110 



EFFECTS OF TEMPERATURE ON FUNGI 



factors, whether favorable or unfavorable to the development 

 of the disease, determines the severity of the attack. 



The destruction of stem tissues of potato and injury to the 

 growing points by Corticium vagum are limited within the range 

 9° to 27° C, [Richards (1921) |, with greatest damage between 

 15° and 21° C. The severity of attack decreases very rapidly 

 above 21° C, and damage is minor at 24° C and above. 



7 C 



100 



80 



ba 



3 

 £ 60 



a 



CO 



O 



40 



20 







100 



w 



80 J 

 I 



60 £ 



so 



.2 



c 

 o 



40 | 

 u 



- 20 







14 17 20 23 26 29 32 



Soil temperature (degrees Centigrade) 



35 



38 



Fig. 6. Relation of the growth rate of Fusarium conglutinans and the de- 

 velopment of cabbage yellows at various controlled soil temperatures. 

 Both have quite the same optima. (After Jones, Johnson, and Dickson.) 



Infection of onions by Urocystis cepulae is governed by soil 

 temperature [Walker and Wellman ( 1926) ] . Abundant infection 

 by this smut fungus occurs at temperatures as low as 10° to 12° C, 

 which is about the minimum permitting germination and growth 

 of onions. Temperatures extending up to 25° C favor infection, 

 but above this point the amount of infection is rapidly decreased. 

 At 29° C and above, the onion seedlings remain free from in- 

 fection. 



Observations of the foregoing type afford a basis in accounting 

 for the seasonal incidence of certain plant diseases and for their 

 geographical distribution. Jones (1924) pointed out that onion 

 smut does not occur in southern Texas, although the pathogen 

 has been repeatedly introduced into this region. The soil temper- 



