266 Journal of Agricultural Research voi. xiu, no. s 



Diseased lettuce leaves were collected from a greenhouse at Grand 

 Rapids on January i6, 191 5. Some of the material was placed in moist 

 chambers where it remained until February 20. A microscopic examina- 

 tion showed an abundance of typical spores. A large drop of water was 

 placed on the spot. After a short time this water was drawn up into a 

 capillary tube. Plates of prune-juice agar were planted by making 

 streaks across them with the capillary tube. Fungus colonies appeared 

 in abundance and were transferred to test tubes. After two days, 

 sporulation which was recognized as typical had occurred, and plates were 

 poured b}^ the loop dilution method, with prune-juice agar. A single 

 spore was located with a microscope, and a pure culture was thus obtained. 

 This was increased, and the subcultures were used for inoculation. 

 Twelve lettuce plants growing in 6-inch pots in the greenhouse were in- 

 oculated by means of punctures along the upper surface of the midribs. 

 Control plants were punctured with a sterile needle. In five days typical 

 lesions of the disease had appeared on the inoculated plants, and the 

 organism was reisolated by the loop dilution method, the spores being 

 lifted from an acervulus by means of a sterile platinum spatula. None 

 of the control plants became diseased. A new single spore culture was 

 thus obtained whose relation to the disease had been established by isola- 

 tion, inoculation, and reisolation. 



In other experiments with pure cultures, typical lesions have been 

 produced by placing material on the uninjured surface of the leaf blades 

 and midribs (PI. 20, C). 



During the course of the work attempts to inoculate species closely 

 related to Lactuca sativa — namely, L. floridana, L. scariola, and Cichorium 

 intyhus — failed; but inoculations of all varieties of L. sativa tested — 

 namely, Black-Seeded Tennis Ball, Prize Head, and Grand Rapids 

 Forcing — were found to be uniformly successful. 



INFECTION PHENOMENA 



Experiments started on April i , in which spores from recently isolated 

 cultures were sown on cover glasses which had been smeared with a thin 

 glucose agar and which were subsequently placed over Van Tieghem 

 rings, proved that the time necessary for germination is very short, 

 ranging from four to eight hours, with an average of six hours at room 

 temperature (25° C). This experiment was continued in order to 

 determine the time necessary for spore formation. This period was found 

 to be very short also, spores being produced from newly formed conidio- 

 phores 30 hours after germination of the original spores and being present 

 in great quantities at the end of 40 hours. On April 18, spores from a 

 fresh culture were sown on the leaf of a young lettuce plant, and the pot 

 containing it was placed beside a compound microscope so that the 

 inoculated leaf could be held firmly on the stage. A thin cover glass 



