DETAILED ACCOUNT OF SPECIFIC DISEASES OF PLANTS 479 



the other pomaceous trees as well, and this may be said of several of 

 the other diseases treated of here that the description of a disease as 

 specifically affecting a certain host, might equally apply to several 

 other host plants. The black-rot fungus not only causes a fruit 

 decay of apples, quinces and pears, but it causes the formation of 

 canker on the limbs of these trees. The fruit rot is the generally 

 recognized form of the disease. The disease begins as. a small spot 

 sometimes near the bud end of the fruit and it spreads until the whole 

 fruit is involved. The apples do not shrink, as in the former disease. 

 The canker form of the disease on the bark of the trees is accompanied 

 by either a roughening of the bark in mild forms of the disease, or in 

 more virulent forms by a destruction of the bark with the formation 

 of depressed areas about which local swellings of the limbs occur. 



The sooty brown, or olivaceous, mycelium penetrates the bark 

 of the tree, hardly extending into the wood. It soon forms pycnidia 

 which are erumpent and surrounded by the remnants of the epidermis. 

 The pycnospores are oblong-elliptic, 22 to 32 by 10 to I4jn, brown in 

 color, and their size varies with the host plant on which the fungus lives. 

 Artificial cultures of the fungus have successfully produced spores. 

 Lime-sulphur solution has been found useful in combating the disease, 

 but pruning and scraping the trees should not be neglected. 



Scab {Venturia inequalis (Cke.) Wint.). — ^The scab also appears on 

 the pear, but mycologists now consider that the scab fungus of the 

 apple is specifically distinct from that of the pear. Earlier mycologists 

 were familiar with the conidial forms of the two fungi, and they 

 were placed under the genus Fusicladium, as F. dendriticum and 

 F. pyrinum, but since the perfect stages have been discovered the 

 species have been put in the genus Venturia. The perithecial stage 

 is saprophytic. Scab is found wherever the apple is grown from 

 Maine to California. 



The fungus mainly attacks the fruit and leaves of the apple, but 

 it has been found on the flowers, flower stalks and twigs. The leaf 

 spots are more abundant on the lower surface, but sometimes also on 

 the upper surface, as a velvety, olivaceous, superficial growth, occasion- 

 ally accompanied by a curling of the leaf. The fruit spots are at first 

 small and olivaceous, and as the mycelium spreads the epidermis is 

 killed and the scabby areas arise (Figs. 164 and 165). Nearly all varie- 

 tes of apple and pear are susceptible, but there is a varietal difference 

 in this susceptibility. 



