Dahlias (Dahlia pinnata) are prized for their large, showy blooms. Native to Mexico, the flowers, which are available in a wide range of shapes and colors, thrive in rich, well-draining soil and in full sun. The flowers grow on bulbs called tubers and grow best in warm, moist soil. In cold conditions, however, the tubers may suffer from rot diseases.
Dahlias with rot problems stop growing, wilt and die. You may see white mold growing on the ground around the stem of the plant, according to Michigan State University's Extension website. If you dig up the tubers, you may see soft, black areas that have rotted away.
Tuber rot on a Dahlia is caused by fungi that thrive in overly wet soils. Although Dahlias need rich, moist soil to bloom well, soil that is too wet will encourage fungi growth. This is especially true in cold, wet soil. For that reason, the soil around Dahlia should be kept just barely moist to the touch, but never soggy or waterlogged. Excessive water in the soil prevents oxygen from reaching the plant, according to the University of Minnesota's Extension website, and leads to tuber rot and other rot diseases.
Always plant Dahlias in locations that drain well and are exposed to full sunlight. This will keep the soil warm and encourage better blooming. Never locate Dahlias where standing water develops or in locations where cold air pools (such as depressions or shaded areas), as this may lead to tuber rot. In addition, do not crowd the tubers; instead, plant each one so that air circulates freely around the plant. If the entire tuber is rotted, you must discard the plant. If only part of it is rotted, you can cut away that part and plant the tuber again in the spring if the healthy part has an eye, or bud on it.