Perennial phlox (Phlox spp.) are popular garden plants that come in a variety of sizes and colors. The blossoms and can be found in white, purple, blue, lavender and pink. The flat face flower has five petals that form a trumpet, and some flowers have contrasting centers. While the phlox is hardy and can withstand most insect pests and diseases, it is important to watch the plants so that any disease introduced to the garden is dealt with as soon as possible.
Leaf spot is caused by several types of moisture-loving fungi. Dark spots or blotches on the leaves of the phlox can be a sign that fungi have started growing. The spots can be black or brown and often have whitish gray centers. Spore-bearing fruiting bodies often appear around the spots looking like black dots. Sometimes the spots come together to form patches. The leaves of the infected phlox plant should be remove and destroyed. Remove the entire phlox plant if is has a lot of damage, plus the surrounding garden soil. At the end of the growing season, do a complete cleanup of the garden--the fungi that cause leaf spot will winter in plant debris left in the garden.
Phlox that look like the leaves are covered with a white to light gray powder are usually suffering from powdery mildew. If the mildew gets too bad, the leaves will discolor and fall off. Fungi that live on the surface of the phlox plant cause the powdery mildew. To prevent it, make sure the phlox has enough air circulation. Thin the plants out if necessary so moisture does not accumulate.
Phlox that are planted in garden areas or soils that do not drain well are susceptible to crown rot. The foliage of the plant will turn yellow and wilt, soon followed by blackening at the base of the stem. Finally, the weakened plant will topple over. Avoid crown rot in the phlox garden by improving the drainage of the soil by adding compost or other organic matter. Don't put mulch near the crown of the plants; the moisture invites bacteria and fungi attacks. If extreme rainfall prevents proper drainage, cultivate the soil around the plants to help it dry out. Pull out phlox plants with crown rot and dispose of both the plant and the surrounding soil.