PATHOGENICITY OF H. SATIVUM 7 



SPECIFIC IDENTITY OF THE PATHOGENE 



Three species of Helminthosporium are known to occur on barley 

 in the United States. These are readily distinguished on the host by 

 the symptoms. H. gramineum Rabh. causes the systemic stripe disease 

 characterized by long, narrow, yellowish to brownish spots on the 

 leaves and sheaths. Many spots often coalesce to form parallel stria- 

 tions which run more or less the entire length of the blade and often 

 down the sheath. Eventually the leaves may be reduced to shreds. 

 H. teres. Sacc. and H. sativum P'.K.B. both cause local lesions which 

 are characterized by peculiar blotches on the leaves. H. teres causes 

 the European blotch or net blotch disease. The spots are yellowish 

 brown in color, irregular in shape, and are scattered on the leaves. 

 When held to the light, a characteristic net work is apparent. H. 

 sativum causes the spot blotch disease characterized by irregular red- 

 dish brown spots on the leaves. The spots are usually longer than 

 they are broad, and, when abundant, may tend to form stripes. 



These three species also may be distinguished readily by their 

 growth on potato dextrose agar. H. gramineum grows slowly, forms 

 a fluffy, aerial mycelium which does not sporulate (at least not readily), 

 and usually gives the medium a reddish or purplish tinge. H. teres 

 also grows rather slowly. The mycelium grows very close to the 

 surface of the agar. The color of the reverse side of the colony is 

 greenish black. Grayish white tufts of mycelium are formed irregu- 

 larly on the surface of the colony. Cylindrical, thin-walled spores are 

 formed, but usually they are not abundant. In. contrast to both these 

 species, H. sativum grows very readily and sporulates abundantly, 

 forming a flat, black or greenish black colony on agar. The abundance 

 of conidia gives the surface a powdery appearance. Organisms similar 

 to the one isolated from typical barley spot blotch have been isolated 

 hundreds of times by workers in this laboratory from various parts 

 of barley, wheat, and rye plants, and from numerous grasses. 



Pammel, King and Bakke (9) described the spores as cylindric 

 in shape, straight or curved, slender, widest at the middle, from 105 

 to 130 microns in length by 15 to 20 microns in width, pale greenish 

 gray to dark brown in color, with 7 to 14 cells. Later workers have 

 found much shorter spores, altho observations on shape agree fairly 

 well. Johnson (7) states that the spores are narrowly spindle-shaped, 

 usually more or less curved. Mrs. Stakman ( 1 1 ) describes the spores 

 of the organism with which she worked as either straight or curved, 

 dark blue-green to brown in color, averaging 41 by 20 microns in/ size, 

 and containing from 3 to 8 septa. Two types were isolated from dis- 

 eased wheat : one a fuscous type measuring 35 by 22 microns and 

 containing from 3 to 4 septa; the other straw-colored to fuscous, 



