Lupins and Fusarium Root Rot? Or something else completely!?

Its still early in the summer and I have few plants in flower, my Lupins being an exception. They are in full bloom and looking amazing as they always do. However over the last couple of weeks one of the plants has started to look really ill. Nearly all the leaves have become yellow and wilted.

As you can see from the picture it is looking in a right state, the flowers are now even starting to go the same way. As soon as I noticed I looked at the RHS website to see if I could find a cause, but powdery mildew and leaf spot the only too listed diseases didn't fit the bill. I may be wrong and being a hypochondriac about my plants the symptoms seem to point to root rot or Fusarium wilting. Two terms when applied to Lupins means absolutely nothing to me.  The literature I have found online has been rather wordy and does not state if there is anything I can do... just the symptoms and the causes. This blog article was the most indepth info i could find and probably inseminated my mind that my lupin has Fasarium Root Rot.

In any case this Lupin looks like it is on its last legs and I will probably dig it out in the next day or two. My other lupins are so fare unaffected and Iwant to keep it that way whatever the reason it is wilting on me.