1 6 BACTERIA IN RELATION TO PLANT DISEASES. 



infections by bacteria, whose further progress in the plant is also impossible. Moreover, the result 

 of an injection into the living plant of bacteria, even those pathogenic to animals and men, is easily 

 predicted: There is no development in the intercellular spaces, or in extensive wound surfaces 

 there is a wholly insignificant and soon extinguished multiplication. The experiments have exactly 

 so concluded and need no further mention. Nevertheless new descriptions of plant diseases caused 

 .by bacteria keep springing up, and, truly, what worthless descriptions and what non-critical experi- 

 ments. That in diseased plants bacteria are often found in great numbers is certain, but they have 

 here always settled down only saprophytically (Metatroph) upon tissues broken down and destroyed 

 by genuine fungi, and now, of course, help in the further work of destruction, and may also lend 

 to the further progress of the disease a special aspect. But exclusive of other injuries, such as frost, 

 animals, etc., the first attack upon the plant is brought about by fungi, not only in sickenings of 

 uninjured plants, but also in case of wound-infections, which often are greatly extended by fungi 

 and are converted into incurable injuries. From the bacillary gummosis of the grape vine to the 

 scab of the potato, all so-called bacterial diseases of plants are of other origin, the bacteria being only 

 saprophytic contaminations (metatrophe Verunreinigungen), not self-conquering parasites (pp. 131, 2). 



Wehmer's views, propounded in 1898, are not essentially different from the earlier 

 views of Frank, or those of Alfred Fischer. They are sufficiently indicated by the following 

 translation from his long paper on potato diseases : 



Thorough investigations of the bacterial rot are not so far to be found in literature. The few 

 contributions which exist take up at random several specific cases and explain the problem with 

 especial regard only to the bacteria, the tuber as a living organism being very little considered. 

 The conclusions reached as to the "pathogenic" characters of the bacteria are indeed generally 

 accepted to-day, but are not yet really sufficiently well grounded. Moreover, they are not perti- 

 nent, as I shall endeavor to show. * * * por us, in this connection, the first sort of decay 

 (primary rot) is of interest practically to the exclusion of the other, and will be somewhat fully 

 considered in various directions. The question arises here especially whether we actually have to 

 do in these cases with Schizomycetes capable of attacking living sound tissue. This supposition, 

 all things considered, is to be definitely denied: There is manifestly no bacterial sickening of sound 

 tubers; consequently in a literal sense also no "primary" rot — this is always secondary. In proof 

 of which I have gathered together a pretty comprehensive mass of experiments. 



Wehmer also regrets that Frank should have abandoned his former safe position to 

 accept the doctrine that bacteria can be independent causes of disease in plants. 



That other factors are truly primary and that the rot with its bacteria is only secondary has 

 been already pointed out by Frank in opposition to earlier statements (Pflanzenkrankheiten, 2 Aufl. 

 Bd. II, 1896, p. 22). For leaving this standpoint there was really from first to last no reason; on 

 the contrary its soundness was rather to be more exactly established by experiment. * * * 



Bacterial decay is only the last stage of the injury begun by environment, and even where it 

 apparently attacks sound uninjured tubers one can demonstrate without difficulty that such is not 

 the case. But of course it is easier from the simple discovery of the bacteria in decayed tissue to 

 infer the pathogenic action of these organisms; in this way without trouble the numerous plant 

 bacterial diseases, such as fill the literature of the day, are established. 



Smith (1899) criticised Fischer's statements and maintained the existence of bacterial 

 diseases of plants, citing numerous experiments by various people in proof of his contention. 



Fischer (1899) answered Smith, maintaining that no one would doubt his having gone 

 over the literature quite carefully; that all of the statements he had thus far examined 

 rested, manifestly, on inexact observations or worse; that, for a book of the scope of his 

 Vorlesungen, his statements were entirely sufficient; and, finally, that "there has not yet 

 been published a single proof for bacterial plant diseases which meets all the requirements 

 of exact bacteriology." 



Smith then made reply (1899 and 1901) to Fischer's criticisms, defending himself and 

 other investigators, the validity of whose statements had been called in question, illus- 

 trating three bacterial diseases by means of numerous heliotypes from photomicrographs. 

 Since 1901 no one has ventured to question their existence. 



Peglion (1899) describes briefly the following as bacterial diseases: spot of hemp stems, 

 mal nero of the vine, tubercle of the vine, tubercle of the olive, and blight of the mulberry. 



