Botrytis is a particularly tricky pathogen. In a previous article, the Delphy Team Cannabis has discussed solutions to mitigate the problem and have botrytis-free cultivation: methods for this could be a careful genetics selection, proper dehumidification, but also alternative methodologies such as UV disinfection, as well as working on making the plant more resilient.
Making plants resilient to pathogens is a very hot topic in the horticulture world, and this is often related to discussions of soil-improving microbes. At the same time, there are more ways to make a plant resilient. For instance, nighttime evaporation or assimilate balance are useful methods to make cannabis more resistant to botrytis.
Night-time evaporation
Botrytis is a weakness-inducing parasite that germinates on weak, damaged, or dead plant materials, in conditions of high humidity (94% or more). Calcium is an element that makes the plant sturdier, which conversely makes Botrytis less likely to develop. Calcium is an immobile element, which means that most of it is transported to the leaves because of radiation and evaporation. However, flowers are the most susceptible to Botrytis, so this is where calcium should be transported to make flowers more resilient. This is difficult to accomplish during the day, because of the lack of radiation-caused evaporation, which makes calcium ends up in the leaves. That’s why this is best achieved in the night when there is no light: this is what’s called night evaporation.
Botrytis in cannabis
To better understand night evaporation, imagine a wet cloth that has to dry in a dark room. When this happens, both to the wet cloth or to a flower, there are three possible scenarios.
Temperature increases
When an object is colder than the environment, then it can be more susceptible to Botrytis. Let’s consider screening against radiation for this. If there is no screening on a clear night, the flower will radiate heat, thus becoming colder. On the other hand, a colder flower will evaporate less, thus transporting less calcium to the flower, making it more susceptible to pathogens. On top of that, there is a chance that the flower will drop below the dew point, thus causing condensation which leads to increased susceptibility.
Convection increase (airflow)
Another method to stimulate night-time evaporation is through having more airflow for the cultivation. If you consider the wet cloth again, it would dry faster when a fan is on it. In a cultivation, you can do that by hanging ventilators across the growing environment. This not only creates a more uniform climate but also generates higher night evaporation.
Lowering the air humidity
A third and often used method to stimulate night evaporation is to lower the air humidity. When the humidity in a space is lower than the wet object in question, this would dry faster as the difference in humidity within and around the wet cloth or flower is greater than the humidity in the environment. The environment can then extract moisture from the wet cloth (or flower) before the air gets saturated. In practice, this is done by airing through the windows or by using dehumidification equipment.
All the above-mentioned factors determine whether there is sufficient nighttime evaporation for the calcium to get to the flowers. Higher night-time evaporation makes the flowers less susceptible to Botrytis. Yet, if it’s too high, the crop will become (too) generative, which will result in smaller flowers and lower yields. In general, it is assumed that evaporation of 15 grams/m2/hour during the night should be sufficient. At Delphy they think this should be higher for cannabis. The ideal amount of night-time evaporation is one of the points that is being researched for the five-year indoor cannabis research at Delphy.
Assimilate balance
Assimilate balance concerns the ratio of sugars produced (assimilation), and the sugars consumed (dissimilation). Plants produce sugar as a result of photosynthesis; at the same time, these same sugars are also consumed to keep the plant going. The consumption of these sugars goes hand in hand with temperature. A higher temperature ensures higher dissimilation. For a healthy and resilient plant, it is important that both assimilation and dissimilation are in balance. Light, CO2, and temperature are the most important parameters for that. When the temperature is too high (for the amount of radiation), then the plant will have a too high dissimilation, thus becoming 'weak'. Weak plants are less resilient and more susceptible to diseases and pests. When a plant grows too weak, two things can be done:
- Increase the radiation = more assimilation
- Lower the temperature = less dissimilation
That’s why it is critical to keep an eye on the relationship between the total radiation and the 24-hour temperature. When a plant has a net amount of assimilate leftovers, it becomes stronger and more resilient. This is because the plant can use the extra sugar to produce healthier leaves, roots, and flowers, as well as higher content substances. All of that ensures that a plant becomes less susceptible to Botrytis.
It is difficult to establish the correct relationship between radiation and daytime temperature, as this is different from crop to crop, but also from cultivar to cultivar. To better understand this, Delphy has developed a tool through which growers can see which 24-hour temperature must be achieved for a certain radiation sum to keep the plant in balance.
Resilient cultivation is one of the points Delphy is researching during the five-year indoor cannabis trials, which will take place at the Improvement Center in Bleiswijk, the Netherlands.
For more information:
Delphy 
+31 (0)10 522 1771
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www.delphy.nl