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"Botrytis still a huge problem in cannabis”

The weeks before harvest are weeks in which the average cannabis grower is on high alert. With the increase in flower volume, the risk of the plants getting Botrytis increases as well. Using chemical agents to prevent this is not possible in cannabis cultivation, so the fungi can rapidly spread after germination, leading to a loss in the harvest. So, what can the grower do to prevent this? Specialists from Team Cannabis from Delphy provide an explanation and tips.

Botrytis in cannabis

Because it is absolutely not allowed to work with chemical agents, controlling the climate and plant resilience is one of the only options growers have to prevent Botrytis. Many growers choose to keep their humidity levels as low as possible for this reason.

However, this is often not enough because the humidity levels in the cultivation room say little about the crops' flowers' humidity levels. In some cases, it is also not possible to lower the humidity levels due to the cultivation location or environmental factors.

In the end, it is about the uniformity of the climate around and inside of the crop. The movement of air can play an important part, according to one of the most recent studies regarding Botrytis in greenhouses cannabis cultivation conducted by Delphy.

Their research looked at the effect of air movement in the Botrytis levels in greenhouse cannabis cultivation. The research chooses to look at a cultivation period from September to November since Botrytis is most present due to the relatively warm and humid weather conditions.

To see the effects of air movement on Botrytis levels, the backs sides of the tables have been provided with fans that create air movements across the length of the table. Fifteen uniform plants have been selected for each table to perform measurements on.  

By the end of the cultivation, all plants have been harvested, and the amount of Botrytis infections have been counted and statistically processed. The results showed that plants that were closer to the fans and were subject to more air movement showed significantly lower Botrytis levels.

From this research, it can conclude that air movement can play a significant role in preventing Botrytis. By dehumidifying in combination with the right amount of air movement, better results can be achieved than by only dehumidifying. However, more factors need to be taken into account in preventing Botrytis in Cannabis cultivation.

Variety sensitivity and plant resilience
In cannabis cultivation, there is a large difference in Botrytis sensitivity for each variety. In one of the first studies conducted by Delphy, seven varieties were selected from seeds to build the mother plant stock for follow-up research.

During this study, the Botrytis sensitivity for each variety was shown to be very different. Some varieties already contracted Botrytis in the early stages, while others, up until the moment of harvest, were not affected. All plants were placed on the same table and were cultivated under the same conditions.

Besides variety sensitivity, plant resilience is also an important factor when it comes to preventing Botrytis. In that case, the focus lies with providing the right nutritional recipe and creating the right climate. This allows the plant to optimally use its assimilation, making it less susceptible to illnesses such as Botrytis.

As mentioned, dehumidifying is an important factor in preventing Botrytis. One of the ways that are used to lower the humidity, and Botrytis levels, is by using active dehumidifiers.

Many appliances and suppliers on the market deliver this, all with a slightly different working mechanism but the same goal; dehumidifying a cultivation area. Some systems blow the cold, dry air from outside to the inside, mixing it with the air already present in the room. Other systems suck the humid air from a cultivation room and have it move past a condenser to blow dry air back into the cultivation room.

Each system can work well in a given situation, given that the grower learns how to use it. No machine or cultivation room is the same, so it will take time to learn the different setpoints and their effect on the climate in a given cultivation room.

It is also important that the air brought in is homogeneously distributed on plant/flower level. We often notice a difference in the measured values in the room and the actual values on the plant/flower level.

Minimum tube
A well-known term in horticulture is the use of a minimum tube. This is a heating tube that is present underneath the crops and provides a constant minimum temperature. Using a minimum tube (perhaps in combination with a growth tube), the air in the cultivation room is heated. According to the physical principles, warm air can contain more moisture, and warm and humid air rises. This warm and humid air can then be moved outside through the air windows, which leads to a more active climate. It is about the supply and discharge of energy, and because of that, moisture. This is the reason Botrytis is often present in greenhouse horticulture in autumn.

During this period, temperature and humidity are relatively high, so less moisture gets discharged during this period. Realizing a higher energy-in- and output due to dehumidification and the use of ventilators could provide a solution in this situation.

Flower temperature
Another important point in preventing Botrytis is the flower temperature. By keeping the flower temperature above the cultivation temperature, you prevent condensation from forming and, with that, the chance of Botrytis forming.

Condensation forms when the flower temperature comes below the dew point. In cannabis cultivation, this is often the moment the lights are turned on. When the lights are turned on, the temperature in the cultivation room will also rise. If the plant is not active enough and stays too far underneath the temperature in the cultivation room, it can get wet, leading to Botrytis.

Another important factor in greenhouse cannabis cultivation is heat emission. When, on a clear night, the screens are not closed on time – which in some cases can already be at the end of the afternoon or the beginning of the night. The heat the flower emits to a clear sky will be greater than the heat emission coming in. This leads to the flower’s heat emitting to the sky and the flower cooling down.

Besides the possibility of condensation, the cooling of the flower also leads to fewer nutrients being transported to the flower. One of the nutrients that are transported during the night and is important in preventing Botrytis is Calcium. Calcium strengthens the cell walls so that Botrytis can not germinate as easily anymore. Using the screens on time, the flower is kept active so that more Calcium can be transported to the flower.

Alternative intervention
In the end, preventing Botrytis in cannabis cultivation is about a combination of factors that can influence this. After all, a chain is only as strong as its weakest link. Even if nine out of ten factors are in order, one weak link that is overlooked can lead to a Botrytis outbreak. Choosing a less sensitive variety is a good start. Plant resilience also needs to be optimal by keeping up the right nutrition- and assimilation balance, which will be discussed in the next article.

Also, the supply and discharging of energy is an important factor to consider when trying to prevent Botrytis. Because chemical agents are not allowed to be used in cannabis cultivation, the search for alternative methods is important. In that regard, cannabis cultivation could be the frontrunner in researching Botrytis.

This is one of the subjects Delphy wants to study during their five-year lasting indoor cannabis research, which will take place in the Improvement Centre in the Dutch town of Bleiswijk starting next year.

Besides conducting research and giving advice, Delphy also is active in turnkey projects and providing cannabis cultivation training. The next training session will start on the 9th of September.

For more information: 
+31 (0)10 522 1771

[email protected]

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