Doctors discovered that the 67-year-old man was infected through a cut in his hand after opening a bag of compost in March.
It follows three other Scottish cases of the same strand of the disease in 2008 and 2009.
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In the earlier instances, the victims had inhaled droplets of water in fertiliser and contracted the disease.
Legionnaire’s has been linked to gardening in several countries, including the United States and Holland, but the four Scots cases are the first in the UK.
Reporting the rare incident in The Lancet medical journal, doctors said compost was known to harbour Legionnaire’s bugs.
The unnamed man, described as previously fit, healthy and a keen gardener, was inexplicably struck down by a serious fever in March.
Doctors saw him in hospital after he suffered eight days of trembling, confusion, lethargy and shortness of breath.
He had a high temperature and an X-ray revealed signs of pneumonia in his left lung.
Dr Simon Patten and colleagues from the Royal Alexandra Hospital, Paisley, wrote: “We treated our patient with oxygen, intravenous fluids and antibiotics, but his respiratory function deteriorated, necessitating transfer to the intensive care unit for intubation.”
The procedure involves inserting a tube down the windpipe to help a patient breathe.
Initial tests for different types of infection proved inconclusive.
However, after washing out his lung, the doctors obtained a sputum sample that confirmed the presence of Legionella longbeachae, a species of Legionnaires’ bacteria.
“When we questioned the patient to find out the source of this infection, we discovered that he was a keen gardener and had lacerated his left index finger two days before the onset of his symptoms, while planting with compost,” they wrote.
“We presumed that this cut was the site of entry of the organism.”
The patient was treated with the antibiotic levofloxacin, which is used to treat dangerous bacterial infections.
The patient’s condition improved and seven days later he was moved to a respiratory ward before being discharged. By May he had made a full recovery.
Legionnaires’ disease is normally caused by the bug Legionella pneumophili, which lives naturally in rivers, lakes and reservoirs, and can also be found in man-made structures containing water, such as air conditioning systems.
Legionella longbeachae is a less common species first isolated from a patient in Long Beach, California.
Unlike its cousin, it is mostly found in soil and potting compost. Infection by this type of Legionnaires’ bug is sometimes called Pontiac Fever.
In the UK, just nine cases have been reported since 1984. Longbeachae infections are much more common in Australia, New Zealand and Japan, where they account for about 30% of all cases of Legionnaire’s disease.
The doctors said previous case studies had shown compost to be a source of the infection.
In 2008, retired gas engineer Drew Murphy, 59, from Bothwell, caught longbeachae after opening a bag of compost. He spent seven weeks in intensive care.
The Royal Horticultural Society has said that bags of potting compost will now carry cautionary statements.
Guy Barter, of the Royal Horticultural Society, said: “The four cases of Legionnaire’s disease all involved compost bags that were being kept in greenhouses.
“Compost bags should never be kept in a place where it is likely to remain warm.”