Can we use chlorine dioxide to prevent diseases?

Can we use stabilised chlorine dioxide continuously in the irrigation system to prevent diseases and biofilm (on a system that recirculates drain water)?

Can we spray chlorine dioxide directly on crop leaves, to prevent diseases in general, specially fungal diseases?

Answer by RICK DONNAN

Answer
You don’t describe whether you have a recirculating water-based system, such as Nutrient Film Technique (NFT). The major alternative is a dripper-fed media-based system where the run-off is collected and recirculated.

Water-based systems
In any water-based system, continuously adding an oxidising agent such as chlorine dioxide would almost certainly soon lead to the death of your entire crop.

Chlorine dioxide is an oxidising agent. Its mechanism is to attack all forms of organic matter (that is, carbon-based material—as in ‘organic chemistry’. Not ‘organic’ as in ‘pesticide free’ etc., produce).

An oxidising agent is not selective as to what it attacks and hence it will destroy all organic matter such as dead roots, bacteria, fungi and otherwise healthy live roots. If the level of oxidising agent is made high enough to ensure a kill of disease bacteria and fungi, it will also be high enough to kill the plants in the system.

Common oxidising agents possibly used with hydroponic solutions are chlorine dioxide, chlorine, calcium hypochlorite (granular pool chlorine), sodium hypochlorite (liquid pool chlorine—especially not recommended), ozone, and hydrogen peroxide. There are other chemicals such as copper and iodine, which are not oxidising agents, but have the same impact.

Continuously adding an oxidising agent to a water-based system results in all plant roots in the system being exposed to damage. In practice, levels as low as only 1 ppm (part per million) have sometimes caused damage.

A classic case that I met was an NFT lettuce grower who had a mild Pythium infection in a mature crop. To get to harvest he dosed his recirculating solution to 5 ppm hypochlorite. He later planted his next crop into the same solution and the young plants all died within a few hours.

Media-based systems
The typical media-based system is periodically irrigated through drippers, usually one dripper per plant. The total irrigation input is controlled to be higher than the plant uptake giving an excess, which is drained from the medium. This drain can either be run off or collected and returned to become part of the feed.

The difference between a dripper-fed media-based system and a water-based system is that the input comes as a single dose at a time. This goes into a limited zone under the dripper and in this zone will be organic matter. By the time of the next irrigation, any oxidising agent in the feed solution will have been deactivated before it is displaced further into the medium by the next irrigation dose. Thereafter it is no longer active, and hence not a danger to the plant roots, but is also useless against any disease in the medium.

As a means of eliminating disease from infected plants in a media-based system, oxidising agents are basically useless. However, their use could still be of benefit. At a reasonable dosage rate, they could eliminate diseases in the recycle stream and hence prevent an infection in one container from spreading through the entire crop, which is a major risk with water-based systems. Their use could also help keep dripper lines and drippers clean and avoid blockages that can sometimes occur due to bacterial build-up.

The downside could be that you are adding additional levels of, say, chloride to the recirculating solution. For crops such as lettuce and tomato, this is unlikely to be a problem because they can take up relatively large amounts of chloride. For other crops, such as strawberry, chloride build-up could be a problem. Never use a compound containing sodium as its build-up has an adverse impact on virtually all crops.

Foliar spraying
Using a strong solution of an oxidising agent is one way to clean a greenhouse between crops. However, to spray a live crop as a preventative is risky.

Firstly, to get a kill of fungi requires a strength high enough to possibly damage the plants. If you are going to the effort of spraying, I think that you would be more effective and safer to spray with a chemical specifically intended for the problem or potential problem that you have.

Remember that good hygiene and good climate control can significantly reduce the risk of infection by fungal or bacterial diseases. That is, prevention is better than cure. Ω

PH&G April 2016 / Issue 166


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