A. K HoRNE 180 



advised tiuit potatoes sliould never be saved i'or seed from plants of a 

 diseased crop. 



Massee^ adopted and elaborated this theory claiming that outbreaks 

 of Phytophthora and epidemics over wide areas are due to hibernating 

 mycelium. He was led to the peculiar view of the wholesale migration 

 of mycelium from the parent tuber to its own offspring : 



" The great bulk of disease is due to hibernating mycelium against 

 which no remedy is known." " I have proved by repeated experiments 

 that when a diseased tuber is planted, the mycelium from such tuber 

 passes into the young potatoes which also become diseased." 



" The produce of a diseased tuber is always diseased." " Potatoes 

 from a crop known to be diseased should never be used for seed." 



These authors were committed to a most pessimistic outlook upon 

 the whole problem. 



Now as a matter of fact, sections of healthy tubers invariably fail 

 to show the mycelium of Phytophthora in its accustomed place — the 

 air spaces — whilst microtome preparations through the junction of 

 diseased and healthy tissue, show that the mycelium present in the 

 diseased portion ceases at the boundary line between the two. The 

 question of dormant mycelium in the sense used by Marshall Ward, there- 

 fore, passes beyond the pale of argument or controversy. The conclusion 

 is reached that the disease is not carried by entirely healthy tubers unless 

 the mycelium reside saprophytically in the skin, and of this there exists 

 no positive evidence. Attention should therefore be concentrated upon 

 diseased tubers no matter how slightly diseased, as a means of causing 

 an epidemic. The first question that naturally arises is this, can 

 the mycelium present in diseased tubers maintain itself through the 

 winter until the planting season begins ? Pethybridge states definitely 

 that it can and the writer can confirm his statement so that a positive 

 answer must be returned. Secondly, does this fact explain the infection 

 of crops each season ? Upon planting diseased tubers the fungus might 

 be returned in a living condition to the soil and remain there to infect 

 the young tubers, but no evidence has been obtained on this point. 

 On the other hand, the fungus might pass from the planted tubers to 

 the young shoot. The evidence available in this connection leaves 

 much to be desired. 



Massee^ claims to have found Phytophthora present upon shoots 

 originating from diseased sets presumed to contain this fungus. 



1 G. Massee in Diseases of Cultivated Plants and Trees, p. 125. 



2 G. Massee in Kew Bull, 4 (1906). 



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