You know... I don't know what it is, but I've had the same problem. Same variety, even! The occasional one or two berries that actually set well and ripen look great, but they fall apart if you glance at them funny and always taste 'off'. I doubt it's the soil in this case, either; I've not done so much extra amending as you (just some composting and mulch, same as everything else here gets), and other items in the space do just peachy.

The wild ones we have in other parts of the yard always do pretty well, and this year they're going gangbusters! I may just rip out the last of the Reds and let the wild ones have the room. They'll probably make better use of it. *sigh*

I know...though after writing this a few days later I noticed some decent looking ones, but they are always so small! Is that typical of that variety? Then I wonder if maybe a virus would be doing it? Other than that, they seem to look healthy. I have some sort of cool looking 'weedy' raspberry fruting now, it's in clusters and is a bit 'sticky' to the touch, but they taste great, they start out yellow and turn red. Heck, I may just put them in instead, they grow like crazy too!

mersiepoo-
I'm sorry to hear that. I hadn't commented previously because I don't grow Latham.
Was the chicken manure composted first? Or was it just straight manure? Perhaps just try simple compost next year to see if that helps?
I think either way, I'd plant another variety to see if that helps. Perhaps a really strong variety like Prelude.
As I mentioned in another post, it sounds like you have Wineberries as the other plant you mention.
Best regards,
-Glenn

I'll BET they need lots more water. They need 1-2" per week.

We have the same crumbly raspberry problem, but ours are not Lathams, they are Cambie variety. We have had so much rain this year that the problem can't possibly be not enough water. They've been like this for years, is it maybe that the canes are too old? Ours are 12 to 15 years old, they look very healthy, tall canes, big leaves, nice dark green colour.

Raspberry leaf curl virus can cause the fruit to be small, crumbly, and seedy. Do the leaves look strange? I'm guessing it might be due to too much rain or it's just a characteristic of the variety (never grown that variety myself).
Kim

Here is a link that might be useful: My Garden

I had a two varieties (Nova & Prelude)of early red raspberries that after a few years of good production began to produce crumbly berries. My extension agent suspected Tomato Spot Rust Virus, vectored to the roots of my plants by the dagger nematode. She took soil samples to run a nematode assay, sent them to Va Tech and the lab techs found that my soil was infested with dagger nematodes. I had to dig all the plants up and throw them out. Once a red raspberry has a virus, it never loses it. You have to destroy the plants and find a planting location where there are no dagger nematodes.

The Cambie variety mentioned above is likely "Canby".

I originally bought my raspberry canes from Aldi many years ago and I don't remember the variety.

When I moved home 7 years ago I brought some of the canes that were growing outside the main row at the old house. I have them planted up against a fence which borders a concrete yard which slopes towards the raspberry bed. The canes have migrated somewhat, some are now growing behind the garage wall but in line with the original row and there are a few canes against a fence which has gravel at the other side. The canes are all from the same stock.

Those which receive the run off from the yard produce crumbly but plump and large fruit. The plants which have migrated behind the garage are much smaller and produce tight juicy and large fruit and the ones near the gravelled area are much more lush plants tham those behind the garage and produce similarly good fruit.

I have come to the conclusion that my poor fruit is as a result of overwatering. The fruit also often looks as though some of the segments are mouldy