94 Journal of Agricultural Research voi. v, nc. a 



this low temperature explains clearly how infected tubers are able to 

 survive the winter season and are in a condition to make rapid 

 growth when placed in the soil. The statements that tubers infected 

 with P. infestans very largely rotted in the ground and that a large 

 majority grew and produced normal plants are both very prevalent in the 

 literature, and the author reports similar experiences in his own experi- 

 ments. These discrepancies, however, may well have been due to the 

 conditions under which the tubers were stored and their state of germina- 

 tion at planting time. Of course, as will be shown later, the influence of 

 moisture and temperature after planting plays an important role. 



From infected seed tubers growing rapidly the greatest number of 

 infected sprouts and shoots were obtained in a saturated atmosphere at 

 a relatively high temperature (23° to 27° C). A temperature of 27° 

 seemed even more favorable than 23° C. This is of interest in view of 

 Vochting's (35) results to the effect that the optimum for the growth of 

 the potato tuber is about 27° and is not out of harmony with the optimum 

 fixed by Jensen (14) for the growth of the myceHum in the potato tuber. 

 How the fungus spreads in the stem and sprout tissues at temperatures 

 between 23° and 27° C. has been described in an earlier part of this 

 paper. The fungus not only traveled up the stem rapidly but also 

 sporulated profusely at such temperatures. In a paper not yet published 

 it is also shown that the growth of liberated zoospores is more rapid at 

 23° to 24° C. than at lower temperatures. This is true also where the 

 vines have been inoculated with conidia and zoospores. Although no 

 experiments have been made to estabHsh the optimum for the growth of 

 the mycelium in the diseased tuber, the data cited above show that the 

 mycelium is very active at 23° to 27° C. Whatever may be the optimum 

 for the myceHum in the tuber, this point is clear: That temperatures 

 between 23° and 27° C. are more conducive to the growth of the mycelium 

 than lower temperatures, other conditions being favorable. 



Although the state of germination of the tuber and the temperature 

 are important, they do not take precedence over moisture. It need 

 hardly be mentioned that P. infestans, by virtue of its phylogeny, is a 

 moisture-loving fungus. To the practical grower it is well known also 

 that an epidemic of late-blight need not be feared in a dry season, while 

 in our northern potato sections a wet season is a sure sign of such an 

 epidemic. The mycelium grows very slowly and absolutely refuses to 

 fruit in a dry atmosphere. It has been shown that the spread of the 

 mycelium is materially retarded when tubers infected with P. infestans 

 are buried in dry soil. Again, the necessity of moisture is well illus- 

 trated in the case of the isolated plantlet referred to. The fungus 

 made little progress in the stem even after reaching the surface of the 

 soil, and it was only by restoring a moist atmosphere that the fungus 

 frmted. It has also been shown that a greater number of the infected 



