juiy2 4 ,i9i6 Disease of Potatoes Known as "Leak" 631 



grown on string-bean and corn-meal agars, Pfeffer's plant agar, and the 

 stems of M. alba. Neither conidia nor oogonia were found when the 

 fungus was grown on the other kinds of media. The fungus produced 

 both sexual and asexual reproductive bodies much more readily in Petri- 

 dish cultures than in tubes. No sporangia or zoospores were seen in any 

 of the cultures made in this study. It is of interest to note that Hesse 

 (7), De Bary (2, 3), Sadebeck (13, 14), and Atkinson (1) are the only 

 writers that to the author's knowledge record having observed the 

 formation of zoospores by this fungus. 



Cultures of the fungus were made from single conidia. To make these 

 cultures some of the agar and mycelium from cultures which were pro- 

 ducing conidia abundantly was ground up in sterile water. Corn-meal 

 agar plates were poured in the usual way. The conidia germinated usu- 

 ally within an hour. The germinating spores were located by examin- 

 ing the inverted plates with a microscope. They were then marked and 

 removed either to agar slants or to Petri dishes. The growth of these 

 single-spore cultures was similar in all respects to that of the original 49 

 isolations of this fungus and to that of the strain of P. debar yanum ob- 

 tained from the Office of Forest Pathology. They produced typical 

 conidia, oogonia, and antheridia in abundance, and the mycelium showed 

 the same characteristics as to branching and the granular structure of 

 the protoplasm. Inoculations were made from these cultures into Bur- 

 bank potatoes with positive results in 90 per cent of the cases. The 

 fungus was reisolated from the rotted potatoes. The results obtained 

 from these single-spore cultures indicate then that only the one fungus, 

 P. debaryanum, was present in all the original 49 transfers. 



The minimum, optimum, and maximum temperatures for growth and 

 the temperatures at which growth was prevented were roughly deter- 

 mined for this fungus. For these experiments Petri-dish cultures on 

 corn-meal agar were made from subcultures of five different isolations of 

 the fungus and from the strain of P. debaryanum obtained from the Office 

 of Forest Pathology. One Petri-dish culture for each constant tempera- 

 ture chamber was inoculated from subcultures from each isolation of the 

 fungus. The growth of the cultures was measured each day for four days, 

 after which the experiment was discontinued, as the culture media in 

 some cases was entirely overgrown with mycelium. 



The minimum temperature at which growth was noticeable in four 

 days was between 5 and 8° C. No growth occurred at temperatures 

 below 5 . The temperature at which growth is most rapid lies between 

 30 and 35 , and the maximum temperature at which growth can occur 

 is between 35 and 40 . The fungus is killed at approximately 40 . 

 Cultures from all five of the isolations from potatoes agreed as to these 

 points, as also did the cultures from the strain of P. debaryanum obtained 

 from the Office of Forest Pathology. The fungus was not killed at tem- 



