Rooting in water does not produce roots that are viable in soil. Those water roots fall off and the cutting makes new roots so your rooting in water accomplishes nothing. No you would kill the cutting trying to sterilize it but do make sure everything to do with cutting and rooting is clean and sterilized. Clean your tools and pots in a mild bleach and water solution.

Do you use rooting hormone? Rooting hormone is fairly cheap and does thousands of cuttings.

Taking cuttings is the same for most plants. First you must make sure everything you use is very clean. In a clean pail mix your soil with warm water. Use a soilless mix like pro mix or one that is formulated for seeds and cuttings. Just make sure it has no fertilizer in it. You want the soil wet but not too wet. Squeeze a handful and some moisture should come out but not a lot. Fill your pots and make a little hole in the center with your finger or pencil or dowel. You do this so you don't knock the rooting hormone off the cutting

Take your cuttings: With a sharp knife or razor blade or scissors cut a 4 or 5 inch piece from the growing tip, cutting just below a leaf. All roots and branches come from a leaf joint. Remove all but the top 3 to 5 leaves. Dip about 1/2 inch of the cut end in rooting hormone. It comes in powder or gel, either is OK as long as it's for soft wood cuttings. I use Stim-root NO. 1. After it's dipped put it in the hole and cover it in, and you're done.

Keep the soil moist but not overly wet. They may look wilted at first but that will change as it starts to grow roots. You will know it's rooted when it starts to show new growth. Wait until it is rooted well before you set it out.

I find it is best to start them in small containers--something like the 4 packs you buy annuals in-- not pots. It is easier to control the moisture level.. If you use a soilless mix weigh the container in your hands before you plant. Now you know what it feels like when it's wet. It will be much lighter when it needs water and that's how you know when to water

Good luck. I hope you have success

I read your post again. Compost is not meant to be used for starting cuttings or planting plants in. If there are roots you'll burn the roots and any cuttings will die.

Hydrogel is meant for holding water in planters. Again it is not a good medium for starting cuttings.

What you need is a soilless mix. There are many. I use pro mix but any soilless mix will do. Change your medium and you should be able to start cuttings. Impatiens root very easily give the right conditions

I seem to recall from the dim and distant past that ifraser is a transplanted Brit. If my memory is correct then use of the term 'compost' would just mean what Americans call 'medium'. In British English compost means both the stuff in your compost heap and the mixtures used for sowing, cuttings, etc.

I'm still waiting for a good answer on this. For more than a year I had good results, but then this disease problem started to appear. It appears to affect mainly Sunpatiens. I wonder if the problem could in fact be the compost/medium. I did in fact say it was a commercial product. Perhaps I should switch to a different one.

Maybe try some other plants in that same medium to see if they have similar results. If they do, you would have a better idea that it is your compost medium.

What size are your cuttings and do you reduce the leaf exposure?

The problem is indeed more serious than I thought. I am now finding established garden plants with the same symptoms, including aubergine and chilli as well as other types of cuttings going down The question is: is it a bacterium or a fungus? The ground is riddled with fungus but most stuff has survived it OK up to now and in any case it can't be the same thing as what is affecting the cuttings and they are not in the same medium. Must be airborne.