Pseudomonas

This disease is caused by a Pseudomonas species of bacteria

The most characteristic symptoms of bacterial blotch are the appearance of dark brown areas on the mushroom cap which are initially light in colour. The affected areas have a greasy appearance and tend to be sticky when touched. The browning is only superficial and rarely goes more than 2 mm below the surface.

The bacterium is very common, especially on peat, limestone, water and compost which are all used for the production of mushrooms.

When the disease is present on a mushroom production unit it can be spread easily by hands, tools, water splash, flies and mites. It is favoured by temperatures of at least 20°C (68°F) together with moisture on the surface of mushrooms which favours its development.

Evidence suggests that the disease will only appear when the population of Pseudomonas tolaasi bacteria reaches a high level. The main aim is to prevent the build up of a large bacterial population. As the bacteria require surface moisture for their development, it is important from the pin head stage onwards that the surface of the developing mushroom is dried off within 2 hours of watering. This is achieved by increasing the air flow through use of a higher fan speed.

Increased air flow is a better means of drying off the crop than increasing the temperatures which usually results in an increased bacterial growth rate and later condensation on cooling. It is thus important to avoid large temperature fluctuations during cropping as these can lead to condensation on the mushroom cap.