormone and inserted into three different rooting mediums. Five each into Canna Coco+, Westland Gro-Sure Cutting Mix and the remainder into just sharp sand. If this does not work then the tubers will be discarded as they may be infected themselves.

No sign of damping off still.
However, and this is a first for me after 30 years or so,some of the cuttings that have rooted have developed flower buds whilst in the propagator. That's from the 2nd pair of leaves. They certainly showed no signs of a bud when I took the cuttings, though I suppose it is possible it was there but to small to be seen.
Very strange.

I am no chemist - but may I suggest if any of you are having problems that you have not seen before - ie (looks like damping off) but you have done nothing different for several years and never had a problem - that you first stop using the multi-purpose compost you are using.

There have been several papers written since 2008 on the affect of composts containing minute traces of Picloram, Amniopyralid and Clopyralid. Some have labelled these as "killer composts", depending upon what crop you are trying to rear as some crops are more susceptible than others. Perhaps the Dahlia and more importantly dahlia soft cuttings are more sensitive to these herbicides.

Sadly with the increase in production of composts by local councils of non screened grass cutting and the like, I can only see this problem getting worse in the short-term.

I would suggest buying top quality "screened" compost from a recognised name and if you do see a problem - get the compost analysed and report it back to the manufacturers. The more of who do this, the more the manufacturer will take notice and react to the problem (hopefully).

This is worth a read - if you are at all concerned: www.the-compost-gardener.com/picloram.html

I hope that helps.