Phlox diseases are one of the biggest reasons these flowers lose their color, wilt, or even die. Powdery mildew, spider mites, aphids, crown rot - these common diseases of phlox often strike tall garden phlox, purple phlox, white phlox, and other varieties leaving plants weak and ugly. The good news? Most problems with phlox can be fixed if you know the signs early. Yellow leaves, webbed stems, odd smells, or stunted blooms all point to trouble.
This guide shows you how to spot each issue quickly and use simple, proven treatments so your perennial phlox stays strong, colorful, and healthy all season long.
Leaves Covered With White Powder Signal powdery mildew on phlox
Powdery mildew on phlox 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 — phlox leaves curling From 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 in large groups of wingless and winged forms on leaves and tender buds. Their feeding retards plant growth.
Common symptoms include phlox leaves curling, phlox leaves turning yellow, and phlox leaves turning brown. These are some of the most frequent issues seen on garden phlox leaves. Leaves may 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 surfaces and disfigure a phlox leaf with black or brown marks. 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 phlox dying From 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, often described as phlox dying rapidly.
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, may be dwarfed or aborted entirely, and sometimes lead to phlox not flowering at all. 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.
Phlox Problems Solved: Your Most Asked Questions Answered
What is phlox plant?
Phlox is a perennial plant grown for its colorful summer blooms and sweet fragrance. The most common type is garden phlox (also called summer phlox or border phlox), a wildflower native to North America that’s now a garden favorite. It grows 3 to 6 feet tall, with green lance-shaped leaves and clusters of pink, red, purple, blue, or white flowers that attract butterflies and hummingbirds. There are also hybrids like phlox flame purple and phlox pink flame. While hardy, phlox plants can face problems like powdery mildew, aphids, mites, or root rot if not cared for properly.
.
What
does phlox look like?
Phlox plants come in many forms, but you can usually identify them by their leaves and flower clusters:
-
Garden phlox leaves: broad, smooth, and green. When unhealthy, they may show curling, yellowing, or brown spots.
-
Creeping phlox leaves: small, narrow, and often needle-like.
-
Flowers: Phlox blooms in clusters of five-petaled flowers. Colors range from pink, purple, and lilac to red, blue, and white.
Garden phlox grows tall with upright stems and pyramidal clusters of fragrant flowers, while creeping phlox forms a colorful, ground-hugging carpet. Both are loved by butterflies and hummingbirds for their vibrant colors and sweet fragrance.
What
are common phlox diseases?
Common diseases of phlox plants include powdery mildew on phlox, crown rot, aster yellows, and fungal leaf spots. These phlox plant diseases cause symptoms like yellow leaves, curling leaves, brown spots, stunted growth, or even entire phlox dying quickly.
Example: Powdery mildew on phlox plants leaves a white, powdery coating on leaves and stems, spreading fast in humid conditions.
How
to treat powdery mildew on phlox?
The best way to manage powdery mildew on phlox plants is to act quickly and combine cultural practices with safe treatments:
-
Spray wettable sulfur every 2 weeks at the first sign of infection.
-
Improve spacing & airflow between plants to reduce humidity.
-
Avoid overhead watering — always water at the base.
-
Remove infected leaves and debris to stop spores from spreading.
Example: If you see a white coating on the leaves, treat right away - don’t wait for it to spread.
Why
are my phlox leaves turning yellow or brown?
Phlox leaves turning yellow may come from aphids, aster yellows, or nutrient stress. Phlox leaves turning brown is often caused by fungal leaf spots, too much sun, or poor watering habits. Overwatering can also create yellow phlox leaves by suffocating the roots.
Tip: Always check the underside of the leaves for aphids or spider mites, they’re one of the most common culprits.
Why
are my phlox leaves curling?
Phlox leaves curling usually signals aphid infestations, but it can also happen from heat stress, drought, or even overfertilization. Environmental stress weakens plants, making them more vulnerable to pests and disease.
Tip: Spray with insecticidal soap to control aphids and avoid using too much fertilizer, which can worsen phlox leaf curl.
Why
is my phlox dying?
Phlox dying quickly can be caused by crown rot, root rot, or aster yellows. Common symptoms include wilted shoots, soft or rotting roots, and collapsed stems. Environmental stress like too much shade, overwatering, or poor soil drainage can make the problem worse.
Example: Crown rot can kill a plant in just a few days, leaving what looks like dead creeping phlox almost overnight.
Is
phlox a ground cover?
Yes, creeping phlox is commonly used as phlox ground cover, spreading into dense, colorful mats of flowers each spring. It’s ideal for rock gardens, slopes, and borders where it creates a carpet-like effect.
Tip: While it’s a hardy choice, watch for creeping phlox problems like browning or dying patches, which can result from poor drainage, drought, or pests.
Why
is my creeping phlox turning brown?
Creeping phlox turning brown is often linked to drought stress, spider mites, or poor soil drainage. In some cases, fungal diseases like root rot or powdery mildew can also be responsible, especially in humid conditions.
Tip: Water deeply but less often, and trim lightly to encourage healthy regrowth. Make sure the soil drains well to prevent waterlogging.
Why
is my creeping phlox dying?
Creeping phlox dying may come from crown or root rot in heavy soil, or fungal infections like powdery mildew and anthracnose. Pests such as spider mites and environmental stress from heat or poor drainage can also contribute to decline.
Tip: Remove dead patches and replant in well-drained soil. Improving airflow and watering at the base helps prevent creeping phlox problems in the future.
What
do phlox leaves look like?
Phlox leaves vary by type, but they share some common features. Most are lance-shaped, grow in opposite pairs, and are deep green. Healthy leaves are smooth, while some wild species may have fine hairs along the edges.
-
Healthy leaves: bright green, smooth, and lance-shaped.
-
Problem signs:
-
Phlox leaves curling → often caused by aphids.
-
Phlox leaves turning yellow → usually from disease or pests.
-
Phlox leaves turning brown → commonly due to fungus
-
Is
phlox deer resistant?
Many phlox varieties, including yellow phlox perennial and hybrids like phlox flame purple or phlox pink flame, are considered moderately deer resistant. In most gardens, deer will avoid them, but during tough seasons when food is scarce, hungry deer may still nibble.
Tip: In areas with high deer pressure, it’s best to combine planting with repellents or protective measures.
