380 Minnesota Plant Diseases. 



fashion typical for the summer spores of the powdery mildews. 

 These spores give to the surface of the leaf a powdery appear- 

 ance. The relationship of this summer spore to its proper win- 

 ter-spore form, has not been determined, but it is probably con- 

 nected with the very common powdery mildew of wild com- 

 posite flowers (Erysiphe cichoracearum) or some closely related 

 species. It has never been reported as appearing in dangerously 

 large numbers in any greenhouse in this state. It would proba- 

 bly yield to the common treatments for powdery mildews and 

 other superficial parasites, e. g., ammoniacal copper carbonate or 

 potassium sulphide sprays. 



The drop of lettuce {Sderotinia libertiana Fcki). This fun- 

 gus has been found very destructive in eastern greenhouses. It 

 attacks many kinds of lettuce and has been found to be the chief 

 enemy of lettuce culture under glass. The fungus is also remark- 

 able in that it is identical with the cause of a rotting-disease of 

 cucumbers. The drop fungus is a cup fungus. The mycelium 

 is parasitic on the lettuce leaves and stem and attacks the plant 

 very vigorously, producing complete collapse and quick rotting. 

 As the rotting of the leaves proceeds, the fungus threads com- 

 mence to form small storage organs, usually on the lower sides 

 of the fallen leaves. These storage organs or sclerotia are about 

 the size of a large pin head, or slightly larger, and are composed 

 of densely woven masses of fungus threads, stuffed with nutrient 

 material. There are sometimes produced considerably larger 

 sclerotia, and these give rise to the cup form of fruiting body, 

 which bears the sacs on the upper surface. Usually, however, 

 the small sclerotia only are produced in greenhouses and these do 

 not produce the cups. They are, however, very resistant bodies 

 and will survive very unfavorable conditions for a considerable 

 length of time. Thus they carry the disease from one crop to 

 another. Freezing or drying, instead of killing them, accelerates 

 their development when conditions are again favorable. In the 

 latter case, the sclerotium fungus threads resume growth and a 

 fine mold-like mass of threads issues from it. These threads, are 

 vigorous and can immediately infect the lettuce plants. The fun- 

 gus thrives best under conditions most favorable to the growth 

 of the lettuce and the greatest amount of damage is done when 

 the lettuce is about mature. Ordinary methods of prevention, 



