Sept. 1894.] 



INJURIOUS INSECTS AND FUNGL 



43 



XII— INJURIOUS INSECTS AND FUNGI. 



Potato Disease. 



The Board of Agriculture think it desirable to give further 

 publicity to the methods which the various experiments 

 reported upon in the Papers [C.-6647 and C.-7138] laid before 

 Parliament in 1892 and 1893, indicate as likely to be useful in 

 checking the spread of potato disease. 



Description of the Fungus. 



The fungus which causes this potato disease belongs to the 

 family designated Peronosporece, and is recognised specifically 

 a? Phytophthora infestans. It attacks the potato plants by 

 means of conidia, or microscopic spores, conveyed either by 

 the wind, by insects, dogs, rabbits, hares, foxes, and by human 

 beings, or by means of spores from the mycelia, or vegetative 

 centres, generated within the growing plants, and coming from 

 infected tubers. In very dry summers, like that of 1893, there 

 is, as a rule, but little disease. On the other hand, it has 

 been observed that the fungus has made rapid progress in a 

 night temperature of from 47° to 50° Fahr. and a day tempe- 

 rature ranging between 57° and 59° Fahr. Generally speaking 

 it may be said that the potato disease is generated, and causes 

 more or less serious injury, in warm, showery weather, especially 

 after electrical disturbances, and makes its appearance between 

 the 25th of June and the beginning of August. 



The first indications of the disease to casual observers are 

 brown spots on the upper surfaces of the leaves. These are 

 caused by the action of the spores of the fungus which have 

 penetrated the under surfaces of the leaves and set up un- 

 healthy action throughout the leaf-structure. On these under 

 surfaces, congeries, or groups, of v/hite silky threads have been 

 formed, from which spores are rapidly generated and distributed 

 by the wind, or other agencies, to infect other potato plants. 

 It is therefore most important to prevent, if possible, the spores 

 from germinating. If this cannot be done, steps should be taken 

 to arrest their progress, to prevent them from forming vegetative 

 cemtres from which may be generated countless quantities of 

 spores, as well as root-like hyphse which, by permeating the 

 leaf tissues, and stalks, cause premature decay, and finally descend 

 to the tubers, making them unsound. 



Means of Prevention and Remedies. 



A review of numerous experiments carried out during the 

 last few years in Great Britain and Ireland, and in many 

 foreign countries, makes it clear that the treatment of the plants 



