Pseudomonas tolaasii, the cause of bacterial blotch is an aerobic, non-spore forming fluorescent bacterium. Pseudomonads are common bacterium easily isolated from field soil, compost and casing. These bacteria are rather closely related and often difficult to distinguish, although a unique feature of the species P. tolaasi is its ability to infect and discolor commercial button mushrooms. The discoloration is pale yellow at the start and darkens to a golden yellow or rich brown color. The blemishes are superficial and decrease fresh mushroom quality for the marketplace. This bacterium is not a threat to human health.
Problems can also occur when conditions get too dry, particularly with low moisture levels in the compost. Mushroom stress can easily occur under these conditions. Subsequent fluctuations in temperature or RH, which in turn result in fluctuating evaporation rates, often result in disease development. Crops are particularly susceptible during later breaks. Heavy pin sets may exacerbate this condition.
Blotch causing organisms are present on the cap all the time growing on nutrients that leak from the mushroom tissue. If the mushroom cap remains wet long enough the pathogen multiplies. A healthy mushroom cap might have 50 million bacteria per cap while a diseased mushroom might have 200 million bacteria per cap. A number of researchers have stated that it is the size of the bacterial population on the cap rather than within the casing that is the determining factor.
If caps remain wet, either intermittently or for long periods there is increased leakage of nutrients that initiates a defense mechanism for the mushroom which results in visible browning of the cap. Under these conditions (cap remaining moist), bacteria can double in population in less than 1 hour, so 50 million becomes 200 million in less than 2 hours and visible disease can occur apparently spontaneously in a healthy crop or during transport or storage. The disease can be spread through compost, casing, water splash, equipment, pickers, insects and mites.
Managing bacterial blotch disease on mushrooms is a matter of chlorinating the irrigation water applied to the crop to a concentration of 150 ppm chlorine, using water that is potable (drinkable) as a source for irrigation water, and, most importantly, inducing the caps of the mushrooms to dry after an application of irrigation water. It is common to include a 2 to 3 hour drying cycle in environmental management after irrigation. During this time the ambient temperature should be raised a few degrees. The humidity should be lowered to below 85%, and the total air flow should remain unchanged or increased by 10-15%. The goal is to lower the humidity in the growing room to induce the mushrooms to dry.
Experience suggests that when the mushroom compost is too dry when it is spawned-less than 60% moisture-the above steps will not eliminate bacterial blotch from the crop. Also, when the source of the peat moss used to case the mushroom beds has changed, bacterial blotch may not be controlled, because some peats foster blotch bacteria more than other peats. Another environmental situation in which bacterial blotch is almost impossible to control is when the external air temperatures are moderate (59-72°F) both day and night, and the air is full of water vapor. In such a situation, the condenser of the air conditioner does not turn on since the air temperature in a growing room is what the grower specified. Since the mushroom growing temperature requirement has been satisfied, the moisture in the outside air is not condensed on the cooling coils. In such instances, placing an electric light close to the air temperature sensor will cause the control system to register that the incoming air is too warm. The condenser will begin to operate, which will remove some of the excessive water from the incoming, ambient air.
There are a few reports that some wild species of Agaricus bisporus possess resistance to bacterial blotch. However, choosing a strain of A. bisporus based exclusively on its susceptibility to bacterial blotch may not be in the best interest of production at a facility. Managing bacterial blotch is not simple and sometimes the best efforts fail. This approach allows a producer to choose a strain well suited for the unique environmental conditions at each facility.