378 



MANUAL OF FRUIT DISEASES 



Symptoms. 



The disease appears in midsummer but is most abundant in 

 fall. Only the leaves are commonly affected ; fruits or other 



parts of the plum rarely 

 show rust-lesions. On the 

 lower surfaces rust-pustules 

 are found (Fig. 110); these 

 are light-brown, small, round 

 and somewhat powdery. 

 They are scattered, or they 

 may be so numerous as to 

 practically cover the leaf 

 (Fig. 110). The develop- 

 ment of these pustules is 

 preceded by the formation 

 of yellowish spots. Later 

 the pustules, or sori, to be 

 observed on the leaves are 

 dark-brown or almost black, 

 but they still retain their 

 powdery nature. 

 Cause. 



The pustules or sori just 

 described are fruit-bodies of 

 the causal fungus, Puccinia 

 Pruni-spinosce. Within the 

 affected leaves of the plum 



the mycelium of this fungus is found. It comes to the sur- 

 face and there forms sori, within which are developed first 

 a crop of spores known as uredospores. These are capable 

 of infecting other plum leaves as well as the foliage, and in some 

 cases the fruit of the peach, almond, cherry, apricot and 

 nectarine. On all these hosts, too, uredospores are developed 

 which are capable of infecting the plum. These spores may 



FIG. 110. Plum-rust, on lower surface 

 of leaf. 



