Powdery mildew in indoor cultivation and downy mildew in indoor cultivation are fungal problems capable of spreading quickly when several factors occur together: high humidity, insufficient air exchange, condensation, and poor cleanliness. Although these two diseases may be confused during the early stages, each one presents different signs that should be identified as quickly as possible.
This article from the Hydroponics Blanes Blog is published for informational purposes only. At Hydroponics Blanes, we DO NOT sell marijuana containing THC. In our shop, you will find everything needed for self-cultivation, including fertilisers, nutrients, environmental measuring instruments, lighting systems for indoor cultivation, ventilation, extraction and climate-control equipment, as well as products intended for plant care.
What are powdery mildew and downy mildew?
Powdery mildew and downy mildew belong to a broad group of phytopathogenic organisms responsible for plant diseases. At first glance, they may appear similar, but the appearance of the leaves, the area where fungal growth develops, and the environmental conditions that favour each infection allow an initial identification to be made.
Fungi on indoor plants do not appear only because of poor cleanliness. Inappropriate relative humidity, sudden temperature changes, wet leaves, irregular ventilation, excessive plant density, or contaminated materials may also encourage spore germination and dispersal.
Detecting the problem during the early stages makes it possible to remove affected areas, correct environmental conditions, and choose a more suitable treatment against powdery mildew and downy mildew. When the infection reaches stems, young shoots, or flowers, control becomes much more difficult.
Differences between powdery mildew and downy mildew
Understanding the differences between powdery mildew and downy mildew is essential to avoid applying an unsuitable treatment. The area where the fungus normally appears provides an important clue, although it is always advisable to inspect both sides of every leaf.
How to recognise powdery mildew
Powdery mildew usually appears as a white or light-grey powder resembling flour or ash. The first colonies may appear as small circular spots that gradually increase in size until they cover a significant part of the leaf surface.
In many species, growth initially appears on the upper side of the leaf. However, some species may develop on the underside, which means that a superficial inspection does not always allow the infection to be detected in time.
The main symptoms of powdery mildew and downy mildew associated with powdery mildew include:
- White or greyish powder on the leaves.
- Spots that gradually increase in size.
- Yellowing, deformed, or weakened leaves.
- Slower growth.
- Spread of the fungus towards petioles, stems, and young shoots.
Powdery mildew can develop under mild or warm temperatures and moderate relative humidity. It does not require a permanent film of water to spread, although condensation, dew, and environmental fluctuations may encourage spore germination.
How to recognise downy mildew
Downy mildew usually begins with yellowish, translucent, or oily-looking spots on the upper side of the leaves. Grey, whitish, or slightly purplish growth may appear on the underside. As the infection progresses, the spots turn brown and the tissues eventually become necrotic.
The most common signs of downy mildew include:
- Yellow or translucent spots on the upper side of the leaf.
- Greyish or purplish growth on the underside.
- Brown or black areas in advanced infections.
- A mosaic-like appearance on the leaf surface.
- Brittle, dry, or drooping leaves.
Downy mildew requires high humidity levels and often appears when leaves remain wet for too long. Night-time condensation, watering over the foliage, an insufficient extractor, or a sharp drop in temperature during the dark period increase the risk.
Quick table for identifying the type of fungus
| Type of fungus | Typical colour | Area of appearance | Spots on the upper side | Can it be removed by rubbing? |
|---|---|---|---|---|
| Leveillula taurica | White | Mainly on the underside | Not always | No, marks may remain |
| Sphaerotheca macularis | White | Mainly on the upper side | Not always | It may partially come off |
| Pseudoperonospora cannabina | Greyish | Mainly on the underside | Yes, yellow or necrotic | No, stains may remain |
This table serves as a visual guide, but a definitive identification may require specialist analysis. It is also advisable to rule out mineral residues, dust, spray deposits, or damage caused by pests before starting any treatment.
