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I must admit that I have always bought commercial potting/seed compost. I'm not the worlds best compost maker and the stuff that I do make is fine for incorporating in the beds, but pretty useless for sowing seed into.
Up until last year all was fine, mainly I think because I always bought peat based compost. Last year I had to get some with recycled stuff in it, and I mean STUFF. I had some problems last year with my tomatoes, some developed dead patches on their leaves, starting at the bottom and working their way up the plant. It wasn't too bad, I got a fine crop, but I was at a loss to explain what was causing those dead patches.
This year, using fresh compost from the same company, I had mushrooms come up in every pot and trough I planted, which sounds like a bonus but imagine a 2" diameter mushroom appearing in a 2" pot of lettuce seedlings.
Then when I started potting up tomatoes and squash disaster happened, those dead patches became dead plants.

Doing a lot of Googling I discovered I'm not the only one, and this recycled compost could well be made from grass cuttings contaminated with a particularly nasty herbicide that stays active for at least 2 years and will even affect cow manure by passing unscathed through a cow's digestive system.
Whether that is what I've got I don't know. It's pretty obvious that this compost contains unsterilised spent mushroom compost, and I would bet that it also contains compost from sprayed grass from (say) golf courses.
I won't publish the company who produce this compost as I may be wrong, but something is not right.

Here are 2 Candy Roaster squash, one planted in my old compost and the other in cheapo B&Q Verve compost.....
.... and here is the same with 2 tomatoes. Both healthy looking ones are 6 weeks younger than the sick ones.

It sounds as if you have hit this problem. The offending chemical is all supposed to have been cleared up and out of the system but if you have hit some old manure (cow or horse) or someone who has been killing dandelions in their lawn and recycling their grass cuttings, this would cause the same problem. I've had this technical conversation at the local garden centre with the manager who is actually a very knowledgeable gardener.

A lot of people don't realise that the offending chemical is still used in proprietary brands that are used for spot killing weeds in lawns. So into the bin go the lawn cuttings after treatment and you get the results that you have seen. A lot of people don't even realise that there has ever been a problem - even the RHS didn't believe it till they had a complete display at one of the big shows wiped out as a result of contaminated manure.

Yes said chemical affects potatoes, tomatoes, beans, peas in the vegetable family as they are all related. And yes it does cause a problem with potting compost as legislation seems to be coming along to get all compost to contain mostly recycled material. Isn't it scary?

The logical step would then be to ban that particular weed killer - but somehow I can't see that happen...

I bought some recycled compost when I tried to grow plants to sell, (I have flatworms so can't use my stuff). I can't tell you how much rubbish I found in it, old bags, bits of footballs, broken glass, pieces of plastic, Bord Na Mona obviously can't afford some machine to sieve it properly, I have no idea what else was lurking in there

Yes, various pieces of shredded plastic of many hues seem to be the norm these days in compost. We've tried many different brands and most of them seem to have this.

We've had your problem in the past, Tony. I have to say it hadn't occurred to me that it was the compost and not our variable levels of attendance. It makes me feel better that it's probably not us then.

OH has just reminded me that we noticed the Verve brand seems particularly prone to white mold too. Others have commented too, when we Googled it.

The problem is of course, which brand of compost is OK ?
Or even worse, the compost that's OK now might not be next week, or month.

I suppose the only option is to make my own.
I used to years ago (and I mean YEARS ago) in the days before peat based compost existed. It was a bit of a palaver and involved laying turves upside down and covering with tarpaulin for a year, and then sieving and sterilising (steaming with a Baby Burco). I seem to remember it was successful and worth doing, but then got lazy and bought this new fangled peat stuff.

At the market garden where I used to work, we even had problems with certified organic compost one year. Definitely no weed killer in that - but they are so limited in what can go into it, it is extremely difficult to find the right balance between ingredients... (I think that year they were too high on something they needed to get the nitrogen into it - rape waste, I think?) Other years that compost was great; much better than any of the non-organic ones I bought.

I used to buy a compost in bulk called Fertile Fibre, I did contact them a while ago and they can ship to Ireland, if you could get a group of interested people it may be worth it. It is based on coir and is certified organic, it was the best potting compost I ever used, they started making different mixes as well, I used to buy in bulk in England and supply to my local organic gardening group

This is the other problem I was talking about.
That mushroom is just under 2.5 inches in diameter, while not too much of a problem in my tomato pot, imagine this coming up in a seed tray.
Proof I think that they are using unsterilised spent mushroom compost.

Actually - I wish I could get some like that - I've never had much success with growing mushrooms!

Too much market demand and it's obviously being met badly ...

will you be having that for your breakfast, looks really lovely. The problem with mushroom compost is that there is SO much to get rid of, there was a place near here and they gave it away, glad to get rid of it, but it did grow thousands of mushrooms, I didn't want any really for that reason

I have no problem with them using spent mushroom compost, I'm a tad annoyed that they didn't see fit to sterilise it.
One thing is for sure, I will NEVER use that company's products again especially as they are one of the most popular in the local garden centres and should know better.

All my tomatoes are in perfect health this year, which prompted me to do a Google search to see if anybody else had the problems I had last year with bought compost containing suspected weedkiller. I found several, but intriguingly there was a link to one of my Ish posts from 2015 and a photo of a comparison between reasonably expensive compost and cheap Lidl compost.
In the photo you can see the bottom leaves of the plants in the (same) expensive compost are looking distinctly sad, where the plants in the cheap compost look OK (apart from needing a good feed)
So I can only assume this manufacturer has been using suspect composted grass cuttings dosed with herbicide for several years and last year it was particularly bad.
Needless to say I won't be using that one again.