234 



PKACTICAL BOTANY 



masses are poisonous, and, as er(/otinf, are sometimes used for 



medicinal jDurposes. A parasite known as root rot (' Thielavia ha- 

 sieola') attcK/ks tlie roots of tobacco, horse- 

 radish, and violets, and of peas and other 

 leguminous plants. ' The rose and peach 

 mildew (iSjiJiwrothvcic jjunnosa'),^ which oc- 

 casionally appears as light-colored downy 

 patches upon the fruit of the peach, attacks 

 the leaves of roses and is very destructive. 

 The wilt disease of cotton, cowpea, and 

 watermelon (^JVecwosmospom vasinfeeta')^ is 

 widely distributed o^'er the Southern states 

 and attacks the vascular bundles in such a 

 way as to cut off the plant's water supply. 

 A common disease of plum and cherry 

 trees is black knot (^Plowriglitia morlosa)A 

 The familiar and very destructive dark 

 and shrunken patches on the fruit of the 

 apple are due to bitter rot (^Grlomerella 

 rufiirimculd}!^)} The value of fruit des- 

 troyed b}' it sometimes amounts to millions 

 of dollars in a single year. 



In addition to the conidial forms alread}- 

 considered m connei'tion with their asco- 

 sporic forms and vised as types of their re- 

 spective groups, there remain thousands of 

 species whose life histories are not known. 



Many are saprophytes and many are parasites, some of '\^'hicll 



are very destructive to crops. 



Fio.189. Ergot which 



has grown on a head 



of rye 



After Duggar 



1 Clinton, G. P., "Root Rot of Tobacco," Conn. uV^r. Exp. Sla., 1906. 



2 "Peach Mildew," Bulletin 107, Cnlu. Agr. Exp. Sla., 1906. 



■s " "Wilt Disease of Cotton, Watermelon, and Cowpea," Bulletin 17, Divi- 

 sion of Vegetable Pathology, U. S. Dept. Ai^r., 1899. 



■• Lodeman, E. G., "Black Knot," Bulletin 81, Cornell University Agr. 

 Exp. Sta., 1894. 



^ "The Bitter Rot of Apples," Bulletin 44, Bureau of Plant Industry, 

 U. S. Dept. Agr., 1903. 



