S. F. Ahmstbong 91 



1919. Rust varied from mere traces up to a mild attack on the hybrids 

 whilst the susceptible parent had a moderately bad attack. 



As far as these results go, they indicate that crosses between a 

 normally immune variety hke American Club and a susceptible variety 

 give rise to hybrids whose inherited susceptibility is of an intermediate 

 nature. 



This, however, does not appear to hold good when some degree of 

 susceptibihty is possessed by both parents. In a further cross (No. 155) 

 between Wilhelmina and Persian Black (a variety of T. dicoccum), the 

 33 Fi plants raised were all very severely attacked. Both parents 

 grown alongside under exactly the same conditions had only a moderate 

 attack. Possibly the much increased severity of attack in this case may 

 be due to the fact that there is a tendency for the hybrids of this cross 

 to be sterile. 



Two other cases may be mentioned: Rivet (T. turgidum — shghtly 

 susceptible) crossed with Persian Black (moderately susceptible), and 

 Rivet crossed with Chinese White — a very early maturing susceptible 

 variety — produced hybrids which were much more severely rusted than 

 either of the parents. 



In the three last-mentioned crosses none of the parent varieties are 

 completely resistant to attack, and as the other differences between them 

 are unusually great and numerous, it is possible that the increased 

 susceptibility of the hybrids may be largely due to the altered meta- 

 bolism of the plants. 



(e) Possible effect of environment upon the fungus. 



In conducting these investigations, it has not been forgotten that the 

 cause of the disease is also a hving organism favoured, or disfavoured, 

 by factors similar to those that affect the host, and that it must therefore 

 also receive consideration in dealing with the question of variation in 

 susceptibility of the host. 



Undoubtedly weather conditions affect the general spread of the 

 fungus according as they favour the vitality, dissemination, and ger- 

 mination of its spores, or otherwise. But apart from this, the writer has 

 had no evidence in the field that the parasite is directly aided in its 

 attacks by one kind of weather rather than another. The spread of the 

 disease in susceptible plants was as rapid during the hot, dry period from 

 May 13th to June 19th in 1919 as it was in the cool, damp period from 

 June "JOth to July 31st of the sa!ne year. 



