The Hidden Variable in UV-C Is the Light That Comes After

Berry and vegetable growers often think about UV-C in terms of the lamp, the dose, and the machine delivering it. Those things matter, of course, but there is another variable that can quietly change the result: the light environment after treatment.

This is where photoreactivation becomes important.

Photoreactivation is the biological repair process that allows some fungi and bacteria to repair part of the damage caused by UV-C when they are exposed to blue, UV-A, or visible light afterwards. In simple terms, UV-C can damage the genetic material of a pathogen, but if the pathogen receives the right kind of light after treatment, some of that damage may be repaired.

That does not mean UV-C stops working, and it does not mean sunlight reverses everything. It means the effect of UV-C can be reduced when timing, darkness, dose, and pathogen biology are not properly considered.

For growers, that distinction matters.

UV-C is often described as a disinfection tool, and technically that is true, but in protected horticulture it should be understood more precisely as a biological dose delivered into a living crop system. The outcome depends not only on how much UV-C reaches the pathogen, but also on what happens after the treatment has been applied.

Many pathogens, including fungi such as powdery mildew and Botrytis, have repair mechanisms that can help them recover from UV-induced damage under certain light conditions. Blue and visible light can support this repair process in some organisms, which is one of the reasons why UV-C treatments applied in full daylight may perform differently from treatments applied during the night.

This is why night-time application is so important.

A UV-C pass during the day may still deliver energy to the crop and pathogen surface, but it is followed immediately by light conditions that can support biological repair. A UV-C pass during the night gives the treatment a longer dark period afterwards, reducing the opportunity for light-driven repair and allowing the dose to have a stronger practical effect.

That is not magic. It is biology.

For strawberry growers, this matters directly. Powdery mildew remains one of the most persistent disease pressures in protected strawberry production, and routine fungicide programmes are under increasing pressure from resistance management, residue expectations, labour availability, and retailer demands. Repeated night-time UV-C treatments have shown meaningful potential to suppress mildew, especially when dose, frequency, canopy coverage, and disease pressure are managed correctly.

But the word “correctly” is doing a lot of work.

UV-C should not be presented as a simple replacement for every crop protection decision. It will not solve soilborne disease, poor hygiene, weak climate strategy, uneven canopy structure, or badly timed crop work. It is better understood as a tool that can reduce pathogen pressure and, in the right system, help reduce dependence on routine fungicide applications.

The practical lesson is clear: do not only ask how much UV-C is being delivered. Ask when it is being delivered, what light follows the treatment, and whether the pathogen has a chance to repair.

This is also why comparing daytime and night-time treatment can be useful for growers, but only if it is done safely and with proper measurement. Any trial should include a known dose, clear treatment zones, worker protection, shielding, warning routines, and close monitoring of plant health. UV-C is valuable, but it is not casual technology.

A simple on-farm comparison could look like this:

Night treatment: Apply a verified UV-C dose after sunset, followed by a full dark period.

Pre-dawn treatment: Apply the same verified dose shortly before sunrise, where the dark period after treatment is shorter.

Control section: Leave a comparable untreated section or continue the current standard programme for reference.

Then monitor disease development, leaf response, fruit quality, and crop safety over the following days. The goal is not to prove a miracle; the goal is to understand whether timing changes the result under your specific crop conditions.

That is the real value of photoreactivation as a concept. It reminds us that UV-C is not only a hardware question. It is a timing question, a biology question, and a systems question.

The real promise of UV-C is not that it kills everything. It does not. Its promise is more specific and more useful: when dose, timing, biology, canopy coverage, and safety are properly calibrated, UV-C can become part of a cleaner and more intelligent crop protection strategy.

Croptiq advises growers and technology providers on UV-C deployment, biological fit, dose strategy, safety, and the practical integration of robotics in protected horticulture.