36 



Amount of Damage Caused by Fungous Diseases of Lettuce. 



No attempt has been made to ascertain, in dollars and cents, the 

 exact amount of loss due to fungous diseases. The proportion of 

 lettuce plants, however, which succumb to disease is anywhere from 

 15 to 85 or go%. The latter percentages are very exceptional, as 

 growers are not content to experience this loss more than once with- 

 out making radical changes in their methods. Practically entire 

 crops have been destroyed by Drop alone to our knowledge, and the 

 majority of growers in Massachusetts have experienced at one time 

 or another a loss of from 15% to 40%. The loss of 25% from 

 Drop is no uncommon experience in a large number of lettuce 

 houses and when we consider that these houses each may contain 

 from 6000 to 12000 plants, worth from 40 cts. to $1.00 per dozen, 

 some idea of the loss may be obtained. In addition to the loss from 

 Drop, there are others arising from such diseases as Rhizoctonia, 

 Botrytis Rot, Mildew, and Top-burn, but the last three diseases 

 need not enter very seriously into an experienced lettuce grower's 

 calculations. 



Lettuce Diseases in General. 



In addition to the various rots which have already been consid- 

 ered, we have at times in this state two other diseases, namely. 

 Mildew, {Bremia Lactucae Regel.) and the physiological trouble 

 known as Top-burn. Two other diseases of lettuce known as 

 Anthracnose* and Leaf-spot** have been described by the Ohio 

 Experiment Station, but they have never been called to our attention in 

 Massachusetts. In considering lettuce diseases as a whole, we can 

 arrange them into three groups based upon the specific causes which 

 give rise to them. These three -groups of diseases may be character- 

 ized as follows : 



First : Those which occur in apparently healthy plants and which 

 are directly due to pathogenic (disease producing) organisms. 



Second : Those which occur in plants that are abnormal, origi- 

 nating from irrational treatment or from inherent weakness, and 

 which are aggravated by the presence of either pathogenic or non- 

 pathogenic organisms. 



Third : Those which are brought about entirely by abnormal 

 treatment, result in physiological disorders, and as a rule neither 

 pathogenic nor non-pathogenic organisms are present. 



*Ohio Agr. Exp. Sta. Bull. ^z. **Ohio Agr. Exp. Sta. Bull. 44. 



