80 PROTECTION OF PLANTS — 1924-25 



With the object of finding out something more about these bodies and 

 under what conditions they are produced the present investigations were 

 undertaken. About the middle of October 1924 the writer collected from the 

 experimental plots of the Botany Department of Macdonald College some 

 potato leaves infected with Phytophara infestans. Attempts at the isolation 

 of the organism on ordinary laboratory media (potato, potato-dextrose, rice, 

 cornmeal) failed owing to the rapid growth of saprophytic organisms and the 

 unsuitability of the media for P. infedans At the same time, however, the 

 precaution was taken against failure by placing bits of leaf tissue from the edges 

 of the diseased are s on slices of potato tubers cut under sterile conditions. 

 These in moist chambers yielded a vigorous growth of the desired fungus, and 

 after a few transfers to fresh slices of potato it was possible to o >tain mycelium 

 that was purely Phytophtora infestans. This was first transferred to bean 

 agar, but the resulting growth was very slow and poor. Oatmeal agar, 50 g. 

 oatmeal, 20 g. agar, 500 cc. water, made without straining and sterilized at 

 I7y2 lb. for 30 min. proved a most satisfactory medium. Excellent growth 

 over the whole surface of the slant was obtained. In making transfers care 

 was taken to use a large quantity of the inoculum, including some of the subs- 

 tratum. The cultures were kept in a closed cupboard in the laboratory where 

 the temperature averaged about 18oC. Inoculations of potato plants with spore 

 suspensions from one of these cultures resulted in typical lesions of late blight. 

 Sliced tubers inoculated with the mycelium after it had been in culture for four 

 months developed an excellent surface growth. After about six weeks material 

 from the pure cultures was examined carefully under the microscope, and 

 revealed the presence of a small number of brown oospore-like bodies. Only 

 two or three t\ibes, (one especially) out of about 20 showed these structures. 

 From the best of these , transfers were made to another series of tubes and 

 these were kept for three weeks in a humidifier in a closed cupboard. The agar 

 used in making these cultures was strained oat-meal (50 g. oatmeal, 20 g. agar, 

 1000 water). Growth on these was just as good as in the others, and at three 

 weeks when the first examination was made the same brown oospore-like bodies 

 were found. Practically every tube yielded them in greater or less quantity. 

 Whereas in the first cultures they were found singly and only towards the upper 

 and thinner portions of the slant, in these tubes they were everywhere, and not 

 only singly, but in twos, threes, fours and more. They were borne terminally 

 on much-branched, thick, light brown, differentiated, clumped masses of my- 

 celium. Only in two instances were antheridia found and the oospores were 

 apparently still immature. A few attempts at germinating these bodies ha-ve 

 been made, but unsuccessfully. Tubes were placed outside during January and 

 left to freeze. It was hoped that the mycelium would be killed and the brown 

 bodies remain uninjured. From this material transfers were made to fresh 

 oatmeal agar and to potato slices. No growth, however, resulted. Hanging 

 drops containing three or four bodies isolated from fresh cultures also gave nega- 

 tive results. In another case they were dried over sulphuric acid for 48 hours, 



