246 DISEASES OF CULTIVATED PLANTS 



PYRENOPHORA (SACC.) 



Perithecia produced under the cuticle, then breaking 

 through, black, setulose, often produced from a sclerotioid 

 base; asci 8-spored; spores ovate-oblong, muriform, honey- 

 colour or smoky. 



Barley leaf stripe. This disease was at one time attributed 

 to Helminthosporium gramineum (Eriksson), which was con- 

 sidered as an entity. Noack, however, has recently proved 

 that the Helminthosporium is a conidial condition of an 

 ascigerous fungus called Pyrenophora trichostoma^ Sacc. 

 ^ Pleospora trichostoma, Fckl.). 



So far as cultivated crops are concerned the fungus 

 attacks barley more especially, forming long brown stripes or 

 patches with a pale border, which sometimes extend the 

 entire length of the leaf. At a later stage numerous clusters 

 of conidiophores grow through the stomata, or burst through 

 the epidermis, and bear myriads of very large, sausage-shaped 

 or spindle-shaped, dark-brown conidia. These conidia ger- 

 minate at the moment of maturity, and being readily dispersed 

 by wind, etc., an epidemic usually follows .its appearance in a 

 field. Infected plants are often stunted in growth, and do 

 not attain to half the normal size. In many instances the ear 

 does not protrude from the leaf-sheath at all ; in others it 

 partly escapes but becomes curved, and the tip is held back 

 in the sheath. When the ear does fully emerge from the 

 sheath it is usually dwarfed and remains erect instead of 

 becoming curved or ' sickled.' Ravn has proved that the 

 conidia of the Helminthosporium stage adhere to the grain, as 

 in the case of smuts, and when the seed germinates the conidia 

 also germinate and their germ-tubes enter the growing-point 

 of the oat seedling. This explains why the first leaf of the 

 seedling may show the stripe disease, and Ravn concludes 

 that infection only takes place in the seedling stage. 



Potter describes a modification of this disease in which the 

 ears pursue the normal course of development up to the time 

 of flowering, and examination showed that pollination had 

 taken place. At this point, however, further growth of the 

 flower had been arrested, and in place of the normal grain 

 the ovary was represented by a blackened mass of dead cells, 

 permeated with the hyphae of Helminthosporium gramineum. 

 Potter considers it highly probable that in such cases infec- 



