Bacterial Blotch (Bacteriosis)

Pseudomonas tolaasii

The bacterial blotch of the mushroom fruit bodies is caused by bacteria called Pseudomonas tolaasii, and also by other species of the Pseudomonas bacteria genus. These bacteria live in all kinds of environments and in an aqueous medium too. That is why there are many ways for them to enter a farm. Usually the spread of bacteria begins from the casing layer but when the sanitary regulations aren’t observed, they can colonize the compost after pasteurization too. The disease mostly occurs summer-autumn time, when the relative humidity and the temperature of the outside air is high enough. Moreover, under improper conditions of cultivation, insufficient ventilation after irrigation causes the mushrooms to remain moist for a long period of time, which promotes the development of the disease. Under favorable conditions bacteria reproduce very fast on the moist mushrooms and due to their vital activity, mushroom caps become covered with yellow-brown spots.

In time, these spots darken becoming chocolate-brown in color and slimy. The damages on mushrooms can be spherical or shapeless, but always superficial and shallow and sticky to the touch. When the infection is severe, spots spread throughout the whole mushroom surface, seriously diseased mushrooms can become deformed, and the caps can decay in places affected by bacteriosis with an objectionable odor appearing. Young pins affected by the disease become brown and don’t develop.

Mushrooms affected by bacteriosis

Bacterial blotch on the mushroom fruit bodies

The initial stage of bacteriosis

Average degree of infection

Severe infection
Researchers discovered that Pseudomonas tolaasii bacteria can move through a layer of water using filaments. If there are droplets of water or water layer on the mushroom cap, nutrients escape the mushroom tissue into the water, which gives the bacteria an opportunity to reproduce in these areas. The amount of bacteria doubles in less than an hour. If mushrooms don’t dry in two-three hours after irrigation and if the condensate formed as a result of a change in air temperature doesn’t disappear during this time, conditions that promote the increase of the bacterial population begin to form. The first symptoms of this disease form on this stage – the yellow-brown spots. And even though the bacterial vital activity doesn’t affect the mushroom productivity at all, mushrooms loose their marketable appearance and become impossible to sell.

On some farms, bacteriosis is an epidemic disease (constant disease). Bacteria can survive between separate batches on various surfaces, in waste, on tools and equipment. When the infection is present, bacteria are easily transferred from room to room on the pickers’ hands and on crates, ladders and other objects. Mushroom flies and mites also spread the disease. Moreover, if the bacteria are present only in one area of the bed, there are easily spread by water at irrigation. The first disease symptoms appear in areas of caps that have been moist for a longest time, that is, the areas where mushroom caps touch.

Bacterial blotch usually appears in the end of the cultivation cycle, when ventilation is weaker and mushrooms are not cared for enough.

From everything above-stated, a conclusion can be made – a mushroom grower’s main goal is to create proper climatic conditions. Since high relative humidity (above 85%) and a temperature higher than 20°C cause the appearance of first disease symptoms, the climate has to be controlled in a way so as to escape such conditions. Droplets of water and condensate that has been formed on the mushrooms after irrigation have to dry in 2-3 hours. For this case, active ventilation and air circulation is used so that the surface of mushrooms dries as quickly as possible. Here, it’s important to make sure that the mushrooms don’t become chapped and scaly. Moreover, in autumn and winter time, the air temperature can be decreased before irrigation, and increased right after it, in order to create shortage of humidity and ensure evaporation from the surface of the developing fruit bodies.

During summer, when the air temperature and the relative humidity are high, there’s a risk of condensate being formed on the surface of the much cooler casing layer and mushrooms. In this case, less water must be used at irrigation.

Don’t allow the temperatures to fluctuate, especially when the relative air humidity is high – constant temperature will not allow the formation of condensate.

Some authors recommend watering mushrooms with chlorinated water (125 ml 10% chlorine solution for 100 l of water per 100 m²) before first break, when the pins’ size is 4-5 mm, and at heavy irrigation, use 10% chlorine solution in the last 100 l per 100 m².

It is assumed that the main source of infection on the mushroom farm is the casing layer that is why it is important for all its components to be «clean». In places where the disease occurs constantly, the treatment of casing layer with formalin or steam might be very necessary. All materials of the casing layer should be stored in non-infected yards before and after they’re mixed.

The disease can be prevented if sanitation and hygiene are maintained on the farm and around it. If the disease occurred anyway, remove all diseased mushrooms and take measures to prevent the spreading of pathogenic organisms by the pickers’ hands and on the inventory, also at irrigation.