Phlox, made popular for its versatility and beauty, has many varieties. Ground cover and tall phlox are just two of the varieties of phlox that add color and depth to rock gardens, borders and other types of landscapes. These colorful, star-shaped flowers are susceptible to a few diseases.
Oedema is a disease that appears when phlox cannot use up water as fast as the roots take it in, according to Cornell University's Plant Disease Diagnostic Clinic. The excessive amounts of water cause pressure within the internal leaf cells. The first signs of this disease are small, swollen blisters that form on the lower leaf surfaces. In time, the blisters can harden and become tan, brown, or like white bumps on the petals and stems of the phlox. Severely infected plants can suffer foliage yellowing and leaf drop. The phlox can become spindly and die. Oedema is common in late winter and especially during times of cool, gray weather. Warm, moist soil and cool, moist air create the perfect environment for this disease. Manage this disease by not watering phlox in winter, when they should be dry. Many times, plants affected by oedema will return to good health with the arrival of spring and summer weather.
This problem is most prevalent in the southern United States, but it can affect phlox no matter where they are planted. There are many kinds of nematodes including the most commonly found culprit, root knot nematode. This parasitic disease causes root rot, poor growth and even plant death. The visible symptoms of parasitic nematodes include foliage wilting and loss of luster, yellow foliage, and stunted, weak new growth. In small leaved plants like phlox, this disease can cause the leaves to turn brown and die. There is no cure for parasitic nematodes, but there are several measures for management. Control of this disease can begin at planting time. Prepare the soil by removing any old plant debris. If you are planting phlox in a flower bed, it may be beneficial to remove the old dirt and replace it with fresh, new soil. This will get rid of any nematodes that have infested the original soil. If phlox is already planted, manage the threat of a nematode infestation by caring and maintaining for the plants without any lapse in upkeep. Take care to always water and fertilize the plants appropriately. Phlox can still be successful in a landscape if cared for properly, even when nematodes are present.
Powdery mildew can be caused by one of many different fungi. This disease appears on the plant leaves as a powdery, white or gray dusting. It begins as round, powdery, white spots that expand to completely cover the leaves and foliage. The spread of this disease is largely exacerbated by humid weather. If left untreated, powdery mildew causes leaf yellowing and eventual plant death. Prevent powdery mildew by removing any fallen leaves during autumn, advises the Plant Disease Diagnostic Clinic from Cornell University. Treat this disease by applying a fungicide according to label directions.