386 Reed : The mildews of the cereals 



Biffen concluded that liability to disease is the dominant 

 character, the hybrids of the first generation being quite susceptible 

 to the attacks of the fungus. In the second generation a portion 

 of the hybrids proved practically immune ; the remaining ones still 

 proved susceptible to the disease. It is interesting to note that 

 Orton,in his work with the watermelon wilt, found that the hybrid 

 of the first generation was immune. 



It must be remembered, of course, that the immunity which 

 plants enjoy against a given fungus is relative, not absolute. Bar- 

 ley plants can be infected with the wheat mildew, if subjected to 

 abnormal treatment. Salmon^^ has mechanically injured barley 

 leaves, subjected them to anaesthetics, alcohol, or water heated to 

 about 50° C, and inoculated them with conidia from wheat. In- 

 fection occurred on such treated leaves. Miss Gibson^* has sown 

 the spores of several rusts on hosts which they are not known to 

 infect. She found that in nearly every case germ tubes were 



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pushed out which passed through the stomata into the mesophyl 

 of the leaf. The parasite may even attain a considerable develop- 

 ment In a plant which it finally cannot fully infect. 



Plants resistant to a particular fungus in one region may be 

 quite susceptible to the same fungus if transferred to another 

 locality. Butler'' states that Farrer's hybrids, resistant to rust in 

 Australia, show a marked Hability to rust in India. Einkorn in 

 general has been found extremely resistant to rust attacks- 

 Howard,^^ however, reports that in India this same wheat suc- 

 cumbed to the attack of Pitcchtia Graminis, due, probably, to the 

 hot season. The plants remained immune until the hot weather, 

 although surrounded by other wheats infected with rust. 



The above-mentioned facts indicate that the external condi- 

 tions of the plant play a prominent part in determining whether it is 

 immune or susceptible to a given fungus-disease. While immunity 

 or susceptibility to disease may be transmitted from parent to 

 offspring, the influence of external factors must also be taken 

 into consideration. 



The study of disease-resistant plants aflTords an extremely 

 interesting field for investigation. As yet we do not know what 

 immunity is. We are unable to state why Common Emmer is 

 free from the attacks of mildew while the Black Winter Emmer 



