Experts sought to solve Nottinghamshire dog deaths
The Forestry Commission wants any experts who have theories about the mysterious deaths of dogs in Sherwood Forest to contact them.
Forester Val Coulton said they are no closer to finding what is behind the death of one dog this year and eight in 2009.
A dog from Lincolnshire died on 21 September after a walk in Clumber Park in Nottinghamshire.
In autumn last year, eight dogs died from symptoms including diarrhoea.
Natural England has ruled out man-made poisoning, and said it believes a natural phenomenon is to blame.
The Forestry Commission said it has posted signs at its main car parks in the area urging pet owners to keep all dogs on leads.
"We simply don't know the cause. And we don't know where to start as we have vast areas of woodland," Mrs Coulton said.
"We don't have the sort of experts needed to investigate the problem," she added.
A mushroom enthusiast has told BBC News that it is unlikely the dogs were killed by a fungus.
Beverley Rhodes said mushrooms must be swallowed, inhaled or absorbed to cause illness in animals and most of the dog owners have said that did not happen.
Eleven dog owners have reported their pets have become ill after walks in the Sherwood Forest area in the past few weeks.
Nottinghamshire vet Janice Dixon has warned pet owners to be vigilant after treating a number of animals for severe sickness and diarrhoea.