What Is Powdery Mildew?
Powdery mildew is the most common plant fungal disease in grow rooms and greenhouses. It presents as a white, talc-like film on leaves and stems and thrives in warm, humid air with dry root zones. Many fungi cause “powdery mildew,” but each species is host-specific—so infections on tomato won’t leap to cucumber (though the same environmental conditions favor both).
Powdery Mildew is not related to household bathroom mildew or downy mildew (a different group with similar looks but different biology).
Quick Symptoms Checklist
- White, dusty patches on upper leaf surfaces
- Progression to grey/black (sporulation → heavy spore load)
- Reduced vigour, off flavours/aromas, lower yields
- Worst in crowded canopies with stagnant, humid air
Why It Spreads in Hydroponics
- High humidity + warm temps (especially in summer)
- Dry media/root zones paired with humid air
- Dense plant spacing and under-canopy shade
- Incoming plant material (cuttings, seeds) without quarantine
- Air intake without fine filtration
Prevention: Your First Line of Defence
- Quarantine all new plants 2–3 weeks; stimulate growth so hidden infections reveal themselves.
- Air management: strong extraction, active circulation, and canopy thinning to prevent humid pockets.
- Humidity control: run your target VPD; avoid humid nights after warm days.
- Irrigation balance: don’t let media swing too dry while the room stays humid.
- Resistant genetics: where available (some crops/varieties have bred resistance; keep notes as resistance can erode over time).
- Sanitation: clean tools, benches, and walls; HEPA/fine filters on intakes; separate mother/prop rooms.
Treatment That Works (Integrated Approach)
1) Activate Plant Defences (Biofungicide)
- Chitosan: a bio-fungicide that triggers plant immune response against biotrophic fungi like powdery mildew.
- Benefits: food-grade, repeatable applications, usable close to harvest, no 30-day downtime typical of conventional fungicides.
- Apply as a thorough foliar spray (both sides of leaves); follow label for dose and interval.
2) Environmental Tweaks
- Drop leaf wetness periods after lights off; improve dehumidification.
- Increase airflow through the canopy, not just above it.
- Avoid over-crowding or heavy under-canopy shading.
3) Cultural & Operational
- Scout weekly with a bright headlamp; isolate hot spots fast.
- Rotate benches/rows so everything gets light and air.
- Seed hygiene/sterilisation when applicable.
4) What Not to Do
- Don’t rely on copper (e.g., Bordeaux mix). It’s a heavy metal, residual, soil-biota-toxic, and undesirable on edible crops.
- Be sceptical of “miracle kitchen cures.” Many studies omit a water-sprayed control; simply spraying water can transiently reduce PM by forcing spore germination and wash-off—so results can be misleading.
Step-By-Step Action Plan (When You See PM)
- Isolate & mark affected plants/sections.
- Prune worst leaves (bag and remove from site).
- Deep clean room surfaces; change pre-filters.
- Apply chitosan foliar per label (full coverage).
- Tighten climate: hit target VPD, increase air changes, thin canopy.
- Re-scout in 3–5 days; repeat applications/cleaning until no new growth shows PM.
Vertical Farms & PM
Even highly sealed vertical farms can see PM due to stacked, humid microclimates and massive plant surface area. Assume it’s possible; engineer filtration, airflow, and dehumidification accordingly and keep a preventive foliar schedule.
Article by Dr Russell Sharp
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