How a fungal infection develops
Spores may enter the growing area through clothing, tools, cuttings, outdoor air, plant debris, contaminated substrate, or contact with infected plants. When they encounter favourable humidity and temperature conditions, they germinate on the leaf and begin feeding on plant tissue.
Once established, the fungus produces new spores capable of moving throughout the growing area. Fans can help prevent pockets of humidity, but a strong, direct airflow aimed at an infected plant may also disperse spores. For this reason, before handling an affected area, it is preferable to temporarily switch off nearby ventilation and remove the material carefully.
Some infections may remain invisible for several days before showing clear signs. When white powder or grey growth becomes visible, other leaves may already be infected without yet displaying external symptoms.
How to prevent fungi in indoor cultivation
The best strategy to prevent fungi in indoor cultivation is to maintain a stable, clean, and properly ventilated environment. An advanced infection may be difficult to control, even with authorised fungicides.
Humidity control in indoor cultivation
Humidity control in indoor cultivation should be carried out 24 hours a day, not only when the lighting is switched on. Humidity often rises during the dark period because the temperature drops while plants continue releasing water vapour.
A thermo-hygrometer with maximum and minimum value memory makes it possible to detect night-time peaks that might go unnoticed during a daytime inspection. During the most sensitive stages, prolonged levels above 70% should be avoided, although the correct range depends on the development stage, temperature, plant density, and air movement.
When the extractor cannot maintain stable levels, a dehumidifier can help remove excess moisture from the air. The unit should have a suitable capacity for the volume of the area and the number of plants.
Air exchange and circulation
The extraction system should continuously renew the air and remove heat and humidity. It is also advisable to install oscillating fans to circulate air between the plants and prevent stagnant areas.
Air should circulate above, below, and throughout the plant canopy without permanently directing an aggressive current towards the same point. A climate controller allows the extractor and air intake system to be regulated according to temperature and humidity.
Avoiding condensation and sudden fluctuations
A rapid drop in temperature can bring the air to the dew point and cause water to form on leaves, walls, or ducts. To reduce this risk, it is advisable to maintain a moderate difference between the lighting period and the dark period.
Cold surfaces, air inlets, and areas near the floor should also be checked. If condensation appears, it will be necessary to improve extraction, increase air circulation, or use a dehumidifier.
Cleaning the growing area
Cleaning helps reduce the number of spores present. Fallen leaves, root remains, spilled substrate, and accumulated water should be removed. Scissors, trays, and tools should be cleaned before moving from one plant to another.
Clothing used in outdoor gardens may carry spores. To reduce the risk, it is practical to wear clean clothing inside the growing area and wash hands before handling leaves, branches, or equipment.
Spacing and plant density
Plants positioned too close together create pockets of humidity and make inspection more difficult. Selective pruning of lower branches and damaged leaves can improve air circulation, provided that clean tools are used.
Removing an excessive amount of foliage at once is not advisable. An overly intensive intervention may cause stress and reduce the plant’s ability to recover.
How to eliminate powdery mildew from plants
To understand how to eliminate powdery mildew from plants, it is first necessary to assess the extent of the infection. A localised spot can be controlled more easily than a white layer spread across leaves, stems, and flowers.
The first step is to isolate the affected plant whenever possible. Next, remove leaves showing visible colonies and place them directly into a sealed bag. Do not shake them or allow them to fall onto the floor.
After removing affected parts, clean the tools and hands. Humidity, temperature, and ventilation must also be corrected. Otherwise, the fungus may return even when the first treatment appears effective.
Solutions used against powdery mildew include potassium bicarbonate, certain microbiological products, and fungicides authorised for each type of cultivation. Potassium bicarbonate changes conditions on the leaf surface and may limit mycelium development.
The dose should always follow the manufacturer’s instructions. An excessive concentration may burn leaves or damage the crop. Before treating the entire area, apply the product to a small section and observe the reaction for 24 to 48 hours.
How to combat downy mildew in indoor cultivation
To understand how to combat downy mildew in indoor cultivation, it is important to remember that this problem is closely related to wet leaves, high humidity, and condensation.
