258 On Forms of the Hop 



inoculations under conditions in which some hundreds of hop-seedlings 

 of the same age became infected. The plant grew during that season 

 to a height of 1 ft. In 1917 this seedling, which was still kept in 

 the greenhouse, entirely lost its immunity. On April 19 there were 

 small but "powdery" patches of mildew on four of its leaves. The 

 leaves were at this date slightly yellowish, as the result of abnormal 

 weather conditions, viz. rapid changes of temperature^. By May 9 

 the stem of the plant, after having been more or less completely checked 

 in growth, was still growing only very slowly; there were now a few, 

 fairly large "powdery" patches of mildew on four of the (upper) leaves. 

 By May 22 the two stems of the plant w^ere only 8 in. high, and still 

 only lengthening slowly; the plant was clearly highly susceptible; the 

 surface of the stem near its apex was covered all round, for a length of 

 11 in., with a continuous growth of the "powdery" conidial stage; 

 several of the young leaves also were infected; and one of the stems had 

 the terminal bud and the two youngest leaves smothered over with 

 mildew and more or less hypertrophied. The plant, which as the result 

 apparently of the severe attacks of mildew never made any very strong 

 growth, reached a height of 2 ft. 10 in. Mildew persisted on it throughout 

 the growing season. This seedling unfortunately died during the winter 

 of 1917-18. It is to be noted that under these same cultural conditions 

 in the greenhouse during 1917 the immune seedlings of the wild hop from 

 Italy all retained their immunity. 



We may ask here what is the explanation of the apparent change 

 from immunity to susceptibility when a plant of Group 3 is transferred 

 from the greenhouse to the hop-garden, and, conversely, of the change 

 from susceptibility to immunity when the plant is brought back from 

 the hop-garden to the greenhouse. The explanation may be sought for 

 in two directions. 



It is possible that there are different strains of the biologic form of 

 *S. Humuli on the hop with different powers of infection, and that the 

 plant when in the hop-garden is attacked by a form which is absent in 

 the greenhouse. The work of Barrus(4) would seem to give evidence of 

 the existence of different strains of a parasitic fungus which are 

 regional in their distribution. Whether such is the case with the Hop- 

 mildew I hope to prove next year by inoculating the same (immune) 

 plant in the greenhouse with spores taken from the same (susceptible) 

 plant in the hop-garden. 



^ The conditions of growtli at tliu time are desciibed more full}' in (_), p. 84. 



