RESEARCH IN PHYTOPATHOLOGY 205 



in Northumberland is apparently responsible for a large number 

 of diseased roots in the fields. But in the decaying roots 

 bacteria in abundance are invariably associated with the hyphae, 

 and so far, considerable difficulty has been experienced in 

 obtaining a culture of this fungus entirely free from the 

 bacteria. Until this has been accomplished the question of its 

 parasitic nature cannot be decided in this case. Wehmer and 

 Frank claim to have grown Fusariitm solani, from a pure culture, 

 as a parasite upon the potato; but these and other published 

 accounts still leave a doubt as to the absolute exclusion of 

 bacteria throughout the entire experiments. Under natural 

 conditions of infection at least, it remains an open question 

 whether the bacteria or the Fusarium is the secondary factor or 

 whether the destruction of the host-cells is due to their combined 

 influence. In the Erysiphacese again one would certainly expect 

 innumerable bacteria to be present on the leaves together with 

 the fungal hyphae but nothing is known as to their action ; 

 apparently these organisms have been entirely left out of con- 

 sideration here, as in the case of many other fungoid parasites, 



Burri (1903) has shown that an actively living bacterial flora 

 is ordinarily to be found on leaves and that these bacteria form 

 a special class quite distinct from those normally present in the 

 air or soil. The number of bacteria actively existent upon the 

 surface of leaves may be several millions per gram, of leaf, 

 whilst the number of those in the resting condition (presumably 

 accidentally deposited) is always relatively very few. No 

 relationship could be established between the number of bacteria 

 and the atmospheric conditions. Duggeli has also shown that 

 certain bacteria accompany dry seeds or fruits and on germina- 

 tion find their way on to the leaf-surface. 



In addition to the well-known epiphyllous fungus Apiosporiiim 

 (Fiwiago), I have found that other fungoid and bacterial germs 

 are extensively present upon the surface of healthy leaves under 

 the ordinary conditions. This was strikingly exemplified by 

 impression cultures of leaves made upon the surface of a 

 nutrient gelatine, in petri capsules. While still attached to the 

 plant the leaves were lightly pressed upon the gelatine in the 

 capsule, which was only momentarily opened for this purpose, 

 and an impression of the leaf-surface was thus obtained. In 

 every case incubation after two to three days showed numerous 

 colonies of bacteria ; and fungi, chiefly represented by species 



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