37 



In the first group can be placed the purely parasitic organisms, 

 such as those which cause Drop and Rhizoctonia Rot. One of the 

 characteristic features distinguishing the disease known as the Drop 

 is that seemingly healthyf lettuce plants fall a prey to it with the 

 same apparent ease as plants presenting slightly abnormal character- 

 istics. We have here therefore a veritable parasite to deal with, and 

 any mode of treatment of this disease must take these facts into 

 consideration. It is quite evident that for the control of diseases 

 in this group we must pay attention to other matters than the green- 

 house conditions, although this class of diseases can be subdued 

 somewhat by changing the conditions under which they are growing. 



The second group of lettuce diseases with which fungi are asso- 

 ciated are brought about primarily by some physiologically abnormal 

 condition, and the fungi in such instances are merely secondary 

 intrusions. To this latter group belong the Botrytis Rot, Mildew, 

 and Bacterial Rot. The first requisite in their treatment or, more 

 properly speaking, their exclusion, consists in understanding lettuce 

 requirements, and paying the strictest attention to the details of 

 heat, light, ventilation, moisture, soil, etc., or in other words to 

 proper cultural conditions of the crop. There are to be sure 

 instances where plants may receive normal treatment from the most 

 skillful gardeners and become diseased, as. for example from the use 

 of poor seed or constitutional weaknesses inherent in the stock. 

 The lettuce Mildew, however, which seldom causes much harm in 

 this State appears from our observations to occur on plants which 

 have received a set-back from transplanting, as it is found largely 

 upon the outer leaves of newly transplanted plants and also upon 

 the older leaves of nearly matured heads. The Mildew was intro- 

 duced into two houses at the Station a few years ago where it 

 existed for only one' season. This fungus propagates by conidia (sum- 

 mer spores) and by oogonia (winter spores). The latter reproduc- 

 tive organs were never observed by us on our plants, and as our 

 lettuce crop was followed by tomatoes, which do not constitute a 



tPlants which have been under cultivation for a miniber of generations cannot be consid- 

 ered in a strict sense as in a normal condition, notwithstanding their capability of respond- 

 ing to skillful treatment in a manner which apparently is normal, or at least not presenting 

 pathological conditions. The modifications wrought in their anatomical and physiological 

 characteristics by long cultivation, influence to a large extent their environmental adapta- 

 tions and render them susceptible to diseases not occurring in their uncultivated state. 



