Last year, four dogs died and another 14 fell ill after being walked in Sherwood Forest in Nottinghamshire.
Now experts fear the disease has returned after one dog died and at least eight dogs were taken ill after visiting the forest.
Warning signs have been put up and the Forestry Commission is advising owners to keep their pets on leads while walking them to reduce the risk.
Pesticides and other man-made chemicals have been ruled out and local vets’ latest theory is that the dogs ate poisonous mushrooms.
Janice Dixon, a vet in Rainworth, Notts, said she had treated several dogs at her surgery in in the past week with “horrendous” symptoms.
Miss Dixon added all the dogs had been walked in or around Sherwood Forest in the 24 hours before becoming ill.
She said: “All the dogs had horrendous diarrhoea and vomiting and were put on IV drips.
“I think this is something naturally occurring in the forest at this time of year because exactly the same thing happened last year, just for a month and then it 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.”
Derek Broughton, of Lincoln, said one of his King Charles Spaniels died on Tuesday after visiting Clumber Park.
Mr Broughton took his two King Charles Spaniels to the forest and said they ran around as normal but he did not see them pick anything up.
He said: “It was just a normal weekend 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.
Stuart Burgess, of the Forestry Commission, said: “It looks like whatever it was last year could be reoccurring this year because we have got similar reports in similar areas at the same time of year.
“We are putting notices up on our sites to say that there is something happening again and to put dogs on a lead and if their dogs fall ill to take them to the vet quickly.”
A spokeswoman for Natural England said: “Natural England has been made aware of recent incidents of dogs being taken ill in woodlands in the Sherwood area.
“The symptoms and timing of these incidents are identical to an outbreak in the same area last year and as a result it is highly likely that they have the same cause.
“It was established that pesticides or other man-made chemicals were not involved in last year’s outbreak and we believe that this latest outbreak is caused by naturally-occurring agents such as ticks or mites.”