Apr. J9. i9i8 Anthracnose of Lettuce 277 



According to the method of determining the thermal death point by 

 subjecting spore suspensions of the organism in thin-walled capillaries 

 to various temperatures for 10 minutes, it is seen that the spores of this 

 fungus have a thermal death point of 40° C. for that exposure. The 

 experiment preceding this one showed that their germination did not 

 take place at a much lower temperature. The failure of infection 

 experiments when high temperatures were involved is thus readily 

 explained. 



RELATION OP OXYGEN TO GROWTH 



A suspension of spores of Marssonina panattoniana was prepared from 

 a freshly transferred culture by filtering the entire contents of the test 

 tube through a sterile glass funnel containing glass wool and cotton. 

 A microscopic examination showed that masses of mycelium were sepa- 

 rated from the spores by this process. These were then introduced into 

 a test tube of sterile prune-juice agar ( + 3, Fuller's scale), liquefied at 

 25° C, and thoroughly shaken. The test tube was then laid away 

 with a slanting surface to cool. After two days the spores at the sur- 

 face had germinated and made a luxuriant growth, but those in the 

 interior of the medium had ceased to grow immediately after germina- 

 tion. It may be concluded from this that considerable free oxygen is 

 necessary for the development of the fungus. 



RECOMMENDATIONS FOR CONTROL 



The experiments reported in this paper have shown the usual source 

 of infection of the anthracnose of lettuce, the type of dissemination of 

 the organism, and the relation of the fungus to certain environmental 

 factors. A rational system of control can be based upon these findings. 



The anthracnose in the greenhouse or the field commonly starts from 

 the trash of a preceding diseased crop. In the field, rotation is possible, 

 and this, if coupled with the avoidance of manure containing lettuce 

 refuse, will eliminate two important sources of the disease. If further 

 work should show that the fungus is carried with the seed, then some 

 simple system of seed disinfection can probably be readily applied. In 

 the greenhouse, rotation is for the most part impractical. Here careful 

 sanitary measures must prevail. The common practice in greenhouses 

 is to turn under the refuse from a previous crop and to plant immediately 

 in the soil. Naturally with this type of culture there is abundant chance 

 for the infection of the growing crop. Careful collection and destruc- 

 tion of lettuce refuse is recommended. The fertilizer value of the leaves 

 turned under is negligible. 



The grower should inspect his beds throughout the growing season, 

 and any plant showing anthracnose, drop, or other severe diseases 

 should be promptly removed from the bed. One plant with the anthrac- 

 nose can become a center of infection which may involve a whole bed. 

 41815°— 18 2 



