Loose Smut of UaU. 103 



CHAPTER XV. 



Loose Smut of Oats. 

 {Ustilaqo avenae (Pers.), Jens.) 



This is a disease which is known wherever oats are cultivated, and caused 

 enormous losses before methods of prevention were discovered. The spores 

 are produced exclusively in the spikelets, and every part of the inflorescence 

 may be attacked, even the glumes and the awn (Plates VIII. and IX). 

 Th3 ovaries are filled with the spores, and the ovary wall remains as a delicate 

 wall around them. The diseased ears may be seen emerging from their en- 

 veloping leaf-sheaths, and when exposed the membrane is soon ruptured, and 

 the powdery spores are blown about by the wind, or washed away by the 

 rain, leaving only the bare axis of the inflorescence with the ragged remains 

 of the envelopes of the flowers. Generally, all the shoots of a plant and all 

 the grains of the ear are attacked, but in some cases the upper spikelets may 

 be free. In a partially-smutted ear, although the tip may be sound, the 

 base is always affected. In no case has the upper portion of the inflorescence 

 been found diseased, while the lower was sound, thus showing that infection 

 proceeds from below upwards, and that when the inflorescence began to 

 elougata, the smut had not reached the upp3rmost pohit of the panicle. 



The spores are scattered before harvest- time, even before all the oat plants 

 have ceased flowering, and thus many of the smut spores are enclosed by the 

 scales, which gradually envelop the ripening grain, or they may fall on the 

 ground, where they will be ready to germinate under the same conditions 

 which favour the sprouting of the seed oats. 



Germination. 



This has been reported and illustrated in comparatively recent times 

 by Brefeld^ in 1883, by Kellerman and Swingle^ in 1890, and by Herz- 

 berg^ in 1895. Fresh material was gathered in November, 190(), and ger- 

 minated during the same month. When placed in water the spores begin to 

 germinate usually in six to eight hours, and it is more rapid in summer than 

 in winter, and with fresh spores rather than with those kept for several 

 months. A germinal tube or promycelium, occasionally two, is produced by 

 an out-growth of the endospore bursting through the exospore. and the 

 protoplasm contained in the spore passes into the tube. The rupture is 

 sometimes so pronounced that the projecting portions of the exospore are seen 

 along each side o' the base of the tube. Then it becomes divided by three to 

 four transverse partitions into equal compartments. From each of these com 

 par.ments, as a rule, little off-shoots or buds arise, generally near the septa, 

 and almost always one is produced at the apex. These buds constitute the 

 promycelial spores or conidia, and they become elongated and ovate, and 

 then fall away. The protoplasm in each segment is used up in the formation 

 of conidia, and if this is exhausted then no more are formed. But some- 

 times the protoplasm is not all used up, and the primary conidium, instead of 

 falling away, remains attached, and gives off at its free end a small bud. 

 which gradually becomes a secondary conidium, although it does not attain 

 the size of the parent. 



The conidia »^hen detached undergo further changes in water. They may 

 either put forth a very narrow-pointed germ-tube, into which the proto- 

 plasmic contents pass, or two adjacent conidia may become connected by a 

 transverse branch, and tlie (■()nt;'nts pass from the one into the other. Then 



