12 BULLETIlsr 1118, U. S. DEPARTMENT OF AGRICULTURE. 



PATHOLOGICAL ANATOMY. 



Microscopic examinations of the lesions, especially those on leaves, 

 show that the fungus forms a compact stroma of convoluted thick- 

 walled hyaline mycelium. From this base numerous hyaline sporo- 

 phorelike bodies arise, but as yet the writer has not found spores 

 attached to these specialized hyphse. 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. XI^O- 



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 

 CoUetotrichum 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 fungus 

 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 



