Pest and Diseases

 Indications of pests and diseases:

  • Some brownish and/or stinking root parts
  • Different colored spots and/or parts appearing to be rather on leave's or flowers' surface
  • Bugs, mollusk and larvae
  • Small bulges all in the same color and located close to each other at certain spots

Counteractions:

  • Keep your growing space as clean as possible
  • Use intercropping (meaning to mix crops and to not put the same varieties next to each other) while keeping sufficient space between each plant
  • Provide the best growing conditions possible
  • Introduce predators fighting your pest or disease
  • Apply mechanical destruction or separation
  • Utilize biological or chemical substances

Like with humans not living healthy, diseases may also spread in plants by "living unhealthy". But of course, pests and diseases are also possible to infest your system despite it being managed perfectly [1]. Although, it is less common than conventionally grown plants [46], it is also important for an hydroponic grower to know some common pests and diseases. So first things first, let us see what we have to deal with:

Many diseases are visible at leaves, roots or fruits, which show likewise insufficiencies different colored or shrunk spots or parts [1]. While with some diseases your overall plants' health is not at danger, others might cause severe damage [1]. Hence, it is important to make sure to separate plants showing specific symptoms from the others, as long as you do not know what you are dealing with.

And with Fungus, insects and others... only a few are beneficial in hydroponic systems, the others are not and hence called pests. While bees may be helpful to pollinate your plants without causing any damage [43, 46], snails or aphids, e.g.,  feed on your plants part by part. There exist different countermeasures, against each pest you may be facing and it is your choice on which to use. Nonetheless it is always a good approach only to apply pesticides if necessary according to integrated pest management guidelines (IPM). And remember, that the best action is always prevention.


Integrated Pest Management (IPM)

It is always preferable to apply an holistic approach for solving problems, which might affect many different critical parts of a system. Therefore, it should be your goal to understand the interlinkages of your hydroponic system as good as possible in order to tackle pests and diseases. An integrated pest management will help you to find the right actions, by making sure you do not falsely identify the cause of your plants' symptoms and their best working counteractions. When discovering a symptom you may hence go through the following bullet points, trying to eliminate as many possible reasons as possible:

  • The pH is in the target range
  • The EC is in the target range
  • The air Temperature is in the target range
  • The light levels are in the target range
  • The nutrient solution Temperature is in the target range (between 17 and 25 ° C [43])
  • The plants receive sufficient fresh air, so that the leaves are visibly moving at least sometime during the day
  • At least same varieties of plants show similar symptoms
  • The crops are free of pests or diseases
  • The fertilizers used are designed for hydroponic gardens and the nutrient solution is not older than it should be according to the following rule of thumb: you should just top up your reservoir with tab water and readjust the target pH and EC ranges with your hydroponic fertilizers when necessary. Renew the nutrient solution, if you in total topped up half of the reservoir's volume [5].  (hereby, the old nutrient solution is by the way a perfect option to fertilize your soil grown plants)

If not even the plants of the same variety show the discovered symptoms, it is likely you are facing some pest or disease (even though you might not see any bugs or similar alarming characteristics yet). Learn in the following how to counteract some of the most common pests and diseases.

Pests

Pest Description Counteractions
 Algae

There exist many different algae species which are also very differently colored. Algae itself does not harm your hydroponically grown plants directly [1], but it might be responsible for indirect negative effects like low oxygen levels within the nutrient solution or clogging pipes, valves and pumps [9].  Additionally, algae is living space and food source for several pest and diseases [1, 5]. Hence, it is important to remove algae regularly from any spots that may be endangered in your hydroponic system – algae will bloom close or within the nutrient solution under warm and humid conditions as soon as they receive some light [9].

You may also discover some algae on your substrate over time, which will tell you to lower the watering. Nonetheless, some algae on the substrate is normally not a big issue you should be worried about [5].

