Certain diseases can wreak havoc in hydroponic systems – guard against them by following this advice.

Plants grown in hydroponic systems are susceptible to many of the diseases affecting field-grown vegetable crops. But because hydroponic systems are grown under shelter (plastic tunnels/shade net), fungal and bacterial diseases of the above-ground parts of the plant may be enhanced by high humidity levels.

Also read: Hydroponic farming: How shade-net and plastic-covered greenhouses affect growth

These can occur if tunnels aren’t adequately cooled or vented, or when daytime temperatures are fairly high but night-time temperatures are low (usually in early and late winter). Under such conditions, diseases such as botrytis, phytophthora and bacterial spot/speck can be destructive to tomatoes and peppers.

When conditions are dry (low humidity) powdery mildew is often a problem in tomatoes and cucumbers, and early blight could affect tomatoes. Infected seedlings are often a very important source of disease; bacterial canker and bacterial spot/speck in tomatoes, for instance, can be introduced in this way.

Pythium – a fungus known as a water mould – can cause a destructive root rot in various vegetable crops in closed hydroponic systems. The disease can build up rapidly and kill many plants. The fungus produces swimming spores called zoospores, which are released in great numbers and swim to the roots, causing infection and root rot. It is essential to control this disease in a hydroponic system.

Various viral diseases can seriously affect hydroponically grown vegetable crops. Tomatoes and peppers are susceptible to many of the same viruses, usually transmitted by an insect vector such as thrips or aphids.

Also read: Hydroponic farming: Shaping your tomato plants

CONTROL STRATEGIES

Always plant healthy, disease-free seedlings. Use a reputable seedling grower who practises strict nursery sanitation. This is especially important in closed hydroponic systems. Disease-free seedlings should have no black or brown spots on the leaves or stems, or show signs of wilting or browning of the roots. Diseases such as bacterial canker in tomatoes, however, show “symptomless infection”, that is, seedlings are infected, but look healthy.

If you’re growing your own seedlings, buy good-quality seed that’s been warm-water treated as well as treated with a fungicide. Use steam-sterilised soil, if possible. Never add peat moss to your growing medium, and don’t re-use your growing medium without sterilising it again.

Also read: Hydroponic farming: How to produce vegetable seedlings

Use the correct chemical control strategies as soon as disease symptoms appear. An initial application has to be followed by regular applications. This procedure is very important for diseases such as early blight and bacterial spot/speck in tomatoes, powdery mildew on cucumbers, and phytophthora rot in tomatoes and peppers.

• Reduce humidity levels by venting or heating in the production area.
• Allow for good ventilation between plants by spacing plants correctly.
• Remove infected plants and plant debris from the production area. Burn or bury infected plants.
• Keep tunnels free of plant debris such as fallen leaves and fruits.
• Control insect pests in the production area, especially thrips, aphids and whiteflies. Scout regularly for insect pests.
• Do not let workers work among plants that are wet.
• Sterilise all cutting implements, such as knives and secatures, regularly.
• Don’t reuse the growing medium in open bag systems.
• Discard or re-sterilise it, and use new bags for new plantings.
• Use municipal or borehole water for irrigation.
• Try not to use water directly from groundwater dams or rivers, but if this is not possible, install and use a sterilisation unit so you can eliminate pathogens from the water.
• Plant resistant varieties wherever possible.

Also read: Hydroponic farming: The basics – popular systems

• This article was written by Alistair Thompson and first appeared in Farming SA.