68 TEXTBOOK OF BOTANY 



spores nor sporidia can infect the wheat, it would seem that 

 the removal of all the barberries within a considerable dis- 

 tance would make the infection of the wheat impossible. 

 But, as a matter of fact, serious epidemics of wheat rust occur 

 year after year in various parts of the world — for example, 

 in Australia and in some of the northwestern United States 

 — in which barberries are very rare or even unknown. So 

 it would seem that the rust is able to survive the winter in 

 some other form than in that of the winter spore. The most 

 probable explanation of this fact seems to be that some of 

 the summer spores live through the winter and infect the 

 young wheat plants in the spring. The summer spores 

 are thin-walled, and apparently most of them are short- 

 lived and easily killed ; but a few are longer-lived and 

 remain able to cause infection for some months after their 

 production. 



Another explanation for the survival and reappearance of 

 the rust in some cases is found in the fact that in warm regions 

 it can live and continue to form summer spores during the 

 winter. So, when new wheat plants appear in the spring, 

 there are fresh spores ready to infect them. The spread 

 of the rust from warmer to colder regions during the growing 

 season of the wheat may be brought about by the wind ; for 

 it has been shown that summer spores can be carried by the 

 wind to great distances. The most promising method of 

 combating the wheat rust seems to be the raising of varieties 

 of wheat that are rust-resistant — that is, are little or not at 

 all affected by the rust. The varieties of durum or macaroni 

 wheat, which are much cultivated in Mediterranean countries, 

 are very resistant to rust, and they are being raised with some 

 success in certain American states. Attempts are also 

 being made, by crossing these with American varieties which 

 are susceptible to rust but are valuable in other respects, 

 to secure new resistant forms ; and already some very 

 promising results have been attained. 



