DETAILED ACCOUNT OF SPECIFIC DISEASES OF PLANTS 485 



from it. The leaf-spots are very small brown with reddish-purple 

 borders, when they first appear, and later, when about 4 mm. in 

 diameter, they become ashen gray at the center with the usual margin. 

 They are scattered over the blade and eventually the leaves blacken 

 and dry up, and as the lower leaves die, new ones are formed above 

 until a characteristic elongated crown may be produced. The gray 

 color of the spots is usually associated with the formation of conidio- 

 sphores and conidiospores. The conidiophores are clustered, arise 

 from a few-celled stroma, and push through the leaf stomata. The 

 conidiospores are elongated and needle-shaped, multicellular, 75 to 

 200/1 by 3.5 to 4.5/4, and under moist conditions, the average length may 

 be exceeded. They germinate readily in ordinary nutrient media 

 and the submerged mycelium in agar grows as a dense colony oliva- 

 ceous in color, while the aerial portion is grayish-green. The disease 

 fortunately can be controlled by the use of Bordeaux mixture (4-4-50), 

 and as the spores retain their vitality for some time, early spraying 

 is important and frequent after sprayings. 



Rust (Uromyces beta (Pers.), Tul.).— The beet rust is known only 

 from California. It is common in Australia and not unusual in 

 Europe. Kiihn thinks that the mycelium may be biennial in the host, 

 forming ascia throughout the year. The spermogonia are found in 

 small yellow groups associated with the aecia, which are white and 

 saucer-shaped with aecidiospores 17 to 36/u in diameter, filled with 

 orange-colored contents. The uredinia and telia are irregularly scat- 

 tered over the leaf surfaces. The urediniospores are obovate, 21 to 

 24/1 by 35/1 with echinulate walls, and two opposite germ pores. The 

 short pedicellate obovate teliospores are 18 to 24/1 by 25 to 32/1, 

 with an apical germ pore piercing a wall scarcely thicker at the apex. 



Cabbage (Brassica oleracea, L.) 



Black-rot (= Pseudomonas brassica (Pam.), Sm., Bacterium cam- 

 pestris (Pam.), Sm.) — The cause of the black-rot of cabbage and other 

 cruciferous plants is a yellow, uni-flagellate microorganism, which causes 

 a yellowing of the cabbage leaves accompanied by a black stain in the 

 vascular system, forming a conspicuous black network on a yellowish, 

 or light-brown, background. The badly diseased leaves are shed, so 

 that the stem may have a terminal tuft of badly distorted leaves. 



