Pethybridge — Observations on PhyoOphthora erythroseptica. 191 



Til. Parts of the Pj.ant attacked by the Fungus. 



Up to the time of publication of the previous paper P. erythroseptica had 

 been found only in the tubers, and in them merely in a vegetative condition. 

 Prolonged and exhaustive search for eonidia or oospores in affected tubers, 

 after they had been removed from the soil, failed to reveal them, even when 

 the tubers were so badly diseased as to be almost or quite rotten. The 

 presence of the fungus and its reproductive organs was also sought for, 

 although in a much less thorough manner, owing to limitations of time, in 

 parts of the plant other thau the tubers, but with negative results up to 

 that time. 



From the available evidence, however, it seemed quite clear that the 

 primary source of infection must be the soil ; and it was natural to suppose 

 that, by some means or other, the soil became charged from an unknown 

 source with the resting spores of the fungus. That the disease is contracted 

 from soil is clear from two experiments carried out during 1913. 



Two small plots were made at Clifden in bog-land which, within the 

 memory of living man, had not previously been tilled. One of them was' 

 contaminated by adding to it a mixture of bruised and cut tubers affected 

 with Pink Rot and soil from a plot which had produced plants bearing such 

 tubers in 1912. The other was not thus treated, and served as a coutrol plot. 

 In each plot ten healthy tubers were planted. In tiie control plot two of 

 these failed to produce plants, but in the contaminated plot seven behaved 

 in this way. Unavoidable circumstances prevented investigations being made 

 at the time to ascertain tlie cause of these failures. 



Every plant in each plot was subjected to close observation, and towards 

 the end of the season was dug and examined in detail with the aid of the 

 microscope. 



In the eight plants from the uncontaminated coutrol plot, with the 

 exception of a slight attack of the ordinary blight due to P. infestans, 

 there were no signs of disease, and no traces of P. erythroseptica were 

 found in any parts of them. 



In the three plants from the contaminated plot, on the other hand, 

 characteristic symptoms of disease (different from those due to ordinary 

 • blight, &c.) were observed in the plants before they were dug. The same 

 symptoms were observed in many other cases in plants growing on land 

 which had been cropped with potatoes for several successive seasons, and they 

 will be described presently. After digging, P. erytltroseptica was found to be 

 present, and bearing its sexual organs in the partially decayed underground 



