A. S. HOHNK 



19:3 



althoujLjh no disease luul been detected in tlie lojiuge. An instance 

 occurred in 1909, on a large field in Durham County. I observed no 

 Phytophthora on the foliage during the season, but at the lifting, there 

 were very few plants without one or more diseased tubers (Fig. 1). The 

 uniform infection of the field seemed extraordinary. It could be ac- 

 counted for in three possible ways : 



(1) Phytophthora may have been ])resi'iit on the plant, but in- 

 conspicuous and escaped observation. 



(2) The soil may have become infected with wind-blown conidia 

 from some potato field in a neighbouring farm. 



(3) The soil may have been infected with mycelium or oospores, 

 if such do exist in the soil. 



Fig. 3. Tuber cut open to show the brown rot caused by Phj'tophthora. 



The whole Phytophthora problem needs very careful study with a 

 view to determining whether there is or is not a definite soil-factor, 

 occasioned by the power of the fungus to rest or live in the soil. If it 

 can be shown that there is not, the prospect of eradicating the disease 

 is not hopeless since effort could be concentrated in the selection of seed 

 tubers and widespread and efficient treatment of the foliage. 



A third line of enquiry which relates to the conditions that favour 

 the development of the blight fungus and involves two issues — (1) 

 blighted tubers, (2) blighted foliage — will be pursued in a later paper. 



