POWDERY SCAB OF POTATOES. II 
markedly under diverse environmental conditions. In fact, many 
cases have come to the attention of the writer where the macroscopic 
characteristics mentioned were not in evidence, and yet the typical 
spore balls were found in the sorus upon making microscopic exami- 
nation. It should be especially emphasized that the three differen- 
tial characteristics pointed out may be totally absent after the 
infected tuber has been harvested and roughly handled through ship- 
ment, 
In Plate II are illustrated what may be called common cases of 
Spongospora and Oospora scab. The upper four potatoes are 
infected with powdery scab and the lower two with common, or 
Oospora, scab. 
FUNCTION OF THE SPORE BALLS AND METHODS OF INFECTION. 
The potato crop probably becomes infected by the spore balls 
present in the soil or on the sets when planted. Just how infection 
takes place is not known. Infection studies are made difficult 
because no one has been able to germinate the spore balls in abundance 
at will Massee (1908) claims that the content of each spore is 
liberated as a whole in the form of irregularly globose bodies with a 
few small projections. These bodies show a slow, sluggish move- 
ment for some time and then come to rest. Each amoeboid body is 
about 3 /i in diameter and uninucleate. Johnson (1908) saw motile 
bodies resembling swarm spores in his cultures which he believed 
were the swarm spores of Spongospora, but he states that he never 
saw them escape from the spore. Instead of being uninucleate, he 
found them to have from one to eight nuclei, like the swarm spores 
of Ceratiomyx. Both Osborn (1911) and Home (1911) have 
attempted to germinate the spore balls without being able to confirm 
either Massee or Johnson. It may be that their germination is sea- 
sonal, like the spores of a goodly number of other fungi, or that some 
special stimulus in the soil is necessary to cause them to become 
active. That they function can not be doubted, because clean seed 
planted in soil infested with Spongospora .spore bails becomes infected 
with the disease, as shown by Home's experiments. 
It has also been proposed by Massee (1910) that the plasrnodia may 
become encysted during the winter and resume their activity when 
the tubers begin to sprout, and Johnson (1909) holds that the plas- 
modium may migrate from the diseased parent tuber into the stem 
and stolons of the young plant and ultimately infect the young tubers. 
As suggested by Home, neither of these investigators has proved 
experimentally that the plasniodium ever assumes such a role. It 
can not help but become obvious that more information as to the 
method of functioning of the spore balls and the method of infesting 
