PATHOGENICITY OF H. SATIVUM 45 



ism was studied under various conditions with regard to its specific 

 identity. The physiology and pathogenesis were studied with special 

 reference to environmental conditions most favorable to the develop- 

 ment of foot- and root-rot. 



The organism is capable of causing disease symptoms similar to 

 those described by Pammel, King and Bakke in 1910. Discrepancies 

 are found between the spore measurements of this organism and that 

 described by Pammel, King and Bakke, but since wide variations oc- 

 curred under different conditions in a single-spore culture of the 

 organism studied, the similarity of disease symptoms is considered 

 sufficient justification for considering the organism to be Hclminthos- 

 porium satwuni P. K. B. 



Variations in the morphology of the spores were found to occur 

 under different conditions of growth. For spores as variable in length 

 as those of //. sativum, it was found necessary to measure 500 spores 

 in order to obtain accurate results. 



On potato dextrose agar, significant differences in mean length of 

 the spores occur when the organism is grown at different temperatures. 

 The shortest spores with a mean length of 55.981^0.35 microns were 

 produced at 28 C. The longest spores, with a mean length of 

 67.32 0.55 microns, were produced at 14 C. The difference between 

 the two means is 14 times the probable error of the difference. 



The greatest differences in length were found between spores pro- 

 duced on different substrata. At 24 C. the mean length of the spores 

 produced on potato dextrose was 65.75+0.37 microns, on autoclaved 

 ripe barley heads 67.74=1=0.38 microns, and on green barley leaves 

 83.14+0.29 microns. The difference between the means of the spores 

 produced on the agar and on the leaves is 37 times as great as the 

 probable error of the difference. 



The temperature relations of the fungus were studied and it was 

 found that the mycelium will grow at from i C. to 37 C., the optimum 

 lying near 28. The spores germinated in redistilled water about equally 

 well at temperatures ranging from 6 to 39, but the length of the germ 

 tubes indicated that the optimum temperature is between 22 and 32. 

 Germ tubes penetrated the tissue of both the coleoptile and the leaf at 

 from 12 to 34, but severe infection occurred through a narrower 

 range, from 22 to 30, the disease developing faster at the higher 

 temperatures. Above 30, however, the development of the lesions 

 seemed to be checked, altho they appeared very soon after inoculation. 

 In general, we may say that rather high temperatures are most favor- 

 able to the growth of The fungus, to spore germination, to infection, and 

 to the development of the disease. 



