A dog is thought to be the latest victim of a mystery illness which killed several pets after they were walked in the Sherwood Forest area.
Derek Broughton, of Lincoln, said one of his King Charles Spaniels died on Tuesday after visiting Clumber Park.
In autumn last year, eight dogs died from symptoms including vomiting and diarrhoea.
Natural England has ruled out man-made poisoning, and said it believes a natural phenomenon is to blame.
Mr Broughton took his two dogs - both King Charles Spaniels - to the forest, along with his daughter's pet dog.
He said: "We took them out into the woods at Clumber Park last Saturday morning. They all had a run around. We never saw them stop and pick anything up at all."
"It was just a normal weekend," he added. "But when I came down to see them Monday morning, all hell had broken loose."
Later that day, all three dogs had collapsed and were almost comatose, he said. One of the spaniels, called Bertie, died the following morning.
Earlier this week, Nottinghamshire vet Janice Dixon warned pet owners to be vigilant after treating a number of animals for severe sickness and diarrhoea.
She said the same thing happened for about a month last year, and then stopped.
"It starts in September, there is more humidity and less daylight and a change in the flora and fauna. I think it could be a mushroom or fungus causing the illness," she added.
Natural England will not be investigating this year as previous tests have established the cause was not man-made.
"It is not poison or pesticide-related, so we have taken the decision not to investigate as we believe it is naturally occurring," a spokeswoman said.
Signs warning dog owners to keep their pets on a lead and to contact a vet if they become ill are being put up in the area, a spokesperson from the Forestry Commission said.