144 



the trunk does not usually extend over I or feet above the ground, 

 "but occurs on the roots for some distance below the surface. 



Trees affected with foot rot appear at first as if they were suffering 

 from lack of nourishment. The foliage becomes light yellow and scanty 

 the leaves developed being smaller than usual ; the tree bears consid- 

 erable fruit, apparently of normal size and good quality. This abun- 

 dant fruiting is in marked contrast with blight, where a profuse bloom 

 is formed, but very little fruit set In trees suffering from this disease 

 the small limbs die first, but as the malady progresses large limbs suc- 

 cumb, thus giving the tree a ragged appearance. Again, in marked 

 contrast with blight and with severe cases of die back, no water sprouts 

 develop from the interior of the tree top or from the trunk. In Florida 

 trees are not usually attacked until they are quite old and have been 

 bearing fruit for a number of years In other regions, however, young 

 trees are said to contract the disease. 



Psorosis a disease (i) known in Florida as "tears" or " gum disease" 

 is often confounded with foot rot, but is unquestionablj^ quite distinct. 

 In appearance it is similar to foot rot, but with it the diseased spots occur 

 on the limbs and occasionally on the trunk, but never on the roots so 

 far as known. Psorosis does not kill the bark entirely, but extends 

 only to the middle layer, the inner bark and cambium layer remaining 

 healthy. 



Varieties of trees attacked. — Probably all species of citrus trees may 

 contract mal-di-gomma, but some are very much more subject to it 

 than others. Sweet seedling orange trees {Citrus Aiirantium) are par- 

 ticularly susceptible, and it is in old groves of these that the most dam- 

 age is done. Sweet orange used as stock ror budding is also liable to 

 the disease. This malady is very destructive to the lemon (C limonum), 

 occuring quite commonly on all varieties. Grape fruit {C. decumana) 

 is frequently affected, but ii much more resistant to attacks than the 

 sweet orange or lemon. Indeed, it is seldom that this fruit is serious- 

 ly damaged. Tangerine and mandarin oranges (0. nohilis) are occa- 

 sionally affected, but the damage is not usually very great. The sour 

 orange (C higaradia) is apparently almost wholly free from mal-di- 

 gomma. The writers carried on a careful search for the disease on 

 thii species, but succeded in finding only two unquestionable cases. 

 The sour orange used as a stock for budding, remains free from attacks, 

 and in Florida its use for this purpose is almost a sure preventive of 

 the disease. It it probable that the disease occurs on the Kme [C. li- 

 metta) and citron {C. medica) also, but no cases have as yet been ob- 

 served in Florida. 



Cause. — Mal-di-gomma has been studied extensively by many inves- 

 tigators, but as yet the cause of the disease is not surely known. It is 

 thought by many to result from the attacks of some parasitic organism, 

 and Professor Brio8i('2) describes and names a fungus {Iiisisportum 

 limoni) which he finds always accompanying the disease. He is in- 

 clined to consider the fungus the cause of the trouble, but is in doubt 

 SIS to whether it is the primary cause. He says, however: "I do 

 not believe there can be any doubt that its presence accelerates the 

 diiorganization of the tissues and aids in extending the disease." 



(1) From the Greek, meaning an ulcer. 



(2) Briosi, loc. cit., 495. 



