PATHOGENICITY OF H. SATIVUM 



After an incubation of 18 hours at 19 C. no germination was 

 obtained at a hydrogen-ion concentration of pH 2.4. Very slight 

 germination occurred at pH 3.4 ; while at pH 4.4 the germination 

 showed a marked increase, rising steadily until a hydrogen-ion concen- 

 tration of pH 7 was reached. From this point a gradual decrease oc- 

 curred, reaching the lowest point at pH 8.2. At pH 9.2 there was a 

 second rise followed by a gradual falling off, until at pH 12 no germi- 

 nation occurred. After incubating for 18 hours longer there was 

 scarcely any change in the amount of germination on the acid side. 

 There was a slight increase on the alkaline side. 



At the higher temperatures the results were very much the same 

 except that the percentage of germination was increased and the point 

 of maximum germination was shifted slightly to the alkaline side. 

 At both 24 and 32 the optimum germination occurred on the alkaline 

 side of neutrality at a hydrogen-ion concentration of pH 7.8. A much 

 greater increase in germination occurred at the higher temperatures 

 on the alkaline side than on the acid. The average germination after 

 1 8 hours incubation at the different temperatures is represented by 

 the curves in Figure 6. 



32* C 

 24- C 



Hydrogen ion concenrrahon - p // 



3 -? 5 6 7 B 9 SO It l 



Fig. 6. Percentage Germination of Spores of Helminthosporium sativnm in Phosphoric Acid 

 Potassium Hydroxide Solutions of Various Hydrogen-Ion Concentrations 



Webb (14) germinated spores of Aspcrgillus nigcr, Pcnicillium 

 cyclopium, Fusarium sp., Botrytis cinerea, and Lcnzites saepiaria in 

 n/5 mannite solutions in which the hydrogen-ion concentrations were 

 adjusted by the use of H 3 PO 4 and NaOH according to Clark and 

 Lub's titration curve for ortho-phosphoric acid. The results obtained 

 with Fusarium sp. are the only ones comparable with those obtained 

 with H. sativum in the wideness of the range of hydrogen-ion con- 



