i62 Diseases of Greenhouse Crops 



stricted as yet to the mushroom caves in St. Paul, 

 Minnesota. The trouble was first described by 

 Tolaas.* 



Symptoms. It is characterized by an unsightly 

 spotting of the caps, the severity of which differs 

 in the cultivated varieties, especially the large white 

 kinds. The spots, which do not extend deep into 

 the flesh, appear while the mushroom is in the but- 

 ton stage, or when the cap is fully expanded. The 

 spots are at first pale yellow, but later become a 

 chocolate brown. Though the disease does not seem 

 to reduce the yield, the market value of the spotted 

 mushrooms is considerably reduced. 



The Organism, Pseudomonas fluorescens is a 

 small rod rounded at both ends and motile by means 

 of polar flagella. It is a facultative anaerobe; pro- 

 duces no endospores, no gas, but liquefies gelatine. 

 On beef and potato agar, it produces a shiny gray- 

 ish white growth accompanied by a greenish pigmen- 

 tation, which diffuses in the substratum. 



Control. Spraying the mushroom caps with solu- 

 tions of benetol, sodium carbonate, or copper sul- 

 phate seems to have no beneficial effect. On the 

 other hand, fumigating the beds with sulphur before 

 planting the spawn insures the production later of a 

 clean crop of mushrooms. The amount of sulphur 

 to use is about one and a half pounds to each tiiou- 

 sand cubic feet of house space. 



'Tolaas, A. S., Pbytopath. 5: 51-53, 1915. 



