Answer: Without seeing the plants I can't tell for certain, but it sounds like a fungal disease. There are several: A wilt disease works its way up from the base of the plant, causing leaves and branches to wilt and die. A leaf spot disease causes circular brown spots on the leaves. Stem rots attack impatiens stems at or near the soil level. Foliage turns yellow, wilts, and dies.
There is no practical treatment for these problems since these annual plants will die at the end of the season anyway. The best strategy is to promptly remove and discard badly infected plants to prevent the spread of the fungus to other plants. To avoid in the future, do not grow impatiens continuously in the same location every year. Control weeds in and around the yard. Sterilize tools after use on infected plants with a spritz of household disinfectant spray or dip them in a solution of hot water and household bleach. Mulching helps prevent splash-borne infection in outdoor plantings. For long term prevention, lighten heavy soil with a mixture of perlite, vermiculite or peat moss and provide good drainage. Avoid overwatering. Space plants further apart to prevent crowding.
Best wishes with your garden!
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