Phlox can be grown as either an annual or perennial, and they are generally used as ground cover, especially short phlox. The plant creates brightly coloured flowers, ranging from blue to lavender to pink to red to white. There are 67 species of phlox, and they are all susceptible to diseases like mildew, fungus and stem canker diseases. Phlox should be planted in areas where they are not crowded and receive adequate air circulation. They should not be overwatered.
According to Garden Crossings, powdery mildew is the most common disease for phlox. This disease creates a powdery white substance on the surfaces of leaves that resembles mildew. It usually occurs at fall, and it will cause the growth of phlox to be retarded and will disfigure the plant's buds, flowers and leaves. Powdery mildew can also cause the infected areas to die by sucking out nutrients. Permanent leaf loss can also occur. This disease is often caused by crowding of plants and too much humidity. Use fungicides to kill powdery mildew, including products that contain Bacillus subtilis, potassium bicarbonate, sulfer and neem oil. Dead leaves should be gathered and destroyed on both outside and inside plants.
Fungal Leaf Spot
Fungal leaf spot is characterised by a circular spot ranging from brown to black with a dry elevated centre point. This disease is caused by too much moisture on plants and poor air circulation. Leaves turn from yellow to brown, and then they eventually drop from the plant. Fungal leaf spot can spread quickly amongst plants in close quarters, especially if they stay wet. Only water in the morning so that leaves can dry, since persistent water on leaves can cause this disease. Do not shape or prune wet plants. Adequately space plants to allow for proper air circulation. Gather up diseased leaves, and destroy them. Prune affected areas, or use a fungicide to stop the disease from spreading. Test fungicide on plant to ensure that it does not do more harm than good.
Stem canker is characterised by small lesions that range from red to dark brown in colour, and these usually are found near the leaf node. Stem canker forms early in the growing season and expands across the plant. It causes the stem to begin to wilt and then to eventually die. Cold, wet conditions can cause the disease to form. Wait to plant phlox until the rainy season is over. Early summer is the best time to plant to avoid the disease. Use filar fungicide to kill the disease. Keep plants well-spaced to stop the disease from spreading. Do not water plants at dusk; water early in the day to allow leaves to dry in the sun.