Disease symptoms can vary, usually dependant on the time of infection. The most commonly observed symptom of Verticillium (or dry bubble disease) is brown spotting on the mushroom caps.
Mushrooms infected with Verticillium can also be deformed with swollen and curved stalks, sometimes with strips of tissue peeling and curling upwards. The infection commonly spreads up one side of the mushroom so the stalk and cap do not develop on that side, resulting in a tilted cap.
Another symptom group is characterized by a puffball-like malformation in which infected mushroom pins develop a stunted or shapeless cap often indistinguishable from the mushroom stem. A dull, grey-white bloom of spores covers the entire structure, thus making the term ‘dry bubble’ very descriptive. Infected mushrooms do not rot and they do not smell unpleasant.
Soil, dust, contaminated casing material, infected trash and spent compost are all sources of infection.
Dispersal of disease sports occurs mainly by two methods – water and air. If mushrooms covered with Verticillium are watered, the spores are dispersed on the bed and often onto the floor. If the floor dries out the spores can be distributed by air currents, particularly if the room is swept. Under certain conditions, transport of spores in the air (on dust) can occur over longer distances through people traveling from room to room and at the end of cropping when forklifts are used to move trays around the farm. Spores can also be spread by pickers and mushroom flies. The sticky spores can also cling to watering hoses, harvesting equipment etc.
Some fungicides are registered for use on mushrooms however, these cannot eradicate the disease alone. Disease control measures involving improved hygiene are always necessary.