Every hydroponic growing system is different in facility size and circulation systems. Some hydroponic tanks are self-contained, while others are connected, allowing water to circulate throughout the confines of the greenhouse. The main root pathogens encountered on hydroponic farms are Pythium and Rhizopus. Here we will focus on Pythium.
Pythium belongs to the class Oomycetes and is a primitive fungus; its main characteristic is the presence of liquid water to survive. In addition to asexual reproduction, it can also reproduce sexually (in the form of oospores). Pythium generally attacks plants when their physiological state is weak, mainly attacking plant root systems, especially those root tips damaged by low oxygen and/or high soluble salt concentration levels, and spreading upward to kill the roots.
Pythium in Hydroponic System
In hydroponic systems, Pythium is a concern because it produces spores that can swim in water. The spore has two flagella that propel and guide the spore along a chemical gradient to attack the damaged root tip. Healthy lettuce roots are white (below, left), while Pythium-infected roots turn dark and soft (below, right).
So, where does Pythium in hydroponic systems come from? The answer is that they are everywhere. Pythium is a soil-dwelling organism and there are many species, but most do not cause plant disease. Highly pathogenic Pythium species, including Pythium.aphanidermatum, Pythium.ultimum, and Pythium.inularare.
Pythium can be introduced anywhere water that comes into contact with soil, such as ponds and streams, or soil that has been intentionally or unintentionally introduced into a hydroponics facility, contaminated shoes, tools or equipment. It is for this reason that the source of water for hydroponics should be clean, preferably well water, unlikely to contain Pythium.
Hydroponic farms should keep surfaces clean, promptly remove plant and soil residues, and follow good farm hygiene practices to reduce the introduction and survival of Pythium. Work area surfaces and equipment should be disinfected each time before and after planting and transplanting seedlings into hydroponic trays.
Especially in deep tank systems, the source of Pythium root rot is often hydroponic trays. The most common material for hydroponic trays is porous polystyrene, into which the plant's roots can grow. Hydroponic trays must be cleaned to remove organic matter and should be heat treated or sterilized before reuse.
However, plant roots can remain embedded in the tray, and new plant root growth tends to follow the channels left by previous plants, so this can contaminate the new roots with pathogens trapped in the pores. Plastic trays can solve this problem and are easier to clean and sanitize, but they are more expensive.
Water and nutrient conditions also affect the survival of Pythium populations. Excessive oxygen content in the pool will reduce the survival of Pythium spores, so hydroponic growers should ensure that there is sufficient ventilation in the water body, so that the dissolved oxygen saturation reaches 8ppm, which can reduce the survival rate of Pythium.
In addition, the hydroponic water temperature should be maintained at 20-22°C, which is most suitable for spinach and lettuce production, and can also delay the activity and infection rate of Pythium. Low oxygen and high root temperatures will increase the problem of Pythium root rot.
Beneficial microbes build up in deep pools over time, which helps reduce the survival rate of Pythium. Some hydroponic growers say their hydroponic systems were filled more than 8 years ago, and that the plants on their farms have good and healthy roots.
Drainage is not necessary as long as the hydroponic system is not contaminated with Pythium or other root pathogens. Changing water in hydroponics can be tricky, so preventing Pythium root rot is key to keeping the entire system safe.
Beneficial microorganisms can be added to hydroponic systems by incorporating biocontrol products, rooting broths, or directly into hydroponic nutrients.
Algal blooms commonly occur in hydroponic systems and reduce the growth rate of plant crops; furthermore, algae can be a habitat for shore flies and mushroom mosquitoes. These insects are known to ingest pathogen spores from plant roots, but Pythium spores are tough enough to survive the insect's digestive system.
So, these insects spread the spores to new planting areas, causing widespread infection. Controlling algal growth and insect populations can reduce the spread of root diseases.
Ultimately, the key to controlling root disease is to reduce the spread of Pythium. Identify and discard infected and symptomatic plants as soon as possible, otherwise the longer the infected plant remains in the hydroponic system, the fungus will increase and spread to more plants.
For Pythium, the spores are released from infected plants into the water and can quickly infect a complete hydroponic system. The interconnected circulatory system of the pools is rarely emptied unless the disease pressure is very high; given such a system, these farms should be equipped with water disinfection systems where filtration and UV sterilization can be very effective. Filtration removes organic matter and algae, while improving clean water quality is an important factor in ensuring UV sterilization.
Once a Pythium infection occurs, treatment is also effective and includes other water treatment options such as chlorination, ozonation, copper ionization, and the addition of oxidizing agents (hydrogen peroxide, peracetic acid). However, these care measures should be used with caution as they can alter the pH of the hydroponic system, interfere with nutrient availability or be phytotoxic to certain crops.
Takeaways in hydroponic Pythium root rot management
- Prevent the introduction of pathogens
- Disinfect surfaces, tools, equipment, especially hydroponic trays and seeding areas
- Wash and sanitize hydroponic trays before reuse
- Promptly detect and discard infected plants
- Add beneficial microorganisms
- Maintain optimal nutrient hydroponics, dissolved oxygen saturation, pH and soluble salt levels
- The water temperature should be maintained at 20-22°C in summer
- Maximum possible control of algae and pest growth.
