Laffeuty — Disease of Cultivated Flax. 259 



V. — Proof of Pathogenicity of the Pungus. 



Experiments which proved that Polyspora is pathogenic to flax, and was 

 the cause of "browning" and "stem-break," were carried out under strictly 

 controlled conditions in the greenhouse. Other experiments were carried 

 out in the field, where, of course, less control of the conditions was possible. 

 These infection experiments will now be dealt with. 



Two small lots of flax seed, previously proved by microscopical examination 

 to be free from fungus infection, were sown in pots of soil previously heated 

 in an autoclave at 140° C. for two hours on each of three successive days. A 

 perfectly healthy and luxuriant crop of seedlings arose, and the plants were 

 allowed to grow until the petals of the flowers had fallen and the fruits were 

 beginning to form. The plants in one pot, which served as a control, were 

 then sprayed with sterile water and at once covered with a large glass 

 bell-jar. Those in the second pot were sprayed with a suspension in sterile 

 water of conidia from a thirty-day-old pure culture of the fungus, and were 

 likewise covered with a large bell-jar. 



The experiment was started in August, and the plants were left 

 in an unheated greenhouse. They were carefully examined daily ; but 

 the bell-jars were not removed for this purpose. The first indications 

 that infection had taken place were noticed on the twentieth day after 

 spraying. Circular brown spots appeared on the leaves, and elongated 

 brown areas on the stems and branches, of the plants which were 

 sprayed with the suspension of conidia, thus reproducing the typical 

 symptoms of " browning " as it occurs in the field. After a further lapse of 

 a few days portions of affected leaves, stems, and branches were removed 

 and examined microscopically, when typical acervuli, made up of the conidia 

 and conidiophores of the fungus, were found on the diseased areas in all 

 cases. 



The conidia present in these acervuli were plated out, and the fungus 

 obtained was proved by comparative tests in pure cultures to be identical 

 with that from which the conidia used for inoculation purposes were 

 obtained. At no time throughout the experiment did the control- plants 

 show any signs of disease whatever. 



Two adjacent plots, containing only healthy flax plants, were selected 

 among those on the land attached to the field laboratory. Each plot was 

 four square yards in area, and when pulling time was near at hand the 

 plants in one plot were sprayed with sterile water, while those in the second 

 were sprayed with a suspension in sterile water of conidia from a sixteen- 

 day-old pure culture of the fungus. 



