Diseases of the Sugar Beet. 



355 



b. Characters of the Fu7igus. 



The leaf spot of beets is caused by the fungus Cercospora beti- 

 cola Sacc. When the spots on diseased leaves begin to look 

 somewhat grayish in appearance, the reproductive or propa- 

 gative parts of the fungus will be found abundantly. The 

 upright blackened leaves often 

 show this ashen appearance over 

 the entire surface. Examining 

 under the microscope a little of 

 this material scraped off with the 

 knife there will be found numer- 

 ous clusters of short erect hy- 

 phae, such as are represented by 

 the darker lower portion of fig- 

 ure 59. These hyphae bear the 

 spores, or reproductive bodies, 

 represented in the upper portion 

 of the figure. These spores are 

 carried about by the wind from 

 diseased leaves to healthy leaves, 

 and from diseased plants to 

 healthy plants ; where they fall 

 they germinate, if the conditions 

 are suitable ; and on growing 

 again within the tissues of the 

 beet leaf they produce the char- 

 acteristic spots. 



The fertile hyphae are about 35 - 



59. — Fertile hyphae and spores of 

 the leaf -spot fungus. 



55x4-5.* The conidia vary considerably in size, but are usually 

 75-200x3.5-4.5,* although much longer if produced in very moist 

 conditions, or in a moist chamber. Pure cultures of the fungus may 

 be readily secured by the ordinary method of dilution culture. The 

 colonies show up well in the petri dishes after a growth of a few 

 da3^s, as shown in figure 60. The mycelium grows in a close mat, 

 deeply olivaceous in color. The aerial growth is alsooblivaceous at 

 first, and later it is grayish-green. Transfers to sterile bean pods 

 have given excellent growths in pure culture. Cultures of the fun- 



Measurements expressed in micromilHnieters. 



