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are much the same on the leaves as on the fruits. There is no forma-. 
tion of a bump below the wart. When the leaves are attacked while 
still very young much the same effect is produced as in the ease of the 
fruit, the leaf surface bulging abruptly outward and causing the warts | 
to appear seated on hollow, conical protuberances. The leaf is often 
considerably thickened where the wart is situated, and the persistence 
of the leaves for at least a year in most cases enables the cork forma- 
tion to proceed further than is usually the case on the fruit. 
Varieties of trees attacked.—Scab attacks only certain species of citrous 
fruits, the sour orange (Citrus bigaradia) being particularly subject to its 
ravages. It was first noticed in the United States on this host. Both 
leaves and fruits of affected trees are often severely injured. The 
ereatest loss, however, is caused by its disfiguring the lemon. It 
attacks the fruits far more frequently than the leaves, and by causing 
the lemons to become bumpy and warty renders them valueless or 
nearly so. On the foliage it is never abundant enough to do serious 
harm. After the sour orange and lemon the Satsuma orange is most 
frequently attacked. This variety, probably a form of Citrus nobilis, 
comes from Japan. On this host scab rarely causes serious damage 
to the crop and is seldom seen on the foliage. In all probability the 
disease was introduced from Japan into the United States on this 
orange. In several localities in Florida the first appearance of scab 
on the sour orange and lemon was shortly after the introduction of 
Satsuma orange trees. The fact of its being usually inconspicuous on 
this host explains why it was not noticed first on this species. 
Scab, or a disease closely allied to it, occurs on very young fruit of 
the lime (Citrus limetta), causing the fruits to fall while still very small. 
It has not been found on the older fruits or leaves of this species. It 
also occurs on the Otaheite orange, a variety of unknown origin, but 
apparently having some lemon ancestry. All the common sweet oranges 
(Citrus aurantium), the Kumquat (C. japonica), tangerine, mandarin, and 
king oranges (C. nobilis), grape fruit and shaddock (C. decumana) seem 
to be nearly if not quite exempt. Certainly no appreciable damage is 
ever caused to these sorts by scab. In Florida, however, it is said to 
occur occasionally on the tangerine and mandarin oranges. In Japan 
it appears to affect the ordinary cultivated oranges, which are largely 
varieties of Citrus nobilis. In Australia the disease is reported on 
orange leaves, but no information has been furnished as to whether or 
not any other sorts are exempt. 
Cause.—Scab is caused by a minute parasitic fungus (Cladosporium 
sp.). So far as known this fungus is found only on the varieties of 
citrous plants subject to the disease in question. It is a very minute 
species and usually forms a delicate, dusky coating, only a few hun- 
dredths of an inch thick, on the surface of the wart. The colorless 
mycelial threads of the parasite creep about in the superficial layers o 
