354 
PRUNE RUST, 
By Newton B. Pierce, 
[Plates XXXIV-XXXVII.] 
The prune, in common with many other drupaceous fruits, frequently 
has its leaves much injured or may be even entirely defoliated by a 
species of rust. This parasite belongs to the genus Puccinia , and is 
known as Puccinia pruni Pers. To Californians the effect of this dis. 
ease on the prune, plum, and the peach is a very serious matter. The 
Pacific coast is known as the home of these fruits, and any widespread 
and detractive disease which affects them all should be well under¬ 
stood by horticulturists. 
DISTRIBUTION AND ACTION ON THE STOCKS. 
Prune rust is widely known, having almost as extended a distribu¬ 
tion as the prune itself. It is found in the United States from the 
Atlantic to the Pacific, and the various countries of Europe are also 
quite generally infected. In the Eastern States it is most injurious to the 
plum and peach. In Texas the peach suffers severely from it, and in some 
sections is completely defoliated. In California the rust has a Avide 
distribution. In the southern part of the State the injury is most seri¬ 
ous in the coast valleys, where both the prune and peach are not unfre- 
quently entirely stripped of their leaves before the wood is matured. 
Much fruit is lost both by the direct and the indirect action of the 
parasite. When the tree is defoliated before the fruit is mature the 
latter shrinks and becomes Avorthless; but the greatest loss generally 
occurs from the nonproductiveness of trees left Avith their wood imma¬ 
ture in the fall. 
As yet only two forms of spores are known for this species of Puc- 
cinia , the uredo or summer spore, and the teleuto or winter spore. 
These spores are produced iu varying proportions on different plants. 
On some hosts the uredospores greatly predominate over the teleuto- 
spores, while on other hosts the reverse is true. It appears probable 
that couditious of food, humidity, climate, and season all tend to vary 
these results. Both spore forms are produced on the under side of the 
leaves, and probably both may, under some conditions, serve as winter 
spores. 
The rust is known to affect the prune, plum, peach, nectarine, apri¬ 
cot, cherry, and almond. In California the prune sustains much greater 
injuries from rust during some seasons than others, depending largely 
on the early or late development of the disease. In 1891, at Santa Ana, 
the rust developed early in some orchards, and much fruit Avas either 
lost or materially injured by the premature fall of the leaves. In 1892, 
m the same orchards, the disease developed later and with much less 
virulence. Where severe enough to defoliate the trees the defoliation 
did not occur until the fruit had been gathered and most of the wood 
