THE POTATO ROT. 107 



if a rain occurs at the proper time, they may be washed into the soil and 

 there germinate and penetrate the tubers, in which they develop and cause 

 the rot. Probably the greatest damage to the tubers from this rot comes 

 from spores which reach them in this way. Tubers placed in the ground 

 have been induced to rot by placing tops, containing spores of the fungus, 

 on the surface above, and applying water. Professor De Barry, who tried 

 this experiment, has repeatedly found the germinating spores in the soil. It 

 is also probable that the mycelium, or vegetative portion of the fungus, is 

 able to reach the tubers from the diseased tops by passing downward in the 

 stem, though it is not certainly known that potatoes become diseased in this 

 way. Certain it is that the fungus threads which live over winter in partly 

 diseased tubers pass upward in the germinating shoots in the spring, and 

 cause disease in the tops. 



So far as is known this is the whole history of the fungus which causes 

 the potato rot. It is produced from spores which germinate on any part of 

 the plant, apparently preferring the leaf; enter, and live mostly within. It 

 passes the winter in partly diseased tubers, from which it reaches the grow- 

 ing tops the next season, where it produces the next crop of spores. No 

 process of fertilization is known, and no true sexual fruit produced, as in 

 most plants, though in certain closely related fungi, one of which lives on 

 the grape, such fruits, called resting spores, are known, and are an important 

 feature in enabling the fungus to pass the winter. Mr. W. G. Smith of 

 England, thinks that he has discovered similar resting spores, and a process 

 of fertilization, in the potato fungus, which he describes and figures in the 

 last (ninth) edition of the Encyclopedia Britannica and elsewhere; but other 

 botanists think that what he has seen belongs to another fungus. 



EEMEDIES. 



A knowledge of the cause may not always enable us at once to provide a rem- 

 edy, and most of the remedies we have for the potato rot were discovered before 

 the true cause was known, and were based on entirely erroneous theories. Still, 

 an earlier knowledge of the cause would have saved the waste of much val- 

 uable time and may now enable us to Judge somewhat of proposed remedies 

 before trying them. 



1. It is evident from the manner of growth of the fungus, almost wholly 

 within the i otato plant, that we can expect little from remedies directly applied, 

 and applications of sulphur, lime, etc., which are destructive to fungi growing 

 mostly on the surface of plants seem to have little effect here. Still, we can- 

 not say that it is impossible that there may be something applied to the leaf or 

 roots which, absorbed into the tissues, will destroy the fungus without injuring 

 the plant. Recent ap[)arent success in France in checking the related grape 

 peronospora, by applications to the leaves, gives ground for hope. So far as we 

 now know there is more reason to expect success from the application of lime, 

 ashes, sulphur and salt to the soil, which, while they assist the growth of the 

 plant, may perhaps destroy the germinating spores of the fungus. There 

 are cases in which the rot appears to have been prevented by such applications, 

 but we need more experiments on remedies of this kind. 



2. Another plan which will suggest itself, considering that the disease seldom 

 appears before about the first of Angus , is to plant early and have the crop as 

 far advanced as possible by thiit time. In this way early varieties can be made 

 practically safe from the rot; for even if they are not entirely ripe and out of 



