Hydroponics knowledge

Pests and diseases in hydroponics

Spider mites, thrips, aphids, powdery mildew, blossom end rot and root rot: how to recognise them, how I prevent them and what really helps for me. With beneficial insects, water and fresh air, plus an honest assessment of all the well-known home remedies.

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Look early

check briefly every day, under the leaves too

🕷️

Predatory mites

eat spider mites and thrips, harmless to you

💧

Spray in the morning

leaves dry off again by the evening

🌬️

Move the air

a fan and airing keep powdery mildew away

Hygiene in hydroponics: keeping pests away

Pests and diseases are also in hydroponics a challenge, even though growing without soil reduces some problems. In closed systems, pests and diseases can spread quickly if they are not recognised and tackled in time. Insects such as aphids and spider mites and diseases such as root rot are especially common.

An important aspect of successful hydroponics is therefore regular monitoring and care of the plants, in order to spot infections early and take suitable countermeasures. Preventive measures such as cleaning the system and using beneficial insects such as predatory mites help avoid many problems.

No panic: most problems announce themselves

Without soil, many problems disappear. There are no slugs, no soil pests and no soil in which diseases persist for years. But even a tower is not completely free of pests. Spider mites, thrips, aphids and powdery mildew occur in hydroponics just as they do in a garden bed.

The good news: if you look early, you get them under control quickly. That is why in summer a quick look at the leaves is part of my daily routine, just like water level, water temperature, pump and air stone. I also turn over a few leaves, because most pests sit on the underside.

My daily round in summer

Check water level and water temperature (18 to 24 °C), are the pump and air stone running, do the leaves look healthy, and is anything sitting under the leaves? A few minutes a day are enough.

Why prevention is the key to success

Various pests and diseases can occur in hydroponic systems that affect the growth and health of the plants. It is important to recognise them in time and take suitable measures for prevention and control. Here are some of the most common pests and diseases and ways to control them:

Contents:

Pests in hydroponics
Diseases in hydroponics
General prevention in hydroponics
My YouTube videos
FAQ

Home remedies, sprays and what helps for me

On this page you will also find the well-known home remedies for each pest, such as garlic brew, neem oil, nettle manure and baking powder, as well as products to buy. I give you an honest assessment of each: how well is the effect proven, and what do you need to watch out for? There are products, but they did not help me. I rely on predatory mites, hosing off, sticky traps and spraying in the morning. Important with every product you buy: dose according to the pack instructions, spray in the morning, never into the tower water, and only use products approved for your plants. Sprays, including neem oil, often hit the predatory mites too. If you use beneficial insects, leave out sprays.

📖I cover spider mites, thrips, powdery mildew, blossom end rot and root rot in detail in my book in the chapter “Probleme lösen” (Solving problems). Just check the table of contents.›

Pests in hydroponics

A quick overview so you know who you are dealing with:

What you see	Who it usually is
fine pale dots, later delicate webbing	Spider mites
silvery shiny patches with small black dots	Thrips
small green or black insects on shoot tips, sticky leaves	Aphids
small white flies that fly up when touched	Whiteflies
small round holes in cabbage leaves, beetles jump away	Flea beetles

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Cabbage white

Butterflies whose caterpillars attack brassicas

Outdoors, the net.

🌿Beneficial insects and home remedies for control:

▸Stretch fine-mesh nets with a maximum mesh size of 0.8 mm over the plants
▸Collect eggs and caterpillars by hand twice a week
▸Use parasitic wasps (Trichogramma evanescens), quantity according to the pack, often 2 to 3 cards per square metre. They lay their eggs in the butterfly eggs, so use them early, as soon as the first butterflies are flying.
▸Nettle manure: ferment 1 kg of nettles in 10 L of water for 10 to 14 days, dilute 1:10. Fermented manure is mainly a fertiliser for plants in soil; its effect against caterpillars is not proven. Against aphids people tend to use a fresh cold-water extract that steeps for only 12 to 24 hours. Neither belongs in the tower water.
▸Garlic brew: boil 200 g of garlic in 1 L of water, dilute with 5 L of water. It acts more as a deterrent; the effect is only weakly proven.

🛡️How I do it

Cabbage whites and flea beetles love kohlrabi and all brassicas. Against cabbage whites I screw a ring 40 cm in diameter onto the lid of my tower and stretch a fine-mesh net with a zip over it. That keeps the butterfly away, and it does not lay eggs on the leaves.

Flea beetles are more stubborn. They are tiny beetles that eat small holes in the leaves. You recognise them by their jumping: tap the leaf and they hop away. They crawl through every gap, even under the finest net. It is worst when it is hot and dry. Chinese cabbage and mustard greens are hit particularly hard for me; kohlrabi gets through better. That is why I plant brassicas very early in spring or only from August. When it turns cool and rainy, they leave you in peace.

