One day your impatiens are fine, blooming and lovely and the next, they are totally destroyed! It's not slugs, it's downy mildew and it strikes fast and hard!
Frank Ferragine, aka Frankie Flowers, addressed this problem briefly on CityLine on the gardening special. Downy mildew can be spotted on the underside of the leaves of impatiens as a white coating, even before the damage is evident. If this is present, it is best to dig out and destroy your impatiens. Do not put them in your compost, but rather burn them. There is no known cure available.
Impatiens downy mildew is a fungus which can strike during wet weather in spring and summer. It causes yellowing leaves and the loss of foliage in impatiens, killing them.
As downy mildew spores can live in the soil for at least a year so, following a year of disease, do not plant impatiens in the same spot. You may plant begonias, fuchsia, or another similar shade/partial shade plant instead. Since it is an airborne disease, it is not necessarily a risk free option to plant impatiens in another location in the garden. The article written by staff at the Royal Horticultural Society suggests you may have better luck growing your impatiens from seed rather than purchasing from a grower/greenhouse as the disease may already be present. The fungus can be latent, without signs of infection, for some time, so be aware that gardeners in Canada began seeing this problem last year, meaning that it may reoccur this year, to an even greater extent. For more information, click on the link below.
Impatiens downy mildew / Royal Horticultural Society