A leap in the dark, applying UV-C after sunset.

Mildew control with UV-C is already well documented in strawberry where repetitive treatments can achieve a remarkable disease control. Mildew is suppressed by the curative action of ultraviolet C light. DNA lesions caused by the high intensity radiation are lethal for mildew and therefore fungal development is arrested.

Fungi, however, developed the ability to repair this DNA damage. Indispensable for the recovery is sunlight, which provides the necessary photon for the enzymatic repair of the damaged DNA. In other words, when UV-C is applied during the day sunshine will activate the fungi to (partly) repair the damage. This means that during the day some of the treatment is undone. In contrast, in the absence of light this repair is prevented and therefore UV-C should be more efficient when applied at night. A first trial was set up to compare the application at daytime and nighttime.

A strawberry cultivation under glass planted in July was subjected to four doses of UV-C light applied either at noon or after sunset.  The UV-C treatment took place on Monday, Wednesday and Friday and lasted from the end of July until the middle of September. Plants were weekly inspected for their degree of mildew infection. The disease pressure of a natural mildew infection increased from the beginning of September, the graph below shows the results.

These results reveal that mildew is indeed more easily suppressed when UV-C is used at night. During the highest mildew pressure on the 18th of September untreated plants showed a disease infection of 25%. During the day, only the highest dose (20 mJ/cm²) could significantly reduce disease infection to 10%. At night, the lowest dose of 3 mJ/cm² outperformed the daytime applications with only an infection rate of 4%. Plants exposed to the highest dose at night barely showed any sign of infection (0,2%).

It is clear that the application of UV-C at night increases the efficiency of the technique which improves energy use of the robot. With this lower optimal dose, UV-C can cover more surface in a single treatment. Furthermore, when applied at night UV-C will not cause any threat to the daily greenhouse activities. An autonomous UV-robot that can perform planned treatments by precise greenhouse localization and automatically recharge its battery will be developed in the course of the project.

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