Nf). 4.] FUNGOUS DLSEASES. 447 



Drop {^Sclerotinia Libertiana, Fckl.). 



This constitutes the most destructive disease of lettuce, 

 and is characterized by the plants wiltintr and dropping into 

 an insignificant mass (see Figs. (5 and 7). This troublesome 

 disease is caused b}^ a sporeless soil fungus, which attacks 

 the stem of the i)lant, and the only effectual remedy is found 



in soil sterilization. 



Ilhizoctonia. 



This is the generic nanu' of the fungus which causes a rot- 

 ting of the lower leaves of lettuce plants, and occasionally 

 works in the stem and head, much to the detriment of the 

 crop. The fungus is a sterile or sporeless type of soil 

 organism, and the same remedy is applicable to this as to 

 the preceding trouble. 



Downy Mildew (^Bremia Latucm, Kegel. ). 



A whitish, downy, mildew growth frequently grows on 

 the lower leaves of young lettuce plants and on the exterior 

 leaves of the mature head. It seldom attacks the normal 

 hej^ltliy portions of the plant, and does not appear to cause 

 a great deal of damage to lettuce crops, consequently little 

 complaint is made in regard to it. We have found that this 

 mildew appears to be incapable of living over the summer 

 season in a house which is empty for a time, and becomes 

 more or less dry. The spores, or conidia, of this fungus 

 are apparenllj^ of very little duration. It is seldom that the 

 funous is troublesome enouiih to call for remedial measures. 



Top Biii'n, or Tip Burn. 

 The above trouble is not caused by any organism, but is 

 due to a lack of proper conditions in the lettuce house. Top 

 burn is merely a wilting of the young, tender leaf extremi- 

 ties, which causes them to dry up and turn brown or black. 

 This greatly disfigures them, and injures to a considerable 

 extent the sale of the produce (see Fig. 8). Amateurs and 

 inexperienced growers are very likely to have top burn. It 

 gives little or no concern, however, to ex[)eit handlers of 

 lettuce crops. The whole matter is one dependent upon the 



