The 4 Reasons Of Black Spots on Hydroponics Vegetables

Black spots are a worrying sight to see on your meticulously raised hydroponics vegetables. Are these normal, a disease, or a symptom of something much more complicated?

Black spots on hydroponic vegetables are due to 1) fungal infection 2) bacteria infection 3) root rot and 4) nutrient burn. Bacterial/fungal infections and root rot can be solved with commercial or home-made fungicides whereas nutrient burn can be addressed by flushing the system.

The causes, remedies, and means of prevention will vary from case to case. If you want to keep your hydroponic vegetables clean and safe, it’s a good idea to know what you’re up against so that it won’t happen again.

Are Black Spots a Bad Sign?

Black spots on hydroponic vegetables are a bad sign and might be the start of the degradation of the plant’s health.

Cultivation using hydroponic techniques, especially indoors, reduces the need for herbicides and pesticides due to the absence of soil where pathogens and pests may thrive. However, in hydroponics, it is of great importance that the presence of adequate aeration and humidity level as the lack of ventilation can trigger many problems (including bacterial and fungal issues).

Hydroponic Black Spot Causes and Remedies?

In general, hydroponic vegetables are vulnerable to the same diseases and disorders found in soil-cultivated vegetables. However, hydroponic vegetables should be less prone to such diseases and disorders because of the clean, sterile, and controlled conditions present in a hydroponics system.

4 Reasons Black Spots Hydroponic Vegetables – Infographic

Black spots should be a cause for alarm since this could mean that the entire system is infected due to fungal or bacterial infection, root rot, or nutrient burn. It may also spread to other vegetables in the system that might not look still affected.

1 – Fungal Infection

Fungal infections are often the most common causes of diseases in hydroponic vegetables. The black spot disease is caused by diplocarpon rosae – a fungal infection which is well-known for affecting roses but it is, of course, capable to infect other plants as well.

Different fungi (Armillaria mellea, Aphanomyces, Clitocybe tabescens, Fusarium, Pythium, and Phytophthora) differ in the symptoms they cause and the negative effects they bring so it’s always important to take note.

Remedy

Fungi can be treated with commercial fungicide available in local gardening stores or in Amazon here. These often come in spray form which can be applied directly to the affected area.

How to create a homemade solution for black spots on hydroponic vegetables?

For a homemade solution, 1 tablespoon of baking soda is mixed into 1 quart of water with liquid soap to make an effective solution that will stick to the affected areas. Spray directly to the affected areas once a week.

For an alternative homemade solution, a hydrogen peroxide solution spray is viable. Mix equal parts of 3% hydrogen peroxide with equal parts of water. Spray the solution of the affected areas once a week.

If the disease has progressed to a point where the affected areas are too severe to be saved, it is best to cut and dispose of the affected area. It must be kept away from other plants to minimize the chances of the disease spreading.

Prevention

A dirty and “overly wet” hydroponics system makes for a perfect environment for fungal growth. Overhead watering is not recommended in a hydroponics system since wet spots on the leaves and stems become sites for fungal growth.

An unclean system will lead to fungal growth in the reservoir, the pipes, and even the growing medium.

Keeping a hydroponic system clean and sterile is recommended to prevent the introduction of fungi, diseases, or pathogens. This can be done by periodically cleaning the system with hydrogen peroxide (H2O2) as this one on Amazon or chlorine bleach solution. Cloudy hydroponics water is an indication that it’s time to clean the system.

Make sure that you always wash your hands when handling different plants as fungi and bacteria may transfer between plants, especially if the other plant is soil cultivated. Finally, make sure that plants are placed in a well-lit and well-ventilated area to prevent fungal growth.

2 – Bacterial Infection

Bacterial infection (often caused by Xanthomonas campestris pv. vitians) can cause black spots on hydroponic vegetables. Bacterial infection often occurs in water-soaked leaf spots which turn darker and unhealthy. Later on, it will cause the collapse of the leaves if left unattended.

Much like fungal infections, bacterial infections grow due to overwatered conditions. Bacteria reproduce most rapidly at 77-86 degrees F and conditions with bad lighting and ventilation. To avoid this rapid growth rate, keeping the plants under grow lamps or under direct sunlight will evaporate the moisture on the surface. Proper ventilation can be achieved by introducing airflow (i.e. opening windows, introducing ventilation fans).

Remedy

Copper fungicide as this one on Amazon is often used to treat bacterial leaf spots, however, it is only effective early in the disease cycle. Bacterial infections have similar remedies to fungal infections.

How to create a homemade solution for black spots on hydroponic vegetables?

A home solution for bacterial infections can be done by mixing 1 tablespoon of baking soda with 1 quart of water with liquid soap. This will make an effective solution that will stay in the affected areas. Spray directly to the affected areas once a week.

For an alternative homemade solution, a hydrogen peroxide solution spray is viable. Mix equal parts of 3% hydrogen peroxide with equal parts of water. As before, a weekly spray on the area affected is recommended.

