Hydrangea

Seasonal Pest & Disease Calendar

Pest & Disease Guide (Homeowner-Friendly)

1. Aphids

Symptoms

• Clusters of soft-bodied insects on new growth
• Curled/distorted leaves; sticky honeydew (sometimes ants)

Treatment

• Strong water spray knocks them off
• Insecticidal soap/horticultural oil if persistent
• Usually cosmetic unless new growth is heavily distorted

2. Spider Mites

Symptoms

• Fine yellow stippling/speckling on leaves
• Bronzing/dull look; leaf drop in heat
• Sometimes fine webbing (heavy infestations)

Treatment

• Hose undersides of leaves during hot spells (reduces mites + dust)
• Reduce drought stress (deep, consistent watering + mulch)
• Miticides only if damage is spreading and mites confirmed

3. Powdery Mildew

Symptoms

• White/gray “powder” on leaf surfaces
• More common mid–late summer with warm days + cool nights

Treatment

• Improve airflow (thin/prune, don’t crowd)
• Avoid excess nitrogen (pushes tender growth)
• Fungicides optional—often cosmetic on established plants

4. Leaf Spot (Cercospora/Leaf Spot Complex)

Symptoms

• Round to irregular purple/brown spots
• Yellowing and premature leaf drop late season

Treatment

• Rake/remove fallen leaves (key)
• Avoid overhead watering; water at base
• Fungicides rarely needed for homeowners unless severe yearly defoliation

5. Botrytis Blight (Flower/Leaf Blight)

Symptoms

• Brown, water-soaked blotches on petals or leaves
• Flowers “melt” or brown rapidly during cool/wet weather

Treatment

• Remove spent blooms and blighted flowers promptly
• Improve airflow; avoid wetting blooms when watering
• Fungicide only if it’s a repeating issue during prolonged wet periods

(Generic blight photos vary widely by host; best field rule is: wet + cool + rapid browning = suspect Botrytis, then sanitize.)

6. Leaf Scorch / Drought Stress

Symptoms

• Browning/crispy leaf edges or tips
• Wilting during heat; foliage may look “burned”

Treatment

• Deep, consistent watering (not frequent light watering)
• Mulch 2–3" (keep off stems)
• Afternoon shade helps a lot on hot exposures

7. Winter Dieback / Bud Kill

Symptoms

• Dead stem tips in spring; delayed leaf-out
• Bigleaf types may leaf out but not flower if buds were killed

Treatment

• Wait to prune until spring green-up (so you don’t remove live wood)
• Prune dead stems back to healthy buds/wood
• Winter watering during warm/dry stretches; mulch roots

Treatment Timing Calendar – Hydrangea (Colorado Front Range)

March–Early April ( )

• Check for winter dieback; wait to prune until you see bud swell
• Water during warm/dry spells if soil is dry

Late April–May ( )

• Watch new growth for aphids
• Light thinning for airflow (don’t shear)
• Treat aphids only if distortion builds

June ( )

• Keep moisture consistent as heat ramps up
• Begin scouting for early leaf spot and mite stippling

July–August ( )

• Deep watering + mulch (critical)
• Watch for spider mites, powdery mildew, scorch
• Treat mites early if stippling spreads; mildew sprays optional

September–October ( )

• Rake/remove spotted leaves; discard diseased debris
• Deep watering before soil freeze

Winter ( )

• Winter water during warm spells (especially exposed sites)
• No pesticide treatments needed

When to Worry vs. When It’s Cosmetic

When to Worry (Action Needed)

• Repeated severe spider mite damage with defoliation
• Major early leaf drop from leaf spot year after year
• Significant winter dieback (large sections not leafing out)

Mostly Cosmetic (Monitor Only)

• Light aphids with minimal distortion
• Mild powdery mildew late summer
• Small amount of late-season leaf spotting