Answer: Your asters may be infected with a leaf spot disease, says Landon Winchester, staff horticulturist at White Flower Farm, perennial flower specialists in Litchfield, Connecticut. During rain or watering, the soilborne pathogens splash up onto leaves and stems, causing the leaves to develop spots, yellow and eventually die. The plant is weakened and flowering is reduced. The best controls are preventive measures. Remove and destroy all the foliage in the fall. In spring, divided the crowns, adding afew shovelfuls of compost per planting hole, and plant them in a sunny, breezy spot where the foliage can dry out quickly, Winchester advises. If the problem persists, spray wettable sulfur in spring after each rainy period as the new growth emerges, completely covering the foliage, he adds.
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