Rose of Sharon is a hardy, easygoing shrub, so when its leaves spot or blacken or its buds fall, the cause is usually one of a handful of familiar problems. Most are surface fungi that mar leaves and flowers but rarely kill an established shrub. Appearances can mislead, though. Some of the most alarming symptoms come from watering, feeding or insects rather than disease, and the few truly serious problems start inside the wood or roots.
Powdery Mildew and Why Shade Invites It
Powdery mildew begins as small, round, white powdery spots on the upper side of older leaves.1Smart Gardener. Calendula: White Chiffonâ„¢ Hibiscus Rose of Sharon Shrub It quickly spreads to cover whole leaves, top and bottom, and the stems too.2Smart Gardener. Calendula: White Chiffonâ„¢ Hibiscus Rose of Sharon Shrub It usually shows up in mid to late summer.3Smart Gardener. Calendula: White Chiffonâ„¢ Hibiscus Rose of Sharon Shrub Unlike most fungi, it doesn’t need wet leaves to spread, so it can appear even in a dry summer, when other leaf diseases stay away.
If you’re not sure what you’re looking at, try rubbing it. Mildew wipes off as a fine white film but returns and spreads within days. Dust or spray residue sits evenly where it fell and doesn’t creep from leaf to leaf.
Powdery mildew is worst in shady, humid areas with poor air movement.4Smart Gardener. Calendula: White Chiffonâ„¢ Hibiscus Rose of Sharon Shrub Rose of Sharon tolerates partial shade, but too much shade reduces flowering and contributes to fungal diseases.5Monrovia. Rose of Sharon: Growing and Care Guide Thinning branches that block sunlight or air flow through the plant is the main fix.6Stutzmans Greenhouse & Garden Centers. Care & Pruning of Rose of Sharon
Because wet leaves aren’t the trigger, watering at the base helps only a little. Opening up the shrub helps next summer more than this season’s leaves, so judge success by the new growth.
This week, pick off the worst leaves and bin them rather than composting them. Thin a few crowded inner branches so air moves through. Fungicide sprays are an option if you want one, but they protect healthy leaves rather than clean up infected ones, so they work best applied early, before the white coating spreads.
Gray Mold on Buds, Flowers and Leaves
Gray mold, also called botrytis blight, makes buds and flowers develop oddly and turn brown.7Extension Plant Pathology. Botrytis Blight Flowers may show irregular flecks and brown spots, and older flowers rot quickly.8Extension Plant Pathology. Botrytis Blight On leaves and stems it causes small, wet-looking spots that grow and cause wilting.9Purdue Plant Doctor. Common Rose of Sharon Diseases and How to Handle Them Infected leaves, petals and soft stems turn dark, soft and wilted, then die.10University of California Statewide Integrated Pest Management Program. Common Rose of Sharon Diseases and How to Handle Them After cool, damp weather the affected parts can be covered with a gray fuzz of spores.11Extension Plant Pathology. Botrytis Blight12Purdue Plant Doctor. Common Rose of Sharon Diseases and How to Handle Them
Other browning has different causes. Rose of Sharon flowers each last only about a day, so a few shriveled spent blooms are normal. Brown, dry, papery petals after a hot, sunny spell usually point to heat or dryness, not fungus. If buds drop whole and stay green and firm, look to watering or feeding rather than mold.
Gray mold can only infect when there is standing water or the air is above 80% humidity.13Purdue Plant Doctor. Common Rose of Sharon Diseases and How to Handle Them Spores travel by splashing water and wind, and infection often starts on flowers or fallen petals before spreading into leaves and stems.14Purdue Plant Doctor. Common Rose of Sharon Diseases and How to Handle Them If the plant dries out after infection, it usually stays as small spots and doesn’t progress to wilting.15Purdue Plant Doctor. Common Rose of Sharon Diseases and How to Handle Them
Start with cleanup. Rake up fallen petals and flowers, since infection often begins there. Snip off brown, mushy buds and flowers and bag them, and clean your pruners between cuts on infected parts. Don’t leave piles of wet debris at the base of the shrub.
Then change how you water. Water the soil, not the leaves and flowers, and do it early so any splashes dry by evening. Cut back irrigation during cool, rainy stretches, and check that sprinklers aren’t hitting the shrub every day.
Leaf Rust, Sooty Mold and Mosaic Patterns
Leaf rust is a fungus that makes yellow-orange pustules on the underside of the leaves.16Texas Plant Disease Handbook. Althea (Rose of Sharon) The infected leaves yellow and develop spots.17Texas Plant Disease Handbook. Althea (Rose of Sharon) Those spots also look yellow from above, which can pass for a nutrient problem until you turn the leaves over. Pick off and dispose of rusty leaves, clear fallen ones from the ground, and improve airflow as you would for other fungi.
A black coating on leaves and stems is usually sooty mold.18San Antonio Express-News. Blackened roses-of-Sharon victims of sooty mold, aphids It grows on the sticky droppings of aphids, small soft insects often seen clustered on new growth.19San Antonio Express-News. Blackened roses-of-Sharon victims of sooty mold, aphids To control the mold you need to control the aphids.20San Antonio Express-News. Blackened roses-of-Sharon victims of sooty mold, aphids Once they’re gone, the mold weathers away over time. The film sits on top of the leaf, so the tissue beneath is usually healthy.
To get rid of aphids, look under leaves and on new tips for clusters of small insects. Blast them off with a strong spray of water every few days. Insecticidal soap works on contact if water isn’t enough. Ladybugs and other predators often help once you stop broad-spectrum sprays.
