Pethybuidgk — Experiments vntli Phytofhthora. 13 



obtained general acceptance at tlie hands of mycologists, and has up to 

 the present not been confirmed by any other worker, success has not yet 

 crowned the comparatively few efforts which have been made to discover 

 such spores. Nevertheless, a more extended and patient search may even yet 

 reveal them. The writer, for instance, has found spores in potato foliage 

 destroyed by Phytophtliora which, judged from a morphological standpoint, 

 might easily be regarded as the oospores of this fungus, but wliich, although 

 experimented with in very various ways over a period of six months, could not 

 be induced to germinate, and so to reveal their nature. Eesting-spores in this 

 case, therefore, must be said to be unknown, but their existence is by no 

 means improbable. 



We also know with certainty that the fungus is found in the form 

 of spawn, or mycelium, within the tubers, and that in them in this form 

 it can pass the winter successfully ; indeed, this is the only way, so far as we 

 know at present, in which tlie fungus can live over the winter. Hence it 

 would appear that, in the absence of resting-spores, diseased tubers must be 

 tlie ultimate source from which the blight starts anew in any given season. 



The question now arises as to exactly how infection of a new crop can 

 occur from the diseased tubers of a former one. It is a well-known and 

 easily demonstrable fact that tlie mycelium in diseased tubers produces at 

 any time, provided the latter are placed in conditions of moderate warmth 

 and moisture, a crop of "spores" on branched hyphae, which grow out 

 from tlie tubers into the surrounding air. Should '' spores " be produced 

 from such a source above ground when the potato plant is in foliage, infection 

 is likely to occur. Even if below ground, there is just the remote chance 

 that such spores might be brought above it by the action of worms, &c. 

 Hence particularly on small farms, where the newly planted crop will not be 

 far away from last year's ground, or last year's pit, around which some 

 diseased tubers are almost certain to be found lying about, there is some 

 probability that infection may occur in this way. But since it seems 

 probable that the chances of infection in this way are not sufficiently wide- 

 spread to explain the constant recurrence of the bliglit, other possible sources 

 of infection have been sought for, and recently tlie view that the potato plants 

 become infected in the field directly from the planted tubers by means of 

 mycelium, and not by " spores," has been revived ; and the object of the 

 present paper is to consider whether this view is supported by any substantial 

 evidence and can be accepted as proved. 



When a potato tuber is affected with Phytophthora, it shows certain 

 characteristic external mnrkings which proclaim the fact. Should there be 

 any doubt in the matter, it is easy to induce the formation of the well-known 



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