The 15-year-old mare had been suffering from a neurological illness, with no prospect of recovery.
Kate, who owned the mare with her fiancé Matt Tarrant, said she was “absolutely gutted”.
She took over the ride in 2008 and had her first run at three-star with Daisy, at Barroca, Portugal.
Kate, who is based at Littleton Manor Equestrian, Surrey said: “Daisy was a one in a million horse who always gave her all and gave me some of the very best rounds of my life.
“I was privileged to be able to ride her and all her efforts have helped establish my career to date. She was a feisty mare who didn’t know how to quit. She will be sorely missed as a ride and a friend.”
Daisy’s eventing career began under the direction of Rachel Wallace before Oliver Townend took the reins in 2007, winning that year’s intermediate championships at the Festival of Eventing, Gatcombe.
The spirited mare won many friends and fans through her performances on the British Eventing circuit and her lively personality around the yard and lorry park.
Daisy had one embryo transfer foal by Grafenstolz who will be competed by Kate in the future.
Photo of Daisy Crazy and Kate Lukas at Lincolnshire County Show Express Eventing 2011 courtesy of Tim Wilkinson.