Garden phlox, or Phlox paniculata, is a perennial that blooms in late summer and comes in a variety of colors. Phlox requires full sun and well-drained soil. Feed the plants in early spring with a low-nitrogen fertilizer for best results. As with most plants, phlox do encounter problems with insects and diseases.
Powdery mildew is a destructive fungal disease that causes powdery white spots on leaves of the phlox plant. Once the spots occur, they quickly destroy any remaining green tissue and then move on to the flowers. Choose disease-resistant varieties to avoid this disease. The plants need plenty of air circulation and sun when planted. If powdery mildew does occur, spraying fungicides such as sulphur or chlorothalonil (sold as Daconil 2787) at the earliest sign of infection can be effective.
Symptoms develop with easily identifiable circular brown spots with white to gray centers. Leaf spot starts on the lower leaves and moves up the stem. The infected leaves curl up and dry. The first defense is to keep the plants healthy by fertilizing and watering. Water in the morning so the leaves have a chance to dry during the day. Remove affected leaves as soon as you notice leaf spot. If desired, apply fungicides in the spring when new growth appears.
Phlox Plant Bug
This bug bites through the tender shoots and leaves of the phlox at the growing tip and sucks the sap. If they become abundant, spray commercial insecticides or insecticidal soaps to treat the problem. Common commercial brands include the chemicals acephate, azadirachtin, or imidacloprid. Always check with a local Extension Office or nursery before using any products.
Twospotted Spider Mite
Twospotted spider mites are the most serious of all phlox problems. These insects suck the liquids from phlox leaves, causing the leaves to turn yellow. Healthy plants are better able to tolerate mites, so make sure phlox are watered and fertilized adequately. An insecticidal soap spray can be fairly effective. If the the problem is more severe, Kelthane, a miticide specifically for flowering perennials, can be found at local nurseries. Apply the spray very early in the day, when temperatures are cooler in the morning hours. Spraying during the heat of the day is likely to cause more damage.
The larvae of the stalk borer tunnel up and down into the stems of a phlox plant. The insects usually go unnoticed until the plants begin to wilt. It's too late for the plant to recover at that point. There is one annual generation with the moths emerging in September and October to lay eggs. The eggs can survive winter and hatch in May or early June. Stalk borers usually cannot be controlled once they have entered the plant. Systemic insecticides may be effective when applied in high damaged areas, but control measures should concentrate on prevention. For more specific control information, check with a local extension office or nursery.