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 
Fic. 110. — Plum-rust, on lower surface 
of leaf. 
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-spinose. 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 
