winter dysentery?

[gavin]

OK, the spotting. It indicates that the bees were stressed - could be Nosema but equally a number of other issues cause it. I just see a few spots on these frames. The mouldy cells in the cleaner comb are, as Gerry suggested, mouldy pollen. It goes that way when unattended and it is in the position on the frame you'd expect to find pollen. Beyond that area I think you just have partially drawn foundation.

The nasty stuff at the foot of the muckier frame needs further investigation. At the very least you have a lot of brood that got chilled when it was being raised in the autumn due to insufficient bees covering frames and which wasn't able to emerge. Could have been Varroa last autumn when that brood was being raised as Jon suggested but there are many ways this could come about. If it wasn't at this peripheral site on the comb I'd be worried about foulbrood (and still am to a degree) with all those discoloured variable cappings and that patchy pattern.

Next time you have frames of that colony to look at, check carefully for scale in the cells in the brood area. Something dried down and glued firmly to the lower part of the cell may be AFB, a loose scale might be EFB, or sacbrood, or perhaps just dried down dead larvae that had been abandoned. Worth a closer look, but most likely all down to insufficient bee power for some other reason when that brood was being raised last autumn. I'd also pick off cappings to see what's underneath.

[gavin]

Strong colonies manage to clear out their winter dead and the chalkbrood fairly well. Weaker ones, not so much.

[Bridget]

Thanks Gavin. I think chilled brood is one reason - I see from my notes of last September that I gave the nuc a frame of brood when I made it up and a few days later noted that I thought the nuc was being robbed as bees were coming out with larvae. Then a couple of days later I checked the nuc and noted that it was probably not robbing but that there were insufficiant bees to keep the brood warm and they had died and were being cleared out.
I am going to give them a clean NUC this week and I will check again for disease as you say.