162 T O R R E Y A 



sis was laid on temperature, other factors such as moisture and soil conditions 

 were determined in the case of cabbage yellows, flax ^^41t, tomato wilt, tobacco 

 root rot, stem canker of potato, and other diseases. An interesting result was 

 obser^-ed in seedling blight of cereals caused by GihercUa saubinetii, an organ- 

 ism causing the disease in both corn and wheat. In corn, severe infection occurs 

 at 16° C. and only slight infection at 24°, while in wheat the temperature rela- 

 tions are reversed. Jones, Johnson, and Dickson (1926) have summarized the 

 investigations. 



Seasonal development influences the reaction of many plants to a particular 

 disease. Waterhouse (1929) found that barley hybrids gave different results 

 in winter and summer months, when inoculated with Puccinia anomala. Some 

 families in winter gave a normal ratio of 3 resistant to 1 susceptible, while in 

 summer the progenies failed to show the expected segregation. Harrington 

 (1931) found that a series of progenies of a cross of Alarquillo X Marquis 

 showed susceptibility as dom.inant with a race of P. graminis tritici at a high 

 temperature, while at a low temperature resistance was dominant. Mains 

 ( 1934) found that hybrids between ^Michigan Amber and Chinese wheat were 

 difficult to classify in their reaction to a race of Erysiphe graminis tritici when 

 grown in the spring, while it was easy to group the hybrid lines when grown 

 in the winter. One parent, ^Michigan Amber, was resistant in the winter and 

 more or less susceptible in the spring. Gordon (1930, 1933) found that some 

 oat varieties showed no significant differences in their reaction to certain 

 physiologic races of Puccinia graminis arenac when grown at four different 

 temperatures from 57.4° to 75.4° F. The Joanette variety, however, was 

 very resistant to some other races at low temperatures and susceptible at high. 

 Peturson (1930) found that Red Rustproof oats was resistant to a race of P. 

 coronata avenae at 57° and susceptible at 70 and 77" . Four other varieties were 

 fully susceptible at all three temperatures, while a fifth variety was resistant. 

 Another aspect of the problem was brought out by the work of Goulden, Xewton 

 and Brown (1930). Some wheat varieties showed no essential differences in 

 reaction to particular physiologic races of P. graminis tritici in the seedling and 

 in the mature plant stage. Other varieties, however, differed markedly in re- 

 sistance in the two stages of plant growth. These results have been confirmed 

 by other investigators. 



4. Diseases caused by bacteria and other organisms. In addition to 

 the diseases of plants caused by fungi, it is now known that many important 

 diseases of plants are caused by bacteria and other organisms. 



Let us recall the fact that Koch (1876) demonstrated conclusively that 

 anthrax of cattle was caused by bacteria. In the period 1878-1883 Burrill car- 

 ried out his studies which showed the relation of fire blight of pears to particular 

 bacteria. Then followed in rapid succession other demonstrations of the rela- 



