CITEUS DISEASES. 79 



but the additional signs are usually absent. Exanthema is not to 

 be considered of any importance at present. 



The cause of the disease is problematical, but it is usually con- 

 sidered to be a mal-nutritional disease, due to an excess of nitrogen 

 supplied in barnyard manure or other organic form. Poor drainage 

 is also thought to be a factor. Instances have, however, been noted 

 in Porto Rico where trees located near stables or heavily manured 

 were in the best of health, and on the other hand trees which had 

 received no organic applications whatsoever showed dieback symp- 

 toms. 



The disease is generally controlled by stopping for a time all 

 cultivation and using mineral fertilizers only. Many growers have 

 claimed that the disease could be cured by the use of blue stone 

 (copper sulphate) applied to the soil around affected trees, and this 

 idea has been apparently verified by experiments carried out in 

 Florida by the United States Department of Agriculture. It was 

 found that from four to eight pounds of copper sulphate in two ap- 

 plications gave most excellent results in restoring to normal con- 

 dition even very sick trees. 



WITHERTIP, AXTHEACNOSE (Colletotrichum gloeosporioides) . 



The withertip or anthracnose fungus is one of the most common, 

 if not tlie commonest fungus, in and about citrus groves. Very exten- 

 sive studies have been made of the fungus and the several phases 

 of disease caused by it. It is one of the forms universally distributed 

 in all citrus-growing districts, and if the view of some workers is 

 accepted, it ia also, the cause of important diseases of apple, guava, 

 mango, avocado, and a wide range of other economic plants. At 

 least all varieties of citrus are very subject to attack by it. Of late 

 years there has been a tendency to consider it more in the nature 

 of a saprophyte or weak wound parasite, than the virulent parasite 

 it has so often been pictured. It is true that it is of universal occur- 

 rence on dead and dying twigs, and in leaf spots, lu fact, citrus 

 leaves and twigs, to all outward appearances normal, will almost 

 invariably develope the fungus, when cxtei-nally sterilized and placed 

 in sterile damp chambers. 



On the other hand cases have been observed where death of 

 branches, or spots on leaves or fruit were quite clearly due to the 

 initial action of this fungus. As a result of observations and studies 

 to date, it is believed that under Porto Rican conditions the fungus 



