Pethybridge — The Verticillium Disease of the Potato. 83 



down the stalk, but in a somewhat less vigorous fashion than where the stalks 

 remained on the plant. 



Some of the tubers produced by inoculated plants in 1914, which investi- 

 gation showed contained the mycelium of the fungus in their vascular tissues, 

 were planted in 1915, and gave rise to diseased plants, which could not be 

 distinguished from similar plants arising from infected tubers produced in the 

 previous season by naturally diseased plants. 



Berthold and Eeinke carried out infection experiments with the fungus, 

 but, of course, not in pure culture. A considerable number of them gave 

 negative results, particularly when the inoculations were made only into the 

 parenchymatous tissue of tubers, or that of the cortex of stems. Positive results 

 were, however, obtained, when the inoculation wound included a portion of 

 the woody tissue, and the inoculated plants showed their type A of the 

 disease. 



The experiments described conclusively show that Verticillium albo-atrum 

 is an active parasite capable of luxuriant growth and development in the 

 larger vessels of the woody tissue of the potato, and that it is the direct cause 

 of the disease described. 



In the cases of the disease studied by the present author the source of the 

 infection of the plant has generally been due to the planting of affected 

 seed-potatoes, and no experiments have yet been carried out to ascertain the 

 way in which primary infection of plants derived from healthy tubers might 

 occur under natural conditions. Eeinke and Berthold showed that when a 

 young root was brought into contact with the mycelium of the fungus, the 

 latter penetrated into the superficial tissues of the root, but apparently it did 

 not reach the vascular tissues. Infection experiments with conidia placed on 

 the surfaces of young roots did not succeed, as the conidia did not germinate. 

 It seems probable that primary infection may occur directly from the fungus 

 living as a saprophyte in the soil ; but further experiments in this direction 

 with pure cultures of the fungus are necessary before a definite conclusion 

 can be drawn. Miss Dale 1 claims to have isolated Verticillium albo-atrum 

 from a sandy soil, and cultivated it on various media. Since it is stated that 

 the fungus in all its stages is a pure white, it might seem improbable that it 

 was in reality V. albo-atrum. Nevertheless, in view of the fact that the fungus 

 when grown for a long period as a saprophyte gradually loses its power of 

 producing black mycelium, it is possible that the same loss had occurred 

 owing to a prolonged period of saprophytic growth in the soil. In a case of 



1 Dale, E., On the Fungi of the Soil. I. Sandy Soil. Annales Mycologici, x. No. 5, 1912, 

 p. 465. 