I show you more about the net against cabbage whites in my video Preventing cabbage whites.

Products you can buy for control:

My assessment: Neem oil contains the active ingredient azadirachtin. It disrupts caterpillars and sucking insects when feeding and moulting, which is well studied. Follow the pack instructions and leave it out if you use beneficial insects.

Fine-mesh insect protection net 2.30 x 4.25 m

Durable construction: handled with care, the insect net can be used for years. Simply remove any dirt with soap and water. Fine net against cabbage whites and other pests. Can be cut to size and stretched over a ring on the tower.

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Angebot

Neem oil with emulsifier 1000 ml

Neem oil is obtained by gentle cold pressing of the seeds of the fruit of the Indian neem tree. Dose according to the pack, only spray in the morning, never into the tower water.

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Whiteflies

Small white insects that stay on the undersides of leaves

Yellow sticky traps show you who is there.

🌿Beneficial insects and home remedies for control:

▸Hang yellow sticky traps near the plants and replace them regularly
▸Use parasitic wasps (Encarsia formosa or Eretmocerus) and predatory bugs (Macrolophus) as natural enemies. Ladybirds, on the other hand, mainly eat aphids.
▸Garlic brew: boil 200 g of garlic in 1 L of water, dilute with 5 L of water
▸Neem oil: mix 5 ml with 1 L of water and washing-up liquid, spray on the undersides of leaves. Dose ready-mixed neem oil according to the pack, and only spray in the morning.

🛡️How to recognise them

Whiteflies sit on the underside of young leaves and fly up in a little cloud when touched. They excrete sticky honeydew, on which a black sooty mould later grows. Yellow sticky traps just above the plants show you early whether they are there. In the greenhouse the parasitic wasp Encarsia formosa helps. It works best if you use it early, while there are still few whiteflies.

Fungus gnats

Fungus gnats are small black flies that buzz over moist substrate. In a tower with foam cubes they have hardly anything to live on. If you start seedlings in coco, coco can attract fungus gnats. Then lay a yellow sticky trap flat directly over the substrate.

Sticky trap	mainly attracts	where to hang it
blue	Thrips	just above the plants
yellow	Whiteflies, fungus gnats, winged aphids	just above the plants, for fungus gnats flat over the substrate

Sticky traps are mainly an early warning system. They show you who is there and whether numbers are rising. Together with a look under the leaves you then know exactly what to do.

Products you can buy for control:

Large yellow sticky traps (15 pieces)

Yellow sticky traps against whiteflies and fungus gnats. Show you early whether something is going on.

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Parasitic wasps against whiteflies (5 cards)

Eretmocerus parasitic wasps on cards for hanging. Use early, while there are still few whiteflies. No sprays with them.

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Neem oil with emulsifier 1000 ml, with spray bottle

Ready-mixed with emulsifier. Dose according to the pack, only spray on the undersides of leaves in the morning.

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Spider mites

Tiny mites that appear in dry conditions

Spider mites and thrips: both love heat and dry air.

🌿Beneficial insects and home remedies for control:

▸Increase humidity by spraying regularly or with bowls of water. Only spray in the morning so the leaves are dry by the evening.
▸Use predatory mites (Phytoseiulus persimilis) as natural enemies
▸Do not place plants too close: a moister microclimate does help against spider mites, but if the plants are too close the air stagnates and powdery mildew comes. Spraying in the morning is the better way.
▸Neem oil: mix 5 ml with 1 L of water and washing-up liquid, repeat every 7 to 10 days. Not together with predatory mites, otherwise you hit the beneficials too.

🛡️How to recognise spider mites

Spider mites are so tiny that you can hardly see them with the naked eye. You recognise them by fine pale dots on the leaves. These appear where the mites suck out the cells. Later the leaves turn yellowish and pale, and with a heavy infestation you see delicate webbing, often on the young shoots. The mites themselves and their eggs sit on the underside of the leaf, often along the leaf veins. A magnifying glass helps with the search.

My cucumbers: an honest account

Cucumbers are demanding. I was able to harvest a few, but there is still work to do before my cucumber growing is really successful. It is not the hydroponics; fertiliser and values are right. The problems are spider mites and powdery mildew, and every gardener has those, in soil too.

On top of that, cucumbers like it more humid than tomatoes. If both are in the same greenhouse, as with me, the air is often too dry for the cucumbers. And exactly this dry warmth is the favourite climate of spider mites. At 35 to 38 °C in summer they have an easy time.

