Understanding Fungus on Crepe Myrtle Bark
Crepe myrtles (Lagerstroemia indica and hybrids) are beloved in Australian gardens for their vibrant summer blooms, striking autumn colour and attractive peeling bark. However, fungus on crepe myrtle bark can mar their beauty and health. This black, sooty coating or fuzzy growth often appears on trunks and branches, especially in humid subtropical regions like Queensland and northern New South Wales.
While not always fatal, untreated fungal issues weaken trees, reduce flowering and invite secondary pests. In Australia’s diverse climates—from hot, dry inland areas to coastal humidity—good management is key. This guide covers identification, causes, prevention and treatment tailored to local conditions.
Common Types of Fungi on Crepe Myrtle Bark
Sooty Mould (Most Prevalent)
Sooty mould, caused by fungi like Capnodium spp. or Aureobasidium spp., is the most common culprit. It forms a black, velvety layer on bark, stems and leaves. Importantly, sooty mould doesn’t directly infect the plant; it grows on honeydew—a sticky excretion from sap-sucking insects like aphids, scale or whiteflies.
In humid Australian summers (25–35°C with high rainfall), honeydew accumulates, providing the perfect medium. You’ll see it most on young shoots or stressed trees.
Bark Cankers and Blight
Less common but serious are fungal cankers from Nattrassia mangiferae (formerly Neofusicoccum mangiferae) or Botryosphaeria spp. These cause sunken, discoloured lesions on bark, often with gum oozing. In cooler, wetter winters (e.g., parts of Victoria or Tasmania), these fungi enter through wounds.
Cercospora Leaf Spot (Spreading to Bark)
Cercospora lythracearum primarily affects leaves but can extend to bark in prolonged wet spells, causing purple-black spots that coalesce into a fungal mat.
Symptoms of Fungus on Crepe Myrtle Bark
Spotting issues early prevents spread:
- Visual signs: Black sooty coating (easily wiped off initially), white-grey powdery growth or cracked, oozing bark.
- Tree health indicators: Yellowing leaves, premature leaf drop, sparse blooms or dieback from branch tips.
- Seasonal patterns: Worse after wet summers or in shaded, poorly ventilated spots.
- Texture test: Scrape gently—healthy bark is smooth/corky underneath; fungal bark feels slimy or powdery.
In arid zones like inland NSW or SA, symptoms are rarer but appear post-irrigation stress.
(Sooty mould on crepe myrtle bark) Typical sooty mould on Lagerstroemia bark in a Brisbane garden.
Causes and Risk Factors in Australian Climates
Crepe myrtles prefer full sun, well-drained soil and 800–1200 mm annual rainfall. Fungi thrive when conditions deviate:
- High humidity and poor airflow: Coastal QLD/NSW (e.g., Gold Coast, Sydney) with 70–90% relative humidity promotes mould.
- Insect infestations: Aphids explode in spring warmth (October–December), coating bark in honeydew.
- Overhead watering: Sprinklers splash spores; use drip irrigation instead.
- Stress factors: Drought (common in VIC/SA summers), root compaction in clay soils or nutrient imbalance (excess nitrogen).
- Planting errors: Crowded trees or shade reduce drying time.
Climate data from the Bureau of Meteorology shows fungal peaks during La Niña wet years.
Prevention Strategies for Healthy Crepe Myrtles
Prevention beats cure. Implement these Aussie-friendly practices:
Site and Soil Selection
- Plant in full sun (6+ hours daily) with 4–5 m spacing.
- Use sandy loam or improved clay; pH 5.5–7.5. Add gypsum (2–3 kg/m²) for heavy soils.
Cultural Care
- Prune correctly: Thin crowded branches in winter (June–August) to improve airflow. Remove suckers at base.
- Water wisely: Deep water (20–30 L per tree weekly in dry spells); avoid wetting foliage.
- Mulch: 5–7 cm organic layer (e.g., lucerne hay) keeps roots cool/moist, suppresses weeds.
Insect Control (Key to Sooty Mould)
- Monitor for aphids/scale in spring; blast with hose or use eco-oil (1:50 dilution).
- Encourage predators like ladybirds with companion plants (e.g., dill, lavender).
Fertilising
- Apply balanced NPK (e.g., 10-10-10) at 50 g/m² in spring. Avoid high-nitrogen feeds that boost soft growth.
Treatment Options for Fungus on Crepe Myrtle Bark
Act promptly but avoid overkill—crepe myrtles are resilient.
Step 1: Remove Sooty Mould
- Manual cleaning: Hose off loose mould with high-pressure (not damaging bark). For stubborn patches, wipe with 1:10 vinegar-water solution (safe, organic).
- Dry thoroughly; repeat weekly.
Step 2: Treat Underlying Insects
- Organic: Neem oil or insecticidal soap (weekly sprays for 3 weeks).
- Chemical: Systemic imidacloprid (e.g., Confidor) for heavy scale—apply as soil drench per label (APVMA-approved).
Step 3: Fungicides (If Needed)
Use sparingly; resistance is a risk.
| Fungus Type | Recommended Treatment | Application Timing |
|---|---|---|
| Sooty Mould | Control insects first; potassium bicarbonate spray | After rain, repeat 10–14 days |
| Cankers | Copper oxychloride (e.g., 20 g/L) | Prune then spray wounds |
| Cercospora | Mancozeb or chlorothalonil | 2–3 sprays, 14-day intervals |
Follow label rates; withhold sprays during bloom to protect bees. In organic gardens, try sulphur-based products.
Pruning Infected Parts
- Sterilise secateurs (70% alcohol) between cuts.
- Remove >20 cm diseased branches; dispose in council green waste (don’t compost).
(Pruning crepe myrtle) Prune to open the canopy for better airflow.
Recovery and Long-Term Care
Post-treatment:
- Monitor weekly; new growth signals success.
- Boost vigour with seaweed tonic (diluted per label) monthly.
- In cooler climates (Melbourne winters), protect trunks with whitewash paint to prevent frost cracks inviting fungi.
Expect full recovery in 1–2 seasons if addressed early. Severely affected trees (>50% bark) may need replacement—choose resistant varieties like ‘Natchez’ or ‘Muskogee’ for humid areas.
Varieties Resistant to Fungal Issues
- ‘Sioux’: Compact, good for pots; tolerant of humidity.
- ‘Biloxi’: Pink blooms, strong bark health in QLD.
- ‘Fantasy’ series: Bred for Australian conditions, mildew-resistant.
Source from local nurseries like Plantmark or Bunnings.
When to Call a Professional
Seek an arborist if:
- Tree >5 m tall or dieback >30%.
- Suspect root rot (wilting despite water).
- In bushfire-prone areas, check for post-fire fungi.
Contact Accredited Arborists Australia for certified help.
FAQs on Fungus on Crepe Myrtle Bark
Is sooty mould harmful to my crepe myrtle?
No, but it blocks photosynthesis and signals pests.
Can I use bleach on bark?
Avoid—harms beneficial microbes.
How long until blooms return?
4–6 weeks post-treatment in ideal conditions.
Does climate change worsen this?
Yes—more humid extremes favour fungi per CSIRO reports.
By staying vigilant, your crepe myrtles will thrive through Australia’s challenging weather. Happy gardening!
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