POWDERY SCAB OF POTATOES. 11 



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 /t 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 balls becomes infected 

 with the disease, as shown by Home's experiments. 



It has also been proposed by Massee (1910) that the plasmodia 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 plasmodium ever assumes such a r61e. 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 