What I do about it

Spray in the morning: I spray tomatoes and cucumbers with water early in the morning. That raises humidity, and the leaves dry off again by the evening.
Never spray in the evening: wet leaves overnight attract fungi.
Use predatory mites: they eat spider mites and thrips, are harmless to plants and people and are simply sprinkled onto the leaves.
Start early: release the predatory mites as soon as you see the first pests. They work best when there are few pests.
No sprays with them: leave out sprays, as they would also kill the predatory mites.
Cut off and dispose of heavily infested leaves.
Against thrips, also hang up blue sticky traps.

I have had spider mites indoors too. I ordered predatory mites, and they ate them all up. That worked brilliantly.

Why spraying helps twice over

Spider mites multiply fastest in warm, dry air. The predatory mite against spider mites (Phytoseiulus persimilis), on the other hand, prefers it moist: below about 60 % humidity its eggs hatch poorly. Spraying in the morning therefore slows down the pests and helps the beneficials at the same time.

Beneficial	eats	good to know
Phytoseiulus persimilis	Spider mites	pure specialist: when there are no spider mites left, it disappears again
Neoseiulus californicus	Spider mites, also thrips	slower, but tolerates more heat and dryness
Amblyseius cucumeris and swirskii	young thrips larvae	best used preventively or very early

Products you can buy for control:

My assessment: Spruzit works with pyrethrins from chrysanthemums and kills pests on direct contact. But it also hits beneficials such as predatory mites and ladybirds. So only use it if you do not use beneficial insects, follow the pack instructions and waiting period, and never spray into the tower water.

Angebot

Predatory mites against spider mites (3 bags)

Predatory mites specifically against spider mites. For me, predatory mites completely ate up the spider mites indoors.

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Hand sprayer 1 litre

For spraying the leaves early in the morning to raise humidity.

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Humidifier 6 litres

Raises the humidity in the room when the heating air is very dry. Spider mites like it dry, predatory mites prefer it moist.

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Neudorff Spruzit pest-free 500 ml

Ready-to-use spray against sucking pests such as spider mites, aphids, thrips and whiteflies. Use according to the pack.

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A few questions about space, location and what you want to harvest, and you get a recommendation.

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Aphids

Small green or black insects that suck plant sap

Outdoors nature takes care of it, indoors the dip bath.

🌿Beneficial insects and home remedies for control:

▸A strong jet of water to wash off the aphids
▸Use ladybirds and lacewings as natural enemies
▸Garlic brew: boil 200 g of garlic, dilute with 5 L of water
▸Neem oil: 5 ml with 1 L of water and washing-up liquid, repeat every 7 to 10 days. Dose according to the pack, only in the morning.

🛡️Outdoors, beneficial insects help

Outdoors you usually do not need to do anything against aphids. Ladybirds and their larvae, lacewing larvae and hoverfly larvae eat aphids in large numbers. You can easily recognise the hoverfly: it hovers in the air like a little helicopter. A strong jet of water also washes many aphids off the plant, and they rarely find their way back. This too is best done in the morning so the leaves dry quickly.

Indoors: check and dip

Indoors there are no beneficial insects. Here you keep aphids in check well with regular checks. I dip small plants once a week into a 30 litre bucket of water. That works as long as they are not too big.

Pull the net pot with the plant out of the tower.
Dip the plant upside down into the bucket.
Shake it off well.
Let it dry in the air, then put it back into the tower.

Dipping once is not enough. Keep at it and dip regularly. Good air afterwards is important so the leaves dry quickly. A small fan helps with this.

What is outside stays outside

Do not bring plants from outdoors into the house. Aphids and other pests come in with them. Even my finest kohlrabi stay outside in autumn. If you bring in side shoots for winter tomatoes, look at them closely and rinse them thoroughly.

In a greenhouse or conservatory you can deliberately introduce ladybirds. The larvae are the most voracious aphid hunters.

I show you a recipe against aphids in my video Recipe for controlling aphids.

Products you can buy for control:

Ladybird eggs and larvae (100 to 150 pieces)

Live ladybird larvae against aphids, for greenhouse, conservatory and garden.

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Lacewing larvae against aphids (5 cards)

Lacewing eggs on cards; the larvae eat aphids in large numbers. Live beneficial insects, use straight after delivery.

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Neem oil with emulsifier 250 ml, with spray bottle

Small bottle to try out. The active ingredient azadirachtin disrupts sucking insects. Dose according to the pack, only in the morning.

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🦟

Thrips

Tiny insects that cause silvery patches on leaves

🌿Beneficial insects and home remedies for control:

▸Spray leaves regularly with water for higher humidity, ideally early in the morning
▸Use predatory bugs (Orius) or predatory mites (Amblyseius swirskii or cucumeris), quantity according to the pack. The 10 to 20 animals per m² mentioned here earlier only apply to predatory bugs; you need considerably more predatory mites.
▸Spray garlic brew on the undersides of leaves
▸Apply neem oil every 7 to 10 days, but not together with predatory mites

🛡️How to recognise thrips

Thrips are slender insects, about 1 to 2 mm long. They leave silvery shiny patches on the leaves, with small black dots in between. That is their droppings. They too usually sit on the underside of the leaves.

