Erysiphe Polygoni. G. 0. Searle. 281 



of the Rape plant had their leaves and stems covered with 

 minute black spots, which, examined microscopically, were 

 found to be wherever minute flecks of mycelium were present. 

 This mycelium was quite easy to trace to the large numbers of 

 conidia to be distinctly seen scattered of%r the surface. The 

 area of infection was quite local as Kohl-rabi plants at more 

 than five to six yards from the Rape plant only showed very 

 few "subinfections." Under the circumstances it seemed to be 

 a permissible conclusion to take the Rape plant as the centre 

 of infection. 



A few days later the same effect was seen in the case of 

 Cabbages, again round a chance plant of Rape growing amongst 

 them. The appearance in this case was exactly similar to the 

 " subinf ection ' on the Kohl-rabi plants and again the Cabbages 

 further away from the Rape plant were unaffected. 



Then the phenomenon was noted on Brussels Sprouts; in this 

 case they were some plants growing near the Swede plots used 

 in the first investigation (A). Here the "subinfections" were 

 much more distinct and generally, though not entirely, confined 

 to the petioles; they showed up distinctly to the naked eye as 

 patches, \ to \ inch across, of minute black spots, the patches 

 being rather symmetrical in shape as though the overlying 

 mycelium was formed by the growth from a single conidium. 

 In every case a few conidiophores and conidia could be seen. 



The last case noted was on the cultivated Brassica known as 

 Marrow-stemmed Kale. In this case the appearance was very 

 distinctive as under the microscope the black spots were found 

 to follow the course of the hyphae at regular intervals giving 

 a dendritic appearance and making it look almost certain that 

 the spots were formed wherever a haustorium had entered or 

 endeavoured to enter the epidermal cells. 



Nov. 2'jth. The weather had been exceptionally warm and 

 moist. "Subinfections " on Marrow-stemmed Kale became much 

 more vigorous and such a large number of conidia had been 

 formed that the characteristic discolouration could hardly be 

 seen. However the mycelium had not spread beyond its former 

 limits. The same happened to a slighter extent on the Brussels 

 Sprouts. 



Dec. 6th. Weather decidedly colder. Some "subinfections " on 

 Marrow-stemmed Kale had spread ^ inch beyond the dis- 

 coloured patch without further discolouration being formed, 

 but very few conidia were to be found on the new mycelium. 

 Some " subinf ections " on Brussels Sprouts had now developed into 

 full infections, in this case also without further discolouration. 



Dec. 10th. "Subinfections" on Marrow-stemmed Kale were 

 observed on specimens exhibited at Messrs Carton's stand at 



