Pethybridge and Murphy — On Phytophthora infestans. 571 



oospores have been found on one of these media — a modification of Clinton's 

 oat-juice agar ; and further than this the study of a new species of Phytoph- 

 thora,^ which produces a new and characteristic type of rot in the potato 

 tuber, carried on simultaneously with the cultures of P. infestans, has enabled 

 us to throw very considerable light upon the mode of development of these 

 organs in the latter fungus. 



Hence Clinton's results are both fully corroborated and substantially 

 am plified. 



II. Notes on Technique. 



The cultures were obtained in the first instance from affected foliage. 

 On keeping this in the laboratory for a day or two under suitable conditions, 

 plenty of aerial mycelium, bearing conidia, is developed. In some cases 

 conidia were allowed to fall on to the surface of a suitable artificial medium 

 in Petri dishes ; and not infrequently a pure culture could be obtained by 

 removing a small portioji of the mycelium developing from a single conidium 

 to a suitable sterile medium. The important thing is to have the infective 

 material as freshly grown as possible, since under these conditions there 

 is less chance of the presence of the spores of other organisms, such as 

 Fusarium, &c. 



In other cases pure cultures have been obtained by lightly touching fresh 

 aerial conidia-bearing mycelium with a sterile, moistened platinum loop and 

 transference to a suitable slant in a test-tube. Of course, some of the cultures 

 obtained in this way are impure, but with care a considerable proportion of 

 them can be obtained pure from the start. 



Another method employed was to prepare, under as strictly aseptic 

 conditions as possible, blocks of living potato-tuber tissue and infect them 

 from the original material. Fhytophthora infestans develops fairly rapidly on 

 the living potato, and by this means the presence of common saprophytic 

 fungi, such as Penicillium, Mucor, &c., can be avoided. 



Since the fungus does not develop very rapidly on the artificial media 

 used, it was soon found that Petri dishes were entirely unsuited for the 

 prolonged cultures necessary ; consequently nearly all our cultures were carried 

 out on slants in test-tubes, or, if liquids were being used, in shallow layers in 

 small flasks. 



In removing portions of cultures from tubes, steel lancet-pointed needles 

 were used which were kept standing in strong alcohol. Immediately before 

 being used they were removed and the spirit on them ignited. By this means the 



'A full account of this new species (P. erythroseptica) will be found in Scient. Proo. Eoy. Dublin 

 Soc, N.S., vol. xiii, No. xxxv, 1913, p. 529. 



