12 BULLETIN 1118, U. S. DEPARTMENT OF AGRICULTURE. 
PATHOLOGICAL ANATOMY. 
Microscopic examinations of the lesions, especially those on loaves, 
show that the fungus forms a compact stroma of convoluted thick- 
walled hyaline mycelium. From this hase numerous hyaline sporo- 
phorelike hodies arise, but as yet the writer has not found spores 
attached to these specialized hyphaB. Beneath the stroma a rather 
loose, coarse, thick- walled hyaline mycelium penetrates the host inter- 
cellularly for a distance of several cells. The host tissue in close 
proximity to the fungus rapidly becomes brown and distorted. Dis- 
tinct hyperplasia is often in evidence beneath the area attacked by 
the fungus, which fact probably accounts for the plainly evident 
excrescence associated with scab lesions (PI. XIV). 
Specialized host tissue can frequently be found separating the 
invaded from the uninvaded parts. This condition occurs in older 
leaves. It has not been observed in leaves incompletely expanded. 
As the leaf ages, secondary fungi, principally Cladosporium and 
Colletotrichum species, invade the lesion and soon partially mask the 
pathogen. This invasion may take place within a few days after 
primary infection occurs, but usually does not become very evident 
until the lesions are several weeks or months old. The older the lesion 
becomes the less conspicuous the causal organism appears. Fre- 
quently the stroma which was conspicuous in young scars disappears 
by fall or winter, leaving only fragmentary bits of the pathogen on the 
host. The stroma which persists in citrus-scab lesions through the 
winter becomes especially thick walled. When that stroma is placed 
under the microscope and a slight pressure exerted on the cover slip 
the fungal mass frequently separates into many thick-walled single- 
celled bodies, indicating that under certain natural conditions this 
stroma may possibly be separated into many sporelike bodies. 
DISSEMINATION OF THE CAUSAL FUNGUS. 
The agencies employed in the dissemination of the citrus-scab 
fungus are doubtless the same as those that aid in the spread of most 
diseases. Frequent relatively high winds accompanied by stormy 
periods occur in the spring about the time the leaves or fruit are 
susceptible to infection. A large number of observations have indi- 
cated that storms play an important part in spreading the furigus 
from tree to tree. This dissemination of the fungus from infected 
trees, however, is hardly comparable with the distinct path of inva- 
sion made by such diseases as cedar rust or scab of apple. The spread 
of citrus scab is very erratic. It may persist for a number of years 
on a small group of trees or even a single tree without increasing its 
distribution, or it may spread very gradually over an orchard, or it 
may suddenly become pandemic over a large, hitherto apparently 
scab-free, isolated property. On the whole, the fungus is gradually 