If the disease has progressed to a point where the affected areas are too severe to be saved, it is best to cut and dispose of the affected area. It must be kept away from other plants to minimize the chances of the disease spreading.

Prevention

Copper fungicide applied early acts as a preventive measure against bacterial infections.

Similar to fungal infections, maintaining a clean and the relatively dry system will minimize the occurrence of bacterial infections and other diseases. Overwatering and overhead watering are not recommended.

Bacterial growth can be prevented by properly covering the reservoir from sunlight penetration.

Tip: Painting a reservoir black or any other opaque color will significantly reduce the chance of bacterial growth.

Maintaining hygiene when handling plants especially if the other plant is soil cultivated since the latter is not grown in a sterile or clean environment. There are more pathogens to deal with in soil cultivation.

3 – Root Rot

Root rot is a disease caused by either oxygen deprivation or fungus infection. Root rot causes growth stagnation, yellowing and blackening leaves, and even plant death if not addressed in time.

Root rot is caused when roots in a hydroponic system are “drowned” and lack aeration. Roots require oxygen to maintain the vital process of cellular respiration. The lack of oxygen causes the root to die, decay, and rot away.

The disease is best identified by dark or brown roots instead of healthy white roots.

Remedy

A homemade DIY remedy for hydroponic root rot is to use baking soda. 1 tablespoon of baking soda to 1 quart of water mixed with liquid soap to make the solution stick to the affected areas. Spray this solution on the roots every 1-2 weeks until the root rot is treated.

A solution consisting of food-grade hydrogen peroxide at a ratio of 2ml per liter in the reservoir can be applied to a running hydroponics system. For a more direct application, a solution of 1 part food grade hydrogen peroxide and 2 parts water can be applied directly to the roots which will be left to rest for 15 minutes.

A chlorine bleach solution of 4-10 drops in 1 liter of water is another remedy for root rot. Uproot the plant from the system and pour the solution over the infected root system. Afterward, leave it to rest for 15 minutes.

Roots already dead and decayed are best cut off to prevent further infection or decay. This is necessary to make sure that the root rot does not spread to other vegetables in the system.

Prevention

The best way to prevent root rot is to make sure that not all the roots in a system are submerged. Aeration is important because lack of oxygen in the root system will inevitably lead to root rot. This prevents root rot caused by drowning and by fungi.

Similar to bacterial and fungal infections, maintaining a relatively clean, dry, and sterile system will help prevent root rot.

4 – Nutrient Burn

Nutrient burn occurs when there are too many nutrients or nutrient imbalances in the system. This can lead to yellowing and darkening of leaves, black spots, and curling of the edges of the leaves.

Remedy

The remedy of nutrient burn is by flushing out the system of excess nutrients. This means draining the reservoir and replacing it with fresh water.

After flushing the system, the electrical conductivity (EC) and parts-per-million (PPM) levels should be monitored. It should be as close to zero since the plants in the system are already full of nutrients. This is the chance for the plants to adjust themselves after being overloaded with nutrients.

Finally, when the symptoms of nutrient burn have subsided, nutrients can be introduced little by little. It’s recommended that only provide half what is usually required to ensure that the nutrient burn will not reoccur after flushing for a short period of time (1-2 weeks)

The table below shows the recommended EC and PPM levels on some popular hydroponic vegetables.

PlantElectrical Conductivity (EC)PPM
Brocolli2.8-3.51960-2450
Cauliflower0.5-2.01050-1400
Celery1.8- 2.41260-1680
Lettuce0.8-1.2560-840
Pak-choi1.5-2.01050-1400

Prevention

Nutrient burn can only be prevented by constant observation of the electrical conductivity (EC) levels and parts-per-million (PPM) levels in a system. Any imbalance should be remedied by adjusting the nutrient content in the system or flushing if need be.

Can You Eat Vegetables With Black Spots?

Vegetables affected with black spots can still be eaten as long as they are properly washed and sanitized. Vegetables and fruits with black spots may not look edible but they are safe to eat.

The bacterial or fungal infections which affect plants do not have any adverse effects on humans even when consumed. Pathogens which do affect humans such as e. coli or salmonella can be present on the surface of vegetables but they are invisible to the naked eye and do not cause any visible changes to the vegetable.

Despite being edible, these black spots are often sites of decaying organic matter which will not taste as good as the unaffected areas. These spots are essentially dead and decomposing organic tissue. For culinary purposes, these black spots should be removed from the rest of the vegetables to maintain good taste and aesthetics in dishes.

Takeaways

  1. Black spots are often a bad sign in hydroponic vegetables because it means there might be pathogen infections by way of fungi or bacteria. It could also mean systemic imbalance by way of nutrient burn.
  2. Bacterial and fungal infections can be remedied with baking soda, chlorine, and hydrogen peroxide solutions applied to the affected areas. Nutrient burn can be remedied by flushing the system and refilling the reservoir with fresh water.
  3. Hydroponic vegetables with black spots are still safe for consumption because bacterial and fungal infections which affect plants do not have negative effects on humans. However, these black spots should be removed because they do not taste good.

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