Minnesota Plant Diseases. ^81 



such as spraying, are not available for reasons that poisonous 

 substances cannot be used on the lettuce and also that the ordi- 

 nary sprays do not affect the sclerotia. The most effective rem- 

 edy is a complete or at least partial sterilization of the soil. A 

 coating of five-eighths or three-fourths inches of sterilized sand 

 or earth will materially reduce the effect of drop, while four 

 inches has in certain experiments completely destroyed all of the 

 disease. The drop can also be greatly reduced by treating the 

 soil with hot water which will raise the temperature of the sur- 

 face to 176 F. to 1 86 F. Lime, sulphur and charcoal applica- 

 tions and coatings of saw-dust and coal ashes have been found to 

 be ineffective against drop. Good ventilation and good drainage 

 will help to keep it in check. 



The grey mold of lettuce (Botrytis vnlgaris Fr.\ This fun- 

 gus appears on greenhouse lettuce, causing a leaf rot. The fun- 

 gus has been described as a saprophyte and it is claimed by recent 

 investigators that it is net at all parasitic. The supposition that 

 it is the summer stage of Sclerotinia libertiana has also been 

 denied. The grey mold of lettuce appears as a fine greyish mold 

 on the rotting leaves. When dry the moldy growth, if shaken, 

 throws off a fine dust of spores which may rapidly spread the 

 disease. The grey mold is probably not responsible for so much 

 damage as is the drop fungus (Sclerotinia libertiana) with which 

 it has been confused. It can be controlled by the same treatment 

 as the drop fungus. 



Leaf spots of violets (Phyllosticta violae Desni. and Cerco- 

 spora violae SaccJ). This is a very common and destructive dis- 

 ease of greenhouse violets. There are two kinds of fungi pro- 

 ducing two kinds of spots, which are, however, not very easily 

 distinguished from each other. The Cercospora spot is, in gen- 

 eral, a cleaner cut spot, while the Phyllo-sticta may be more dif- 

 fused. Both are whitish and have dark centers where the spores 

 are formed, in the former case, in a loose weft, in the latter, in 

 dark spherical receptacles or capsules of very minute size. The 

 two diseases may be intimately associated. When the spots are 

 numerous, the leaf may be killed off, and the entire plant some- 

 times dies. The fungi are both plants of the Imperfect Fungi. 



Bordeaux has been found useful in combating the leaf spots, 

 though it does not entirely prevent them. A quarter strength 