Mosaic viruses show as mottled patches of light and dark green on leaves, often looking variegated.21Smart Gardener. Calendula: White Chiffonâ„¢ Hibiscus Rose of Sharon Shrub Unlike rust or sooty mold, the pattern is part of the leaf itself and can’t be rubbed off. Depending on the plant and the virus, the effect ranges from almost none to stunted growth.22Smart Gardener. Calendula: White Chiffonâ„¢ Hibiscus Rose of Sharon Shrub A mottled but vigorous shrub can usually be left alone. There is no cure for a virus, so remove only a badly stunted shrub, and don’t take cuttings from mottled plants.
Verticillium Wilt, the Serious One
Verticillium wilt is a soil fungus that blocks the plant’s internal water-carrying tissue, so branches wilt even when the soil is moist. Peeling back the bark of a wilting branch can point to it, because infected wood often shows dark streaks running along the grain, though a lab test is needed to be sure.23Garden Basics. Summer Brings Verticillium Disease in Plants A cut across the branch shows a discolored ring or patches in the wood.24University of Maryland Extension. Verticillium Wilt Disease of Trees and Shrubs
Drought wilting affects the whole plant and recovers after watering. Gray mold wilting starts from soft spots on the outside. Verticillium wilt often hits one side or a few branches while the rest look fine.
Symptoms often appear in summer, when heat puts more demand on the plant’s water-carrying wood. Leaves on the affected branches may yellow, wilt and drop while the rest of the shrub still looks normal.
No spray cures verticillium wilt. Prune out affected branches back into clean wood, and sterilize your pruners after cutting infected wood. Keep the shrub watered in dry spells and avoid heavy feeding, which helps it outgrow the infection. Some shrubs recover and put on healthy new growth once the affected branches are gone.
If most branches die over a season or two, the shrub is unlikely to recover. The fungus lingers in soil, so don’t replant a susceptible plant in the same spot.
Yellow Leaves and Buds That Drop Before Opening
Too much or too little water or fertilizer can make buds drop before they open.25Gardening Know How. Common Rose of Sharon Diseases and How to Handle Them Look first at recent dry spells, heavy rain or a new feeding schedule. Soggy soil and drop after rain point to overwatering, while wilted, crispy edges after heat point to underwatering. A recent heavy dose of fertilizer can also trigger drop. Steady deep watering and light, regular feeding is the fix.
To check moisture properly, push a finger a few inches into the soil near the roots, and water deeply when it feels dry at that depth. A layer of mulch evens out moisture swings.
Long dry spells followed by heavy soaking often trigger bud loss. A deep soak less often beats a light daily sprinkle, because it encourages deeper roots. During a heat wave, check the soil more often than usual.
Yellowing shrubs often can’t get enough iron from the soil.26Gardening Know How. Yellowing Rose Of Sharon Leaves – Why Rose Of Sharon Has Yellow Leaves Leaves with yellow tissue but green veins are the classic sign, especially in alkaline soil. It is usually corrected by applying iron chelate, a form of iron plants can absorb, following label directions.27Gardening Know How. Yellowing Rose Of Sharon Leaves – Why Rose Of Sharon Has Yellow Leaves
Root Problems from Nematodes
Root-knot nematodes are tiny soil worms that cause roots to form swollen knots.28Wisconsin Horticulture. Root-Knot Nematode The roots can’t move water and nutrients properly, so plants may be stunted, wilted and yellow, like a nutrient shortage.29Wisconsin Horticulture. Root-Knot Nematode30UC Statewide IPM Program. Nematodes / Home and Landscape / UC Statewide IPM Program The knots can also crack open and let soil disease organisms in.31UC Statewide IPM Program. Nematodes / Home and Landscape / UC Statewide IPM Program
Damaged roots can’t take up what the soil holds, so adding fertilizer may not make the leaves any greener. Symptoms often show up in patches or on part of the shrub first, and may worsen in hot, dry weather when water demand is highest.
Suspect the roots if yellowing persists after you’ve fixed water and iron. Gently dig at the edge of the root zone and look for knobby swellings. Your local extension service can test a soil sample. Meanwhile, keep the shrub well watered and mulched to reduce stress.
Is It Dead or Just Winter Damaged
Rose of Sharon leafs out late in spring, so bare branches in April are often normal. Wait until late spring before judging the shrub. Then lightly scratch a branch about halfway down to look for green under the bark.32Ask Extension. Winter Damaged Rose Of Sharon
Green and moist means the branch is alive and may only have winter injury.33Ask Extension. Winter Damaged Rose Of Sharon Brown and dry means that part is dead, so prune down to where you see green.34Ask Extension. Winter Damaged Rose Of Sharon Test at several heights and on several branches, because a branch can be dead at the tips and alive near the base, and damage is often partial. Shrubs cut back hard usually regrow from living wood and still flower that summer.
Keeping Rose of Sharon Disease Free
Full sun is best, since too much shade reduces flowers and encourages fungi. Thinning branches each late winter guards against powdery mildew, gray mold and rust together.
Remove crossing or crowded branches so light and air reach the interior. Cut back to a main stem or to an outward-facing bud rather than leaving stubs. Doing it in late winter, while the shrub is bare, makes it easy to see which branches crowd the center.
A simple yearly routine covers the rest:
- Late winter: thin crowded and crossing branches.
- Spring: check for winter damage and aphids on new growth.
- Summer: water deeply at the base, never overhead in the evening.
- Fall: rake up fallen leaves and petals.