2 
Journal of Agricultural Research 
Vol. IX, No. i 
The blackened areas frequently spread down the petioles of the leaves 
into the twigs upon which the leaves are borne. If such a twig is young 
and actively growing, the diseased area spreads quickly and the whole 
twig becomes blackened and shriveled. In young orchards, where there 
is much new succulent growth, these blackened, shriveled twigs, bare 
of leaves, are very common; and in this condition there is little simi¬ 
larity between such affected parts and frost injuries. The disease has 
never been found spreading down into the mature wood. 
In March there are periods of several days in which the weather be¬ 
comes very warm and the orchards dry up to some extent. Such weather 
causes the affected tissues at the base of the leaf blade and the affected 
leaf petioles to dry up, become stiff and rigid, and the leaves to hang to 
the tree. The trees in such cases resemble pear trees badly affected with 
pearblight in the young succulent twigs (PI. 2). 
In many cases the lesion spreads from the petiole of a leaf down into 
twig tissue which is still soft but not actively growing, and a small, black 
area around the base of the petiole is formed. After the leaf has dropped 
off and the dry season comes on, brown blister-like scabs are formed over 
such affected areas (PI. A, 1, 2). As the twig continues growing, these 
scabs become loose and may be cast off during the succeeding fall. On 
trees which have been affected in the spring, however, such blisters are 
frequently found in the following winter, and these are presumably the 
sources for the dissemination of the disease. 
DISTRIBUTION OF THE DISEASE 
The disease was first observed in 1912 at Oroville, Cal., by Mr. R. W. 
Hodgson, of the University of California. Since then specimens have 
been received from time to time from the Oroville and Orland regions, 
and it has now been seen or reported in almost all the Citrus-growing 
regions of northern and central California, both on the orange (Citrus 
aurantium U) and the lemon (1 Citrus limonum Risso). It has not been 
observed in southern California, and, so far as is known, no one has 
reported it from there as yet. 
ISOLATION AND IDENTIFICATION OF THE ORGANISM 
On January 24, 1916, fresh material of Citrus blast was sent in from 
Palermo, Cal. Sections through the lesions were made, and the tissues 
were seen to be filled with motile bacterial organisms. Isolations 
made from this fresh material in +10 standard peptone-beef bouillon 1 
produced a clouding on the second day; on plating, these yielded sev¬ 
eral sorts of colonies. One of these types of colonies, on being inocu¬ 
lated into orange trees in the greenhouse, gave positive results con- 
1 All references to acidity are expressed in terms of Fuller’s scale. 