  • Clean your hydroponic system regularly with dish soap to also eliminate algae hotspots
  • Use a nutrient solution reservoir and distribution system that are not translucent to prevent algae bloom
Fungus Gnats Fungus gnats look a little similar to mosquitos [5]. In the adult stadium, they are harmless to your plants but rather just annoying. Whereas in the larvae stadium, they take the plants' roots as food source, causing several direct damage and increasing the susceptibility to other diseases [1].
  • Remove all dead organic matter and algae
  • Balance the fungus gnat infestation by introducing nematodes like Steinernema feltiae
  • Use bacteria based (e.g., Bacillus thuringiensis israelensis) pest-control products
  • Use organic pesticides against fungus gnats
  • Use conventional pesticides against fungus gnats
Aphids You might know aphids as plant lice, which are often visible around the stems as accumulated green, gray or black dots [9]. And if you do not spot them immediately, you may discover some honey-alike film on your plants' leaves [5]. Aphids extract substances from the leaves, leading to limited metabolism and growth of the plant. However, aphids will not kill plants directly but carry and spread other diseases with them [1].
  • Balance the aphids infestation by introducing nematodes
  • On the internet you easily find some simple organic receipts explaining how to cook your own organic pest repellent
  • Use organic pesticides against aphids
  • Use conventional pesticides against aphids
Thrips Thrips are very small and hence hard to see with the naked eye, although they are able to kill your plant. They feed themselves from the leaves inner substances and cause severe damage, which starts to become visible with blackish dots around the leaves edges, followed by spreading a yellowish area from there and lastly drying out the whole leaf [1]. They are hard to fight, but not impossible to vanish [9].
  • Balance the thrips infestation by introducing nematodes
  • On the internet you easily find some simple organic receipts explaining how to cook your own organic pest repellent
  • Use organic pesticides against thrips
  • Use conventional pesticides against thrips
Spider Mites Spider mites are very small and hide well, the reason because they are very hard to see with the naked eye [5]. Though a trick is to softly wipe a tissue along the downside of your plants' leaves and check it then for signs of blood – if you find blood, you probably just killed some spider mites, meaning that there are more which have infiltrated your hydroponic garden [9]. They seem to prefer dry conditions as well as fruiting and heavily fertilized crops and cause limited and deformed growth along with leaves turning yellowish [1]. Heavy infestation is also characterized by small webs around leaves and flowers and leaves falling off after turning yellow [5].
  • Increase humidity a little if possible
  • Growth plant varieties less susceptible to spider mites
  • Balance the spider mite infestation by introducing nematodes
  • On the internet you easily find some simple organic receipts explaining how to cook your own organic pest repellent
  • Use organic pesticides against spider mites
  • Use conventional pesticides against spider mites
White Flies White flies are mostly annoying but easy to detect and to identify: they are very similar shaped to moths but white [9]. They cause white spots and yellowing on your plants' leaves [1].
  • Balance the white fly infestation by introducing nematodes
  • Use organic pesticides against white flies
  • Use conventional pesticides against white flies

Diseases

Disease Description Counteractions
 Root Rot Root rot causes your plants to become yellowish and to wilt, while the roots are turning brownish and mushy [27]. Most of the time, it's reason are too much water or pathogens infesting your hydroponic system.
  • Minimize the chance of root rot infestation by adding root stimulators during early growth stages
  • Reduce watering and keep your plants' root zone dryer
  • Use organic pesticides against pathogens
  • Use conventional pesticides against pathogens
 Gray Mold Gray mold is caused by a fungus and therefore more common under humid conditions [1]. It starts as dots on the leaves turning ash gray and spreading over the whole plant, till it is killed [9].
  • Do not keep your humidity levels around your hydroponic system too high
  • Use organic pesticides against pathogens
  • Use conventional pesticides against pathogens
 Powdery and Downy Mildew Powdery and downy mildew are caused by a fungus and therefore more common under very humid conditions [1]. They start as white dots on the leaves' upside for the first or underside for the latter [9]. They cause stagnated growth and turn the leaves yellow till they fall off. Without counteraction, these mildews are also likely to kill your plants [5]. 
  • Do not keep your humidity levels around your hydroponic system too high
  • Use organic pesticides against pathogens
  • Use conventional pesticides against pathogens
 Rust Rust is caused by continued very high humidity levels and is very contagious [9]. It starts as red bumps on the leaves' underside, which then cause the leave to turn to yellow, then brown and ultimately to die [1].
  • Do not keep your humidity levels around your hydroponic system too high
  • Use organic pesticides against pathogens
  • Use conventional pesticides against pathogens

Link to References