Bacterial blotch

Bacterial blotch regularly occurs in the months between September and December, mainly following a spell of warm, humid days. The cause of bacterial blotch is always an evaporation problem. Evaporation problems can occur at various times.

The bacterial blotch seen currently mainly occurs in a later flush. The problem has then often been caused by sprinkling to maintain moisture levels after the first or second flush. It is important that casing soil no longer glistens with moisture a day after sprinkling. Not glistening means the reflection of the lamps in the growing room can no longer be seen in the casing soil. In the winter and during dry spring days this usually happens automatically. Sprinkling causes the room RH to rise. The computer will automatically increase ventilation to remove excess moisture and allow the room to dry. In humid weather this process takes longer, so the beds also stay wetter longer.

How can you prevent casing soil remaining wet for too long?

Firstly, try to give water in the shortest space of time possible. Make sure the water is absorbed by the casing soil. Instead of waiting 1 hour between sprinkling, water again after 30 minutes.

Secondly, use the drying program on the computer. In the drying program various settings can be changed for a certain time. For example:

• set the ventilation position 5% higher
• setpoint R.H. 5% lower
• temperature 1^o C higher.

Change these settings for a 3-hour period. This will dry the casing soil surface quicker and reduce the risk of bacterial blotch.

Mark den Ouden, C point