Cobweb is a fungal disease caused by a number of closely related species of Cladobotryum. In North America, no highly pesticide resistant varieties have been identified to date although resistance seems to be increasing.

Cobweb is characterized by the growth of coarse mycelium covering affected mushrooms. Mushrooms can be attached at any stage in their development. Affected mushrooms eventually turn brown and rot. Sometimes the mycelium of the pathogen may change color, becoming pink or red, and the cobweb appearance is replaced by a mat of mycelium. Also in second or third flushes white dense powdery mycelial growth may occur on the casing surface. Often the most damaging symptom is cap spotting which can occur at any stage if the environment is favorable.

Cladobotryum species are soil inhabiting fungi. The original source of the pathogen on the farm continues to cause debate however in many instances once it is on the farm the spores can recycle and existing crops become the source of newer crops if adequate measures are not in place to prevent spread.

Spores are the main agents of dispersal for Cobweb and Cobweb spore are very readily airborne, particularly during treatment and if an untreated area is watered. Switching off the fans before salting and watering can localize the spread during these procedures. salting is only effective when done in conjunction with a damp tissue. Crops should be inspected and treated regularly (including weekends). Even salting will distribute spores if not done carefully. It is most effective if the disease patch is very carefully covered with a minimum of air movement using a piece of damp paper. The salt is then applied to the paper.

Fungicides legally permitted for use include Bravo® and Mertect® although neither can be effective without a strong sanitation program.

Successful disease control relies upon the management of the inoculum of the disease. The absence of inoculum results in no disease. The presence of low quantities results in much less disease than the presence of large quantities.