Apr. 29. i9i8 Anthracnose of Lettuce 269 



for a week, after which they were removed. The spray was then cut off, 

 and after normal greenhouse conditions had been restored, another set of 

 20 plants was inoculated and placed in the case for the same length of 

 time. The ordinary greenhouse temperature (65° to 75° F.) prevailed 

 during the entire experiment. No appreciable difference could be ob- 

 served in the rapidity with which the fungus spread through the leaf 

 in the two cases compared. Apparently, once the infection occurs, it is 

 quite as easy for this fungus to spread within the host under ordinary 

 conditions as under humid conditions. Aside, therefore, from the relation 

 of water to dissemination and germination, it is evident that temperature 

 plays a more important role than the moisture relations of the host. 



DISSEMINATION OF THE DISEASE 



The source of this trouble in greenhouses is somewhat uncertain. 

 The disease was noticed in 191 6 by Dr. G. H. Coons in an isolated planting 

 of lettuce on new ground which had never borne a crop before. It has 

 occurred suddenly in greenhouses where no previous outbreaks were 

 known. This indicates the possibility of seed transference of the fungus, 

 or else the existence of the fungus as a soil saprophyte. The experiments 

 reported under "Physiological relations of the causal organism" showed 

 that the spores were not able to survive drying on glass for five days. 

 They might, however, be more resistant in the mucilaginous coat of 

 lettuce seeds. The fungus might also be transported in bits of trash 

 carried with the seed. The presence of the fungus in the seed or in the 

 soil has not as yet been demonstrated. 



Natural infection proceeds from the outer, low^er leaves to the inner 

 ones, so there is good ground for assuming that the common source of 

 infection in greenhouses is the soil, or rather diseased trash, which is left 

 to overwinter in the ground. Additional e\'idence to support this view 

 is that diseased leaves which are exposed in the field, packed in sand 

 in cans from January 20 until April i, were found to contain ^aable 

 spores. The fungus spreads to the inner leaves either by local infection 

 or by the actual penetration of the mycelium. Upon stripping a leaf 

 from the head it can be seen that the newly infected areas of the inner 

 leaves are adjacent to the old spots on the outer leaves. The disease 

 thus spreads step by step inward until the whole plant is affected. Spots 

 high up on the leaves come from infection by spores. A careful examina- 

 tion of leaves at the early stage of infection reveals minute discolored 

 spots at a considerable distance from diseased areas. By the aid of the 

 microscope a spore can usually be found on the surface of these minute 

 spots, with its germ tube boring into the tissues between the cells. Dur- 

 ing a dashing rain these spores are possibly splashed high up on the plant 

 from old spots which have begun to sporulate ; or they may be carried by 

 some of the many insects which infest the lettuce plant. The fact 

 before mentioned, that the spots are more numerous in the lower part of 



