4 BULLETIN 1118, U. S. DEPARTMENT OF AGRICULTURE. 
of which are scabby lesions, others coalescing to form more flattened 
scabs. In severe infections numerous lesions cause a serious dis- 
tortion of the leaf (PL VIII, Fig. 1; PL IX, Fig. 2), which sometimes 
drops as a result of the infection. Later, the lesions become warty 
(PL VI, Fig. 4) and assume a pinkish and still later a dirty olivaceous 
color, largely as a result of invasion by saprophytic fungi. 
Fruit. — The incipient stages of citrus scab on the fruit are essen- 
tially the same as on the leaf, except that the outgrowths are solid. 
The lesions soon become cream-colored or pale yellow-orange and 
decidedly raised, resulting in more or less distortion of the fruit (PL 
III, Fig. 1). In a few months these spots lose their cream color and 
usually assume a dusty gray hue. On the grapefruit these protu- 
berances when few in number eventually flatten out, and the fruit 
regains its normal shape (Pis. X and XI). Lesions on the sour 
orange and the lemon seldom flatten, but ultimately produce ugly, 
corky, slightly raised warts (PL V, Fig. 2; PL VI, Fig. 2). The out- 
growths on tangelos are higher and more pointed than on grapefruit 
and do not flatten as the fruit grows (PL XII, Fig. 1). 
Twigs. — Infection of twigs takes place for the most part on very 
succulent growth, such as rapidly growing nursery stock and involves 
only the most susceptible varieties (PL VIII, Fig.l; PL IX, Fig. 1). 
The lesions appear as small cream-colored slightly raised warts, 
later becoming somewhat pinkish and still later taking on a dusty 
color, due to secondary invaders. 
Several types of injury may be confused with citrus seal). Prob- 
ably the most frequent confusion is between the so-called thrips 
injury and old flattened scab lesions. The scars produced by the 
former are never raised and are much smoother than those caused 
by scab; they frequently have a characteristic silvery sheen, and the 
position and pattern are usualty distinctive. Another type of injury 
which causes confusion in the field, even to pathologists, is wind 
injury to young leaves caused by blowing sand or rubbing effects. 
This condition is found only on trees exposed to sweeping winds in 
such positions as are found near a wide expanse of open land or a 
lake front. Frost injury at a distance may also be mistaken for 
citrus scab. The minute features are quite distinct. 
Citrus canker has been confused with citrus scab because of its 
warty and corky appearance. An individual citrus-canker lesion 
grows to be much larger than one of citrus scab. It extends much 
deeper into the spongy tissue of the fruit rind and entirely through 
the leaves. The canker spots are surrounded by a yellowish oily 
zone, which is very distinctive. Citrus canker readily attacks 
sueculent twigs of the common varieties of citrus. 
