Leaves Covered With White Powder Signal Powdery Mildew
Powdery mildew is caused by fungi that live on the surface cells of a plant, coating infected leaves with a white or ash-gray powdery mold. Badly infected lower leaves become discolored and distorted, then (drop)off. It is the most common disease problem in phlox. The disease is encouraged by crowding and by warm, humid weather. It is unsightly, but rarely kills phlox. Spray healthy plant foliage thoroughly with wettable sulfur every 2 weeks, starting as soon as the whitish coating of the fungus appears tp control its spread. Read and follow the instructions on the product label. Allow ample spacing between plants for good air circulation and try to avoid wetting foliage when watering. Collect and discard all aboveground debris in the fall so fungal spores do not overwinter. For more information see file on Dealing with Fungal Disease.
Leaves Discolored, Webbed Over From Spider Mites
Spider mites are about 1/50 inch long, barely visible to the unaided eye. They're not true insects, but are related to spiders, with four pairs of legs, piercing-sucking mouth parts, and very compact bodies. They may be yellow, green, red or brown. Inspect the lowest leaves on your plant. Upper leaf surfaces stippled with small yellow dots or red spots suggests the presence of mites. Leaves, and adjacent stems may be distorted or covered with fine webs. Mites are most abundant in late summer's hot, dry weather. Start control measures as soon as you notice the first stippling of the leaves. Spray the pests with insecticidal soap every 3 to 5 days for 2 weeks. Read and follow the instructions on the product label. For more information see file on Dealing with Mites.
Leaves Are Curled And Distorted Due To Aphids
Aphids, also called "plant lice," are soft-bodied, pear-shaped sucking insects about the size of the head of a pin. They often cluster gather in large groups of wingless and winged forms on leaves and tender buds. Their feeding retards plant growth. Phlox leaves may turn yellow or brown. They wilt under bright sunlight, or sometimes curl and pucker. Spray the insects with an insecticidal soap product as directed on the label. Spray stubborn infestations with a pyrethrin/pyrethrum insecticide according to label instructions. Do not overfertilize plants. For more information see file on Dealing with Aphids.
Spindly, Leggy Plants; Stalled Blooming Are Due to Cultural Conditions
Spindly, Leggy Plants Indicates Not Enough Sun
Phlox need lots of direct, bright sun to flourish. If they are shaded, they are forced to stretch to reach the light, causing them to develop thin stems. Trim away nearby overgrowth that may be blocking their access to light. Transplant them to a sunnier spot in the fall.
Blooming Stops Because Plant Has Set Seed
It is important to snip off the faded flower heads from phlox stems promptly. If the flowers begin to develop into seeds, the plant will no longer produce flowers, since its life cycle has been completed.
Flowers Are Deformed By Phlox Plant Bugs
Adult phlox plant bugs are reddish-orange, with a black stripe running down their back. Adults and nymphs sting flower buds, causing them to be deformed. They also suck plant sap from tender leaves and buds, producing light spots and distorted growth. The eggs, laid on leaf stalks, hatch in May. Spray the pests with a pyrethrin/pyrethrum insecticide product, taking care to follow label directions. Cut and discard dead growth and debris after the first frost to eliminate overwintering eggs. For more information see the file on Controlling Plant Bugs
Leaves Spotted Or Blotched By Fungal Leaf Spots
Many kinds of leaf spots are caused by fungi that thrive on moist leaf surfaces and cause transparent to brown or black spots that sometimes disfigure phlox leaves. Some fungal spots are surrounded by flecks or black dots, their spore-bearing fruiting bodies. Often spots come together to form larger patches of dead tissue. Pick off and discard infected leaves. Remove dead plant debris promptly from the garden to reduce overwintering spore populations. Dig up and discard seriously infected plants together with the soil of their root ball. Mulching helps prevent splash-borne infection in outdoor plantings. Spray healthy foliage with garden sulfur fungicide according to label directions to prevent the spread of the fungus. For more information see file on Dealing with Fungal Disease.
Crowns and Roots Rot; Odor Present Signals Crown Rot
Crown and root rots on phlox are caused by soil-dwelling bacteria and fungi Bacteria attack a few spots on the crown--where the stems meet the roots--just below the soil surface. Then they spread to the entire crown, turning it into soft, smelly mush which may then be invaded by fungi. If the plant is already in leaf, the lower leaves are discolored and the young shoots begin to wilt. The roots are blackened, rotten and covered with white fungal threads. The whole plant dies in a few days.
This disease is most likely to occur in the late winter thaw when dead leaves decompose on the ground and harbor bacteria and fungi which spread to healthy tissue. Remove and discard the infected plants and the adjacent soil in the trash. Disinfect tools by dipping them in a solution of hot water and household bleach to avoid spreading the disease. Keep winter mulch away from plant crowns to reduce problems. For more information see file on Dealing with Fungal Disease.
Discolored Flowers; Stunted Growth Is Caused By Aster Yellows
Aster yellows is caused by a mycoplasma-like organism, similar to bacteria. It is spread by leafhoppers and aphids. Phlox plant parts or entire plants may turn greenish-yellow, and be stunted or dwarfed. Leaves are often spindly. Flowers turn yellow, and may be dwarfed or aborted entirely. Plants may wilt and die early. Remove and destroy plants known to be infected. Spray remaining plants with a pyrethrin/pyrethrum insecticide product, as directed on the label, to kill pest insects that carry the disease.