Remove affected leaves, reduce humidity immediately, and increase air exchange. Inspect the underside of all nearby leaves, as greyish growth may remain hidden when observation is carried out only from above.
Preventive treatments based on copper and other fungicides may be authorised for certain crops, but not every product is suitable for every plant or growth stage. Always read the label and follow the dose, maximum number of applications, and safety interval.
Do not mix products without first checking compatibility. Certain combinations may cause phytotoxicity, especially when oils, sulphur, soaps, or alkaline products are applied with insufficient time between treatments.
Treatment against powdery mildew and downy mildew step by step
- Stop unnecessary handling of the plants.
- Examine the upper and lower sides of the leaves with a magnifying glass.
- Isolate affected plants whenever possible.
- Temporarily switch off the nearby fan before removing leaves.
- Place contaminated material in a sealed bag.
- Clean scissors, hands, trays, and surfaces.
- Measure humidity and temperature during light and dark periods.
- Correct extraction, ventilation, and condensation problems.
- Apply only an authorised product, following the label instructions.
- Inspect the crop again over the following days.
A treatment against powdery mildew and downy mildew should not be limited to simple spraying. The most important stage is removing the environmental conditions that allowed the fungus to develop.
Recommended products for preventing and controlling fungi
At Hydroponics Blanes, we recommend preparing the growing area with equipment that helps anticipate environmental problems:
- Digital thermo-hygrometer: measures temperature, humidity, and maximum or minimum values.
- Climate controller: automates the extractor, air intake, and other environmental equipment.
- Air extractor: removes humidity, heat, and stale air.
- Oscillating fans: prevent pockets of stagnant air between plants.
- Dehumidifier: reduces humidity peaks during the dark period.
- Air inlet filters: reduce dust, insects, and particles coming from outside.
- Pruning scissors: make it easier to remove affected leaves precisely.
- Cleaning and disinfection products: help keep tools and surfaces in good condition.
- Authorised fungicides: should be selected according to the type of fungus, plant species, and cultivation stage.
- You can view the products available in our online shop: PEST CONTROL
Before purchasing a treatment, check the composition, authorisation for use, and compatibility with the development stage. During advanced stages, it is especially important to avoid residues on flowers and respect the safety interval indicated by the manufacturer.
Common mistakes when combating powdery mildew and downy mildew
One of the most common mistakes is treating only the visible spots. The fungus may remain present on other leaves or reappear if humidity stays high.
Another common mistake is significantly increasing fan power without first removing infected leaves. This action may spread spores throughout the entire area. Mixing homemade products without measuring the quantities correctly is also risky.
Applying more than the recommended quantity does not improve control. An excessive dose may cause burns, stop growth, or result in irreversible damage. Spraying while the lighting is switched on or when temperatures are high is also not advisable.
When to remove an affected plant
When the infection covers a large part of the canopy, reaches the stems, or appears on the flowers, keeping the plant may put the rest of the crop at risk. In this situation, removing the affected specimen may be the safest decision.
Infected material should not be added to household compost or left near the growing area. It should be placed in a sealed bag and disposed of in accordance with local regulations.
Monitoring after treatment
After cleaning and applying the chosen treatment, inspect the plants every day for at least one week. Pay particular attention to older leaves, inner areas, lower sections, and places where air circulation is limited.
Record temperature and humidity to check for any peaks. An infection that returns after a few days generally indicates that some contaminated areas were not removed or that the environment remains favourable to the fungus.
Early detection, consistent hygiene, and a stable environment provide the best defence against powdery mildew in indoor cultivation and downy mildew in indoor cultivation. Good planning, climate-control equipment, and regular inspections can considerably reduce the risk of these fungi damaging plants.
At Hydroponics Blanes, you will find products for ventilation, extraction, environmental measurement, dehumidification, nutrition, and preventive crop care. Our team can help you choose equipment suited to the dimensions of the area and the existing environmental conditions.