CHAPTER I 



BLIGHTS 



STEM BLIGHT OF ALFALFA 



Pseudomonas medicaginis Sackett 



HISTORY AND DISTRIBUTION. The disease has been known in Colo- 

 rado since 1904 and was described briefly by Paddock in 1906 and more 

 fully by Sackett in 1910. It is distributed generally over Colorado, and 

 is reported to occur in Utah, New Mexico, Arizona, Nevada, Nebraska 

 and Kansas. 



SYMPTOMS. The disease is primarily a stem infection. In the 

 earliest stages, the stems have a watery, semi-transparent, yellowish to 

 olive green appearance along one side. Soon there oozes from the dis- 

 eased tissue a thick, clear, viscid liquid which spreads over the surface 

 and collects here and there in little bead-like droplets. The exudate al- 

 so dries in a short time with a glistening finish, and gives the stems very 

 much the appearance of having been varnished, and where the liquid 

 has collected in little amber-colored scales and has hardened, it looks 

 as if the varnish had run and dried. Stems in this condition have a dry, 

 slightly rough feel to the touch. The exudate also dries uniformly over 

 the surface or just beneath it, and there produces a dark brown, resinous 

 surface which blackens with age. Such stems are very brittle and 

 easily broken, which fact makes it almost impossible to handle the 

 crop without an immense amount of shattering. The leaves attached 

 to the blighted stems usually show the disease, and sometimes they 

 exhibit the infection independent of the stem. In this case, the 

 petioles become watery and pale yellow, then droop. The malady 

 may be confined to the petiole and base of the leaflet, or it may involve 

 the whole of the blade. Occasionally leaves are found where the 

 inoculation has been made, apparently, in the margin of the leaflet, 

 and the infection has proceeded toward the middle. In such instances, 

 the tender tissue has a watery look, as if it had been bruised. 



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