Dec. 10, 1913 
Foot-Rot of the Sweet Potato 
267 
abundance of pycnospores are produced on corn meal and rice. On 
beef agar and bouillon, on the other hand, the pycnidia are compara¬ 
tively few and mostly sterile. The development of pycnidia on steamed 
sweet-potato stems was very similar to that found in nature, except that 
the beaks were longer. A fair growth only was made on string beans 
and Irish-potato and sweet-potato cylinders. Com meal and rice, how¬ 
ever, are the only media tried on which the growth could be regarded as 
showing typical development, it being quite apparent in most other cases 
that the conditions were quite abnormal. 
GERMINATION OF PYCNOSPORES 
Germination begins in about 6 % to 7 hours in hanging drop cultures. 
Growth in sterile or hydrant water is slow at first, the germ tube reaching 
only about one-half to one and one-half times the length of the spore in 
24 hours at room temperature (21.5 0 tO22.5°0.). In nutrient media a much 
better growth is made. At the end of 48 hours the germ tube reaches a 
length several times that of the spore and begins branchings. (PI. XXV, 
H.) Growth is fairly rapid thereafter, although if compared with certain 
Fusaria it would be regarded as a slow-growing organism both in arti¬ 
ficial culture and on the host. Preliminary to germination, the pycno¬ 
spores swell up, lose their original shape, and become more nearly 
spherical. 
INFLUENCE OF TEMPERATURE ON THE GERMINATION OF PYCNOSPORES 
The minimum, optimum, and maximum temperatures for germination 
of the pycnospores were determined by inoculating about 2 c. c. of rice 
water in test tubes with spores of the fungus from a young culture on 
corn meal. One set of cultures was placed in each of six thermostats, 
the range of temperature being indicated in Table II. Another set was 
similarly inoculated and placed in the laboratory room as a check. The 
cultures were examined at the end of 18 hours, and those that had ger¬ 
minated freely were thrown out. The other cultures were continued 
for 24, 42, or 48 hours, as necessity required. 
Table II .—Limiting temperature for the germination of pycnospores. 
Thermo¬ 
stat. 
Temper¬ 
ature. 
Time in hours. 
18 
24 
42 
48 
VI. 
°C. 
11.5 to 12.0 
15.0 to 17.0 
21.0 to 22.0 
25.0 to 26.0 
35*5 to 36.0 
37*4 to 37.5 
40.0 to 40.5 
No germination. 
Slight germination 
Fair germination.. 
VIII.... 
Room .. 
X. 
Slight germination... 
Fair germination. 
Good germination.... 
XII. 
XIII. ... 
XIV. ... 
Fair germination. 
Fair germination.. 
No germination. 
