Downy and powdery mildew are two fungal diseases that can destroy a bed of phlox plants. Yellow patches that show up on leaves, along with distorted stems with altered growth, are symptoms of downy mildew. White or gray mildew on the lower or top surfaces of leaves and new stem growth is also a sign of downy mildew. Powdery mildew is a powdery gray substance that is found on leaves of the phlox plant. It starts in the lower region of the plant and spreads up the leaves until it reaches the top section.
Inspect phlox plants for signs of fungus. Identify what type of fungus it is: powdery mildew or downy mildew.
Remove from a greenhouse any plants with signs of fungus. Do not plant them in a flower bed.
Purchase fungicide sprays and strictly follow the directions to apply the material to the infected plant. Choose liquid seaweed spray or a proven organic method to fight the fungus without using chemicals in the garden.
Apply the fungicide or organic spray throughout the season at least two times or according to the product directions. Alternate applications of more than one product to prevent the possible buildup of resistance to the fungicide. Spray the bottom and top of the leaves. Stop spraying when the moisture begins to bead up and run off of the plant.
Reduce moisture in the area where the phlox plant is growing. Wet conditions add to the spread of fungus. Water plants only when needed early in the morning to allow the moisture to dry before evening.
Remove all plant debris at the end of the season to prevent fungal growth during the next growing season.