This fungal disease is caused by the fungus Mycogyne perniciosa and is common although often not serious. When it develops early in a crop and if not controlled, it can cause considerable crop loss.
The most characteristic symptom is the development of distorted masses of mushroom tissue, which are initially white and fluffy but become brown as they age and decay.
Small amber to dark brown drops of liquid develop on the surface of the undifferentiated tissue, especially in conditions of very high relative humidity. It is this wet decay and amber drops on the affected mushroom tissue that gives this disease its common name.
In dry conditions, the distorted masses remain dry in appearance and are very similar to that of Verticillium Disease or dry bubble.
In addition to the distortion symptom, small fluffy white patches of mycelium may occur on the surface of the casing, following the infection of a developing mushroom below the casing surface.
When mature mushrooms are attached, only the base of the stalk may be affected and the cap can develop symptoms on only part of the gills. The later infection, the less distortion.
Contaminated casing material is a common primary source of the pathogen. Generally, symptoms in the first flush indicate contamination of the casing. Compost is not an important source. Spores may survive on the surfaces of buildings or may be carried by crop debris and in this way can contaminate crops.
The disease spreads by spores and mycelium. Once the pathogen is established in the crop, the main means of spread is by water splash and by excess water running off the beds. Pickers may also spread the pathogen on their hands, on tools, cartons and clothing.
One of the most important means of control is the elimination of the primary sources of the pathogen. This can be achieved by paying strict attention to hygiene. It is particularly important to ensure that the casing materials are stored in a area that will not become contaminated by debris and dust from the growing rooms. Once the pathogen becomes established in a crop, spread must be minimized. All affected mushrooms should be carefully removed. Covering affected mushrooms with a cup, alcohol or salt is an alternative to removing them. As water is one of the most important methods of spread, this should be done only after all diseased mushrooms have been removed. If plastic pots are used to minimize spread, it is essential to push the pots well into the casing, preferably down as far as the compost surface, otherwise sideways drainage of water will disperse the spores of the pathogen.