38 



host for this fungus, and as the soil underwent desiccation, the 

 conidia apparently died out and hence no further infection has been 

 noticed. 



Under the third group of diseases peculiar to lettuce we can 

 include the Top-burn, the cause of which has no special reference 

 to a specific organism, it being brought about by the failure or per- 

 version of the normal physiological activities of the plant. Micro- 

 organisms, however, may not infrequently accompany the diseased 

 tissues. If a sharp knife is plunged into the soil to some depth and 

 passed around in proximity to the lettuce plant so as to cut off the apices 

 of the roots we can produce artificially the characteristic symptoms 

 of Top-burn. This simple experiment teaches us that Top-burn is asso- 

 ciated incidentally at least with root absorption, but there are other 

 elements which come into play in practical greenhouse management 

 which bear upon the prevalence of this disease. Indeed it is pos- 

 sible to fill a house with Top-burn and ruin a crop within the short 

 space of twenty-four hours by disregarding temperature and light 

 conditions. Top-burn is most commonly brought about in the fol- 

 lowing manner. If the night and day temperatures are run high dur- 

 ing a period of cloudy weather and this period is followed by bright 

 sunshine. Top-burn is quite sure to follow. The reason of this is 

 that a high temperature accompanied by cloudy weather causes 

 active growth, which results in producing tissues of an extremely 

 delicate character. When exposed to intense sunlight and rapid 

 transpiration, the edges of the young leaves wilt, collapse 

 and turn black. The remedy for this group of diseases consists 

 therefore in paying strict attention to the details governing healthy 

 plants and, in case of Top-burn in regulating the day and night 

 temperatures in accordance with the conditions of the crop and 

 weather. Skillful lettuce growers understand quite well the cause of 

 the Top-burn and are able to control it successfully. Inasmuch as 

 lettuce plants make the most growth at night, the danger from high 

 temperature at this time is the greatest, especially when the crop is 

 approaching maturity. High temperature at night on immature 

 plants is allowable and during clear days the temperature can go 

 frequently as high as 80° or 90" F. either on mature or immature 

 plants without causing any injury. 



