What a timely question. Careful Peter, timely in time of year, not that we have seen any sign of the dreaded Impatiens Downey Mildew. In fact we are growing beautiful Impatiens this year.
So beautiful in fact we are treating this tray (Summer Berries and about 30 of her friends) as not up to our standards but that's just because she's too big to offer shelf life in a retail garden centre.
I have written about Impatiens Downey Mildew previously, so you can find lots of information here. Unfortunately I can't say the Downey Mildew has gone away or that we have found a miracle cure. We are being careful not to bring any Impatiens plants into our nursery and we grow our seedlings very 'dry' which prevents the fungi spores germinating.
There was a research program done some years back (Wow 1996) on Downey Mildew in Brassica Vegetable seedlings, a closely related but different strain. Both diseases have similar symptoms to each other but they are very specific to the host plant, they are not interchangable. The recommendation of this research was to NOT water our plants between 9.00 & 11.00am. This is the time when the spores are most active and the addition of water just provides a perfect environment for germination. I favour 'early' watering to ensure time for evaporation through the day. Water sitting on foliage over night is just bad news as it encourages any number of fungal & bacterial diseases.
Unfortunately the other critical cultural element is to remove any diseased plant material and ensure that Impatiens are not grown in the same space again at least for a season or two. This is of course the ancient crop rotation system. Sad really, I clearly recall cousin Fred Wood lamenting that Impatiens seedlings would last so long that they had ruined the autumn flower seedling sales season... perhaps there's a conspiracy! Anyway, if Kay hasn't grown Impatiens for a few years she is a good chance to make a fresh start so long as she chooses 'clean' seedlings. This applies to potted plants as well, If you find the disease don't fight it. Remove the infected stock and clean containers thoroughly with disinfectant.
Actually, the best advice I can offer is to grow New Guinea Impatiens. The Impatiens Downey Mildew disease is so specific that it doesn't even effect this close relative of Impatiens wallerana.