194 AMERICAN JOURNAL OF BOTANY [Vol. 9, 



at 53 F. it approaches the average for the higher temperatures. The 

 highest temperature at which the rust will produce infection in wheat was 

 found to be 80 F. under the conditions of the experiments. The figures ob- 

 tained by Johnson (1912) are cited in evidence that it is not failure of the 

 spores to germinate which determines the infection limits observed in the 

 experiments. Johnson (p. 48) found the optimum temperatures for the 

 germination of the uredospores of the common cereal grain rusts to be low 

 12 to 17 C. helping to explain such observations as the difficulty of 

 keeping rust in culture in the greenhouse in the summer, when the incubation 

 period of the rust is shorter than at any other time of the year but it is re- 

 markably difficult to obtain infection ; the difficulty of finding viable uredo- 

 spore material in the spring, the larger number of the spores having already 

 germinated; and the favoring of rust development and epidemics by sub- 

 normal temperatures at the critical infection periods in the life of the host 

 plants. 



Stimulants and Depressants ; Toxic Agents 



Observations on chemical and physical agents stimulating or depressing 

 the vitality of the host plant indicate that susceptibility to rust infection 

 is affected in like manner. 



Sheldon (1903, p. 44) found that in the case of the asparagus rust and 

 the cai nation rust complete immunity to infection can be produced by 

 lowering the vitality of the host an end the failure to achieve which in 

 the case of the brown rust of the bromes caused disappointment to Ward. 

 Concerning the asparagus rust, Sheldon states (p. 44) : 



Attempts have been made repeatedly not only on asparagus but on several species of 

 pinks, to inoculate them when they are not growing well. It was tried on repotted plants, 

 those attacked by insects, and young seedlings. A failure was the result in nearly every 

 case; while with vigorous, growing plants which had become established there were few 

 failures thirty-seven out of forty-two inoculations made at one time being successful in 

 one instance approximately 90 percent. 



In his experiments with the carnation rust Sheldon (p. 83) found attacks 

 by thrips a very disturbing factor, as it was almost an impossibility to 

 secure infection where the thrips had worked to any extent either before or 

 after inoculation, while vigorously growing plants which were free from 

 thrips were readily inoculated. 



Spinks (1913, p. 243) and Voelcker (1912, p. 319) have made observa- 

 tions on the susceptibility to rust of the wheat plants grown in the pot- 

 culture experiments on the fertilizing effects of small quantities of the salts 

 of the heavy metals conducted at Woburn, England. They found that 

 the lithium salts of the 1911 experiments depressed susceptibility, with the 

 exception of lithium nitrate which gave increased susceptibility to rust. 

 The experiments of 1912 were with zinc salts, and they were all found to 

 produce increased susceptibility to rust, zinc nitrate seeming particularly 

 notable in this respect. 



