300 Agricultural Experiment Station, Ithaca, N. Y. 



separated by a narrow strip in which the green has been destroyed,, 

 and the brown has not yet appeared. It consists of a colorless or at 

 most a very pale yellow line in which the growth of the fungus is 

 probably very active. But during periods which are unfavorable to 

 the development of the parasite this line cannot be discerned, and 

 the green and brown tissues are apparently in contact. Under such 

 circumstances the identification of the disease without the aid of a 

 microscope is an exceedingly difficult matter. Let us suppose that 

 the fungus has succeeded in gaining an entrance, and that it has ad- 

 vanced a limited distance in the leaf tissues. If at this time the 

 weather should turn dry and hot, the development of the parasites 

 would be checked, and the result would be the formation of a small 

 brown spot or area perhaps near the edge of the leaflet, and if 

 several such spots exist the injury might be ascribed, without careful 

 examination, to what is commonly known as the early blight fungus. 

 The name " downy mildew " has been given to the potato rot 

 disease from the fact that there appears, under favorable circum- 

 stances, a downy or mouldy growth upon the under surface of the 

 leaves. This is white in color and may be of considerable 

 density. The upper surface of the foliage does not show it, but 

 whenever this frost-like growth appears on the under side, it is 

 almost certain that the potato rot fungus is present, especially if 

 the other conditions mentioned above are also present. This 

 external growth consists of spores and of the parts bearing them. 

 The spores, or conidia, mature very quickly, and have the power 

 of immediately propagating the fungus. They are small and 

 light, and may be carried long distances b}^ winds. It is largely 

 owing to these bodies that the progress of this potato disease 

 is so rapid. They are produced in countless numbers and are 

 very energetic in attacking healthy tissue. It appears to be very 

 probable, ylso, that these conidia, or summer spores, are the cause 

 of the rotting of the tubers. After maturing upon the leaf, some 

 fall to the ground and by means of w^ater and other mechanical 

 agents they are brought in contact with the tubers growing under- 

 neath the surface of the soil. Here they germinate and 

 elfect an entrance in the same manner as occurs above ground. 

 The color of the affected parts also changes, a brown, dry rot 

 taking the place of the normal white color (see Fig. 98). The 

 more slowly the tubers decay, the less is the amount of moisture 



