Potting mix has taken its toll on five people hospitalised with legionnaire's disease

Gardeners in the upper South Island are being warned to protect themselves after four people in Tasman and one in Marlborough went down with the disease this month.

They bring the total to 10 cases so far this year - up on last year's eight.

Nelson Marlborough Health's Dr Stephen Bridgman says legionnaires can cause pneumonia and, in some cases, death.

One sufferer spent 21 days in hospital.