At all the infected properties there are restrictions on the movement of plants or other risk materials off the sites.
They are being treated with fungicide, at-risk plants are being safely destroyed, and surveillance is under way in the areas surrounding the properties for signs of the disease.
Yard said growing conditions at plant nurseries were ideal for the fungus with many vulnerable young plants in sheltered, warm and damp environments.
"There has been a large amount of communication with the nursery industry and growers have been particularly vigilant in checking their plants."
MPI asks members of the public to report any suspected signs of myrtle rust to its Exotic Pest and Disease Hotline on 0800 80 99 66.
They advise people not to touch the rust or the plant, but to note the location and take photos of the symptoms and the plant.