Mo. G. Department of agriculture. t49 



Under iioi-mal conditions, the unaffected parts retain a deep green 

 color, while the diseaised area may be yellowish-brown, dark brown 

 or nearly black. But whatever the color, each area is sharply out- 

 lined. There is no gradual merging of one into the other, but a 

 distinct change of color marks the progress of the disease. Occa- 

 sionally another peculiarity may be noticed. If the leaves are closely 

 examined it will be found that the green and the brown areas are not 

 directly in contact with each other; they are separated by a narrow 

 strip in w'hich the green has been destroyed, and the brown has not 

 yet appeared. It consists of a colorless or at most a very pale yel- 

 low 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 the microscope is an 

 exceedingly difficult matter. Let us suppose that the fungus has 

 succeeded in gaining an entrance, and that it has advanced 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 dis- 

 ease from the fact that there appears, under favorable circumstances, 

 a downy or mouldy growth upon the under surface of the leaves. This 

 is white in color and may be of considierable density'. The upper sur- 

 face of the foliage does not show it, but whenever this frost-like 

 grow^th appears on the under side, it is almost certain that the po- 

 tato rot fungus is present, especially if the other conditions men- 

 tioned above are also present. This external growth consists of 

 spores and of the parts bearing them. The spores, or conidia, ma 

 ture very quickly, and have the power of immediately propagating 

 the fungus. They are small and light, and may be carried long dis- 

 tances by winds. It is largely owing to those bodies that the pro- 

 gress of this potato ddsease is so rapid. They are produced in count- 

 less numbers and are very energetic in attacking healthy tissue. It ap- 

 pears to be very probable, also, 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 water and other me- 

 chanical agents they are brought in contact with the tubers growing 

 underneath the surface of the soil. Here they germinate and effect 

 an entrance in the same manner as occurs above ground. The color 

 of the affected tubers also changes to a brown, dry rot taking the 

 place of the normal white color. The more slowly the tubers decay, 