Against thrips, predatory mites and blue sticky traps help me. The soil predatory mite Hypoaspis miles, on the other hand, does not feed on the leaves. It lives in the substrate and catches fungus gnat larvae there and the pupae of thrips that pupate in the soil. In a tower with foam cubes you hardly need it; in coco or soil it can help.

Products you can buy for control:

My assessment: Sprays such as Spruzit or neem oil also kill predatory mites. So decide: either beneficials or sprays. I choose predatory mites.

Angebot

Predatory mite mix against thrips and spider mites

Live predatory mites to spread on the leaves. Use early, as soon as you see the first pests.

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Blue sticky traps against thrips (60 pieces)

Blue sticky traps attract thrips. That way you see early whether there are any, and catch some along the way.

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Soil predatory mite Hypoaspis miles

Predatory mite for the substrate: eats fungus gnat larvae and thrips pupae in coco or soil. It does not help on the leaves.

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Neem oil with emulsifier 1000 ml, with spray bottle

Ready-mixed. Dose according to the pack, only in the morning, not together with predatory mites.

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Angebot

Neudorff Spruzit pest-free 500 ml

Spray against thrips and other sucking pests. According to the pack, only without beneficial insects.

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📖Which pests turn up in the tower and what I do about them, I explain in my book in the chapter “Schädlinge im Tower” (Pests in the tower). Just check the table of contents.›

Diseases in hydroponics

🍂

Root rot (Pythium)

With an air stone it hardly ever occurs.

⚠️Symptoms

Yellow, wilting leaves and brown, slimy roots. Foul smell. Plants lose vitality.

✅Prevention:

•Air stone for continuous aeration of the nutrient solution
•Use clean water: tap water is fine; with very hard water use soft or reverse osmosis water. Distilled water is not necessary.
•Disinfect all components after every harvest
•Keep water temperature between 18 and 22 °C

🛠️Control:

•Remove and dispose of affected plants immediately if heavily infested
•Disinfect the system with 3% hydrogen peroxide solution, without plants, dose according to the pack
•Let it circulate for 30 minutes, then rinse thoroughly
•You do not need special fungicides for the nutrient solution in hobby growing; practically nothing is approved for that anyway. Air stone, cleaning and fresh nutrient solution help.

Healthy roots are white or pale and strong. Rotten roots are brown, soft, slimy and smell musty. The plant then often wilts even though there is enough water. A common cause is a water mould called Pythium. It mainly attacks roots that are already weakened by lack of oxygen or too much fertiliser.

The cause is usually water above 24 °C, light or dirt in the container, or too much fertiliser. Warm water holds less oxygen, and at the same time roots need more of it when it is warm. With an air stone I hardly had any problems even at 38 to 39 °C in my greenhouse.

How I go about it

Cut off brown roots. Lots of white roots are a good sign; never cut those off.
Clean the container.
Mix fresh nutrient solution: first EC, then pH.
Let the air stone run continuously; it does not go on the timer.
Provide plenty of air movement around the plants and put the tank in the shade.
Remove heavily infested plants completely so the pathogen does not spread in the water.

Water temperature	What happens
below 18 °C	The roots work more slowly, the plant grows slowly.
18 to 22 °C	ideal; up to 24 °C is still fine
from 25 °C	less oxygen, more stress for the roots, root rot becomes more likely

Products you can buy for control:

My assessment: Hydrogen peroxide breaks down quickly in water into water and oxygen. It kills germs, but in too high a quantity it also attacks the fine root hairs. That is why you clean the empty system with it and rinse thoroughly afterwards.

Angebot

AquaForte Hi-Flow V-30 air pump

Powerful air pump for the tank. Runs continuously for me, even on hot days.

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Solar air pump for outdoors

Oxygen pump with solar panel, for towers outdoors with no socket nearby.

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Pontec PondoAir Set 450

Complete aerator set with hose and air stones for larger containers.

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Continuous monitor for pH, EC and temperature

Stays in the container. You see water temperature, EC and pH at a glance at any time.

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Hydrogen peroxide 3 %, 1 litre

For cleaning the empty system. Dose according to the pack, then rinse thoroughly.

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Disposable gloves (100 pieces)

For clean hands when cleaning and handling infested plants.

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Angebot

Syringe set with blunt needles

For measuring small amounts precisely, for example hydrogen peroxide or fertiliser.

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More on this on my pages about Algae in hydroponics, the EC value and the pH value.

🥀