39 



Summary. 



In the lettuce forcing industry, which is of great importance in 

 Massachusetts, much loss is experienced by various diseases of the 

 crop, of which rotting is the worst. The amount of loss due to this 

 cause is very commonly 25% and occasionally a whole crop is 

 destroyed. 



This trouble has been prevalent for some time, but its real nature 

 has been very little known. It has been found to be caused by sev- 

 eral -"different fungi entirely distinct from each other, and differing 

 very much in their mode of development and relations to the crop. 



Botrytis vulgaris, the fungus to which the trouble has generally 

 been ascribed, occurs very rarely upon well grown lettuce as a real 

 parasite and is of minor importance. It is commonly associated 

 with the troubles known indefinitely as " Damping Off," " Mildew," 

 " Black Root," and " Rot." 



By far the worst trouble is that characterized by a rotting of the 

 stem and sudden and complete collapse of the whole plant, which is 

 known as the " Drop." 



This disease has been found to be caused by a fungus called 

 Sderotinia Libertiaiia, which has not previously been described on 

 lettuce, but is well known as the cause of many similar diseases of 

 other plants. 



The spread of this fungus in the greenhouse is almost entirely by 

 growth in the soil, where by means of special organs called sclerotia 

 it is able to exist indefinitely between crops and resist all the ordin- 

 ary influences of Nature. 



Another undescribed disease of lettuce has been found which is 

 caused by a species of Rhizoctonia. This is much less prevalent 

 than the Drop and is characterized by a rotting of the leaf blades. 



No serious bacterial rot of lettuce is prevalent in this State. 



By sterilizing the soil either wholly or in part, the Drop and Rhiz- 

 octonia can be completely eradicated or suppressed. 



Experiments show that | in. or | in. surface coverings of steril- 

 ized sand or earth gave a reduction of 47% in the amount of Drop. 

 One inch of sterilized sand or earth gave an average reduction of 

 87% ; \\ in. of sterilized soil an average of 93%, and 2, 3, and 4 

 in. gave 100% reduction or no Drop whatsoever, when not contami- 

 nated by infected material. 



