190 Potato Diseases 



Pethybridge^ found that one plant out of nine raised from diseased sets, 

 planted in a cool greenhouse " produced a shoot only 2 or 3 inches high 

 which quickly became diseased from below upwards and soon died. 

 Doubtless the mycehum had entered the shoot from the parent set." 

 The plant was removed but Phytophthora was subsequently detected 

 on several plants in this house. Pethybridge states " although 

 absolute proof is lacking, it seems practically certain that the plants 

 whose foliage became diseased must have become infected by means of 

 ' spores ' from the single diseased sprout sent above ground by one 

 of the diseased sets." It is not made evident from the description 

 that Phytophthora had been actually observed upon this single diseased 

 shoot and its upward course from the tuber traced, so that the experi- 

 ment as described does not demonstrate that the fungus developed with 

 the developing sprout. This important point still needs to be settled and 

 evidence obtained from experiments in greenhouses would need to be 

 supported by evidence from extensive field experiments. 



That diseased tubers may produce healthy plants is a matter of 

 common knowledge. The crucial question in relation to the infection 

 of crops is whether and, if so, in what proportion, will diseased tubers 

 yield diseased plants. A single diseased plant as everyone knows 

 becomes a centre of infection and the possible source of an epidemic, 

 since the fungus possesses the power of rapid growth, rapid sporulation 

 and the spores are rapidly disseminated. It is important to realise 

 that a single diseased plant in, say, a twenty-acre field of potatoes 

 might start an epidemic. What then are the chances for, or against, 

 the occurrence of one bad plant in a twenty-acre field ? Results which 

 show complete failure to obtain Phytophthora in the aerial parts 

 produced by diseased potatoes must be received with some reserve, 

 owing to the small number of tubers planted and since information is 

 usually lacking as to whether the fungus was actually observed or 

 demonstrated to be living or even present in the tubers when planted. 



There are of course other possible methods whereby an outbreak in 

 spring might be occasioned but these have either no positive evidence in 

 favour of them or they have not yet been investigated. 



Payen- in 1853 records that he had observed Botrylis infestans on 

 the tomato and since Payen's time the fungus has also been recorded 

 as occurring upon various wild Solanums. According to Howard S. 



' G. H. Pethybridge in Sci. Proc. Boy. Dub. Soc, xiii (N.S.), No. 2 (1911). p 21. 

 ^ Payen in Dcs maladies des pommes de terre. Paris (1853). 



