New York AGRICULTURAL EXPERIMENT STATION. 401 
an inch. In July, several black specks of the size of a small pin 
head appear at the center of some of the spots, on the upper side 
of the leaf. Many spots, however, never show the black specks. 
Affected leaves fall prematurely. In severe attacks the trees 
may be almost completely defoliated by July 1. The disease is. 
most troublesome in wet seasons. It is more prevalent on Long 
Island and in the Hudson Valley than in other parts of the State. 
Treatment.— Leaf-spot is only partially prevented by the treat- 
ment recommended for scab. See page 404. Where the disease 
is troublesome we suggest that this treatment be preceded by a 
spraying with Bordeaux mixture (1-to-11 formula) just as the 
buds show green at the tips. 
RUST. 
(Gymnosporangium spp. Syn. Roestelia spp.) 
Description, etc.— Rust is a disease in which circular, orange- 
colored spots about one-fourth of an inch in diameter appear on 
the leaves in June. It also produces yellow spots on the fruit at 
about the same time. 
It may be caused by several species of fungi belonging to the 
genus Gymnosporanguum. The species of this genus are peculiar 
in that in one stage of their life cycle they live upon the apple 
and some other closely related plants, while in another stage they 
inhabit the red cedar and its relatives. Gymnosporangium macro- 
pus is the most common cause of apple rust. Its other stage oc- 
curs on the red cedar where it produces the so-called cedar apples. 
In New England and some parts of the South apple rust is a 
destructive disease, but in New York it is of rare occurrence ex- 
cept on Long Island. Some varieties are attacked much more 
severely than others. Rust should not be confused with russeting. 
Treatment.— Rust is difficult to combat. Spraying seems to 
check it but little. Since the red cedar harbors one stage of the 
fungus which is the chief cause of rust, it is good policy to destroy, 
so far as practicable, all red cedars in the vicinity of the orchard. 
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