Apr. 29. jgis Anthracnose of Lettuce 279 



plants, as they say, "on the dry side." This measure is of general benefit 

 with other diseases as well. 



High temperature will also check the anthracnose. Lettuce, however, 

 will not make its best growth under such conditions. Hence, the utili- 

 zation of this factor to prevent the occurrence of the disease is probably 

 of but slight benefit. No doubt in cases of epidemics the disease could 

 be effectively checked by raising the temperature to 30° C. (80° or 90° F.), 

 especially if this were coupled with good ventilation. 



So far, the measures outlined have been preventive. These are cheap 

 and readily applied and in line with the best commercial practice. There 

 is left a direct method of fighting this disease. It is possible to protect 

 leaves of lettuce from this disease by spraying them with a protective 

 coat such as Bordeaux mixture or ammoniacal copper carbonate. The 

 latter, while not so effective as fungicide, avoids the staining which 

 accompanies the use of Bordeaux mixture. This method is to be used as 

 a last resort and should not be necessary if the other preventive measures 

 are followed. 



SUMMARY 



This paper gives the results of experiments with the anthracnose of 

 lettuce and its causal organism. 



The signs of the disease consist, in general, of distinct lesions on the leaf 

 blade and midrib. This is frequently accompanied by dwarfing and 

 wilting. Previous workers had found Marssonina panattoniana {M. 

 perforans) associated with this disease and assumed its etiological rela- 

 tion. This relation has been proved. 



A study of the conditions of infection showed that germ tubes were 

 produced from spores in the average time of six hours. Penetration 

 takes place at a point of contact of the walls of the epidermis. Tem- 

 peratures above 32° C. prevent germination and infection, but excessively 

 wet conditions are not necessary for infection. 



Germination phenomena and the relation to various nutrient media 

 were determined. The spores withstand desiccation on glass only four 

 or five days. The fungus does not grow at temperatures above 30° C. 

 The thermal death point was found to be 40° for 10 minutes. The 

 organism does not grow submerged in agar. 



The source of the organism in new locations is as yet unsolved. The 

 trash from a previously diseased crop is undoubtedly the chief agent in 

 carrying the disease over from year to year. Splashing, as from a hose 

 in watering, proved to be a ready means of spreading the disease. 



Control measures, chiefly prophylactic, based on the findings of this 

 paper are given. 



