Bacterial blotch in the first flush

Under favourable conditions, bacteria multiply very quickly, depending on the type of bacteria, in as little as 20 to 60 minutes. So what is the trick to preventing bacterial blotch? Ensure that the conditions are not favourable for bacterial blotch, but are favourable for our “own” mushrooms. How do you do this?

What are the right conditions for bacteria? Warm and humid. These are also the conditions for mycelium to grow. However, I believe there is a big difference. A humid climate does not mean that it is wet. That you see free water. And that is precisely the biggest difference.

How do you recognise free water in the air?

When the climate unit requires humidification, steam or fine water mist is added to the air that is blown into the cultivation room via the air duct. This mist, visible as vapour, comes out of the air hose and is particularly visible at the point where the air enters the room.

This visible vapour consists of free water: small water droplets that have not yet been absorbed by the air. A clear sign of this is the wet floor under the air duct, where the air enters the room. With a strong torch, you can often still see water droplets floating in the air (see also page 79 of Mushroom Signals).

These droplets eventually end up on the growing bed, precisely in the places where the mycelium or, later in the cultivation process, the pinheads form. This keeps the surface too wet. This creates ideal conditions for bacteria, allowing them to multiply rapidly and en masse.

But what happens in the climate unit?

If the RH falls below the set minimum setting, the climate computer opens the humidification valve. The following then happens in the climate unit:

• The air is cooled significantly to control the air temperature in the room.
• The temperature in the cooling coil can fall below the condensation temperature.
• This is visible because water flows out of the climate unit’s drain: this is condensed moisture from the air.

After the cooling coil the air is humidified , but cannot absorb this extra moisture. After all, air that is cooled to or below the condensation temperature already has a RH of 100%. Without heating, no extra moisture can be absorbed into the air.

As a result, the added humidification water condenses in the supply air. This is visible as fine water droplets that remain suspended in the air.

Why does this occur mainly at the front of the room?

Water droplets are heavier than air and therefore fall relatively quickly. This happens mainly immediately after the air is blown into the room, causing free water to settle mainly at the front of the room. This situation leads to localised free water on the surface, which increases the bacterial pressure. This causes bacterial spots to form, particularly in the front part of the room.

What can you do about this?

• Wet the floor of the room (see this blog). This ensures natural evaporation, humidifying the air without water droplets floating in the air. This evaporation also has a cooling effect.
• Increase the fan setting. This causes a larger volume of air (more m³) to be sucked through the climate control unit. The temperature of the air in the cooling block rises as a result, meaning that the air does not need to be cooled as much. Less cooling means less condensation on the cooling block, so more moisture automatically remains in the air.
• Check the RH settings. If the RH in the room is lower than the minimum setting, the climate unit will start to humidify; if the RH in the room is higher than the maximum RH setting, the system will start to dehumidify. If the minimum and maximum RH are set too close together, the climate unit may perform both actions simultaneously. This is due to the delay in the control required for a stable climate. Therefore, a difference of at least 2% between the min. and max. RH is necessary; in some situations, 3% is even better.
• Keep the compost temperature under control. In short schedules or in situations where the activity of the compost is not yet under control, a lot of cooling is required. This increases the risk of dehumidification and requires extra attention in the climate control.

Pay attention to the following when using higher fan settings:

• The air supply temperature is stable. It should not fluctuate by more than 2 °C. If the temperature fluctuation is greater, the climate control unit settings are incorrect. In that case, have them checked by your advisor or installer.
• Work with a slightly higher RH setting (e.g. +1%). Bear in mind that at higher fan speeds, you will be working with higher wind speeds in the room. In that case, it is advisable to use perforated (holed) plastic along the two lower beds, where the air hose is located.

Mark den Ouden | Mushroom Blog