Answer: It sounds as if your plant has two different diseases. The black dots are probably Alternaria leaf spot disease, but the plant collapsing is due to Rhizoctonia stem canker, says Margery Daughtrey, plant pathologist at Cornell University. Alternaria fungus is mostly a cosmetic problem. The spores are spread by wind and germinate on leaves in moist conditions, causing the black dots, says Daughtrey. But it rarely kills the plants. Rhizoctonia stem canker, on the other hand, is a widespread problem that often kills plants. Impatiens are particularly susceptible to this disease because their dense leaf canopies create humid conditions around the succulent stems - the perfect environment for the fungus to thrive. Gardeners usually don't notice any problem until the whole plant collapses and dies, says Daughtrey. Rhizoctonia stem canker is a soilborne fungus, and once it's in your flower bed it can infect other impatiens. The best control is to increase air circulation around the plants and keep the leaves dry by watering only in the morning. Instead of daily light waterings, the plants should be thoroughly watered two or three times a week. Remove any mulch and cultivate the soil regularly. To improve air circulation, increase the space between plants by mixing other shade-loving annuals, such as begonias, in with the impatiens.
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