The Rose of Sharon belongs to the plant species Hibiscus syriacus and is a medium-size, treelike shrub that typically grows 9 to 12 feet tall and 6 to 10 feet wide. Rose of Sharon shrubs are beloved for their large, showy flowers that bloom during late summer in a variety of colours and double or single blossoms. The Rose of Sharon is susceptible to several different diseases, including fungal and bacterial leaf spots, grey mould, root and crown rots, and hibiscus chlorotic ringspot virus. Diagnose the disease before attempting treatment.
- Skill level:
- Moderately Easy
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Things you need
- Pruning shears or loppers
- Bleach or alcohol-based solution
Diagnose fungal leaf spots by looking for small to prominent dots or raised spots on the Rose of Sharon's leaves. The leaves might have yellowish or brownish spots on the surface, and the leaves may fall off.
Identify bacterial leaf spots or blight caused by the bacterium Pseudomonas syringae by inspecting the Rose of Sharon's leaves for large, irregular or angular spots on the leaves that are brown to black in colour. The Rose of Sharon may have stem-tip and blossom dieback, as well as blackening leaf veins and stem cankers. Bacterial blight typically occurs during wet, rainy weather in spring.
Look for wilted foliage and discoloured, stunted leaves on the Rose of Sharon to diagnose root and crown rots caused by Phytophthora species fungal diseases. The leaves might drop, and a streak, stain or canker may develop on the main stems or branches. The infected bark areas might ooze a reddish-black sap as well.
Check for discoloured or spotted flowers to identify the fungal disease grey mould, also known as Botrytis blight. Gray mould occurs during high humidity, typically causing the flower buds to rot, the leaves and shoots to wilt and drop, and the twigs to die back.
Diagnose hibiscus chlorotic ringspot virus by looking for stunted and distorted leaves that have streaked or spotted discolouration. The discolouration on the leaves will appear chlorotic, meaning pale-coloured or yellowish. Hibiscus chlorotic ringspot virus also causes round, ringed spots on the leaves.
Diagnose the Disease
Pick off and discard or destroy all leaves infected by bacterial and fungal leaf spot. Reduce excessive moisture on the foliage by watering early in the morning, avoiding the use of overhead sprinklers. Fungal leaf spot diseases rarely require treatments with fungicides.
Prune away and discard all leaves, branches and stems that display symptoms of bacterial leaf spot and bacterial blight. Make the cuts with clean, sterile pruning shears or loppers. Prune the Rose of Sharon when conditions are dry to prevent spreading the disease, and avoid over-fertilising the shrub. If cankers appear on the main stems or trunk, the shrub probably will die and have to be removed.
Cut away all branches infected by root and crown rot. Cut back any branches that have cankers to help control the spread of the rot disease.
Prune away and discard all dead or dying leaves, stems, twigs, flowers and buds to treat grey mould. Clean up and discard all fallen leaves and flowers. Prune the shrub's branches to thin out the top canopy to improve air circulation, and don't use overhead watering.
Water, prune and fertilise your Rose of Sharon properly to help improve the shrub's health and vigour if it has hibiscus chlorotic virus. No curative treatment exists for this virus, so the Rose of Sharon might need to be removed if the infection begins to severely affect the shrub's growth and appearance.
Tips and warnings
- Another disease that can affect Rose of Sharon shrubs is Armillaria root rot, which causes similar symptoms as other root and crown rots. Armillaria root rot also causes branch die back around the top of the Rose of Sharon, mushroom clusters at the shrub's base and black or reddish-brown "shoestring" spore structures around the shrub's roots. There is no cure for Armillaria root rot.
- Don't confuse grey mould or bacterial blight with poor cultural care, all of which can cause bud drop on your Rose of Sharon. If your Rose of Sharon isn't displaying any other symptoms other than bud drop, the shrub could be suffering from under-watering or over-fertilisation.
- When pruning your diseased Rose of Sharon, disinfect your pruning tools before and after use by dipping them in a bleach or alcohol and water solution to prevent spreading the diseases to your other landscape plants, trees and shrubs.
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