Phytophthora infestans in Irish Potato 



89 



healthy and diseased tissue, the hyphae could be readily seen ramifying 

 between the cells, as shown in figure 3. The mycelium can usually be 

 found higher up in the stem in the cortex than in the pith cells when the 

 disease is growing up the stem from the infected parent tuber. When the 

 cortex has been destroyed it may be found in the pith cells. So far the 

 author has seldom found the mycelium in the vascular system or the 

 wood cells. Histological studies indicate that the mycelium of P. 

 infestans spreads up the stem most rapidly in the cortical region when 

 conditions are favorable 

 for its rapid growth. 



DEVELOPMENT OF* EPI- 

 DEMICS OF PHYTOPH- 

 THORA INFESTANS 



One argument used 

 persistently against the 

 theory of resting myce- 

 lium being the means of 

 perpetuation of P. in- 

 festans is the sudden 

 and almost simultane- 

 ous outbreak of the dis- 

 ease over wide areas. 

 It has seemed more 

 plausible to many to 

 imagine that some form 

 of resting spore func- 

 tioned in spreading the 

 disease rapidly each 

 year, as is known to be 

 the case in related spe- 

 cies. Massee (20) has 

 questioned the capac- 

 ity of the conidia of P. 

 infestans to start an epidemic. He believes that epidemics start from 

 mycelium of the fungus latent in the tissues which becomes active with 

 the advent of favorable weather conditions. 



In the fall of 191 1 the following experiment was made at Madison, Wis., 

 to learn something as to the development of an epidemic of P. infestans 

 under field conditions, with special reference to the role played by conidia. 

 It should be mentioned that this fungus seldom occurs in the vicinity of 

 Madison, and, so far as known, it was absent from the State in 191 1. The 

 writer is sure it did not occur in the vicinity of Madison that year, and 

 therefore his results were not complicated by its presence. On the even- 



FiG. I. — Cross section of a potato plant, showing the mycelium of 

 Phytophthora infestans, which has killed the cells of the cortex and is 

 a later stage than that shown in figure 3. The mycelium is present 

 among the pith cells. The plant from which this cross section was 

 made became infected like the one in figures. 



