Yellow foliage on marguerites

The bush did sooo well when I first planted it, lots of flowers, it grew like insane. But now the top foliage is turning slightly yellow and the flowers do not develop fully - all the white bits falling off before opening, leaving only the yellow centres.
Is it dying? Or could I save it by pruning back real hard and see what happens? Could it be the wrong soil - we are quite alkaline here (well, the test said 8.0, at the very most a 7.5...
Such a shame because in the beginning it did wonderfully and I so fell in love with it! :(

 

Lift it out the ground and examine the roots to see if you have chafer grub or vine weevil. From what you describe I suspect a root infestation of some sort.

If that's not the problem then it may be suffering from mineral deficiency if it has exhausted the soil. Margurites are normally robust though and will tollerate most soils. Clumps can become congested so divide up if the centre of the plant appears matted and has no flowers.

 

Hm, root infestation can be the problem - not soil exhaustion or clumping, as the plant has only been in there 6 weeks or so, after digging in lots or organic stuff and even some slow-release fertilizer granules, so lack of feeding should not have been the cause. I read that marguerites can suffer from over-watering? If that's the case then the torrential rainfalls we had recently would not help really... *sigh*

Thanks frogesque, if it is not raining this afternoon after work I will dig it up and take a look at the roots...

 

Nope, no vine weevil. Had the bush up over the weekend and checked the roots. The only worms I could find there looked yukk enough - about half an inch long, skinny, orangey yellow things, mostly forming a question mark shape and with tiny spikes. Anyone recognize the description? I seem to have quite a few of them in the garden (had a - normally - red coloured palm like plant called "Red Star" up as well as it is also turning increasingly yellow, again could not see any creepies except for these half-inchers!).
I need to get into the habit of having a camera with me when I am in the garden...

 

These sound more like some kind of centipede, which usually eat other insects, rather than plants.

 

In the S.E where I am there is a lot of leaf miner infestation, which on close examination is a series of greyish trails just below the surface of the leaf with very small grubs at the end. The effect is of weakening and thinning of growth, and I have definitely had these problems with the petals as well. Either way, you could probably give it a dose of Provado which will deal with that as well as any soil borne pests, though not very fast. Hope this helps