31 
' | farasknown. 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 citrous trees may 
| | contract mal-di-gomma, but some are very much more subject to it than 
' | others. Sweet seedling orange trees (Citrus awrantiwm) are particu- 
| larly susceptible, and it is in old groves of these that the most damage 
| is done. Sweet orange used as stock for budding is also liable to the 
| disease. This malady is very destructive to the lemon (C. limonwm), 
occurring quite commonly on all varieties. Grape fruit (C. decumana) 
| is frequently affected, but is much more resistant to attacks than the 
| sweet orange or lemon. Indeed, it is seldom that this fruit is seriously 
| damaged. Tangerine and mandarin oranges (0. nobilis) are occasionally 
| affected, but the damage is not usually very great. The sour orange 
| 
| (C. bigaradia) is apparently almost wholly free from mal-di-gomma. The 
writers carried on a careful search for the disease on this species, but 
succeeded 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 
is probable that the disease occurs on the lime (C. limetta) and citron 
| (C. medica) also, but no cases have as yet been observed 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 Briosi' describes and names a fungus (Husisporiwm 
limoni) which he finds always accompanying the disease. He is inclined 
| to consider the fungus the cause of the trouble, but is in doubt as to 
whether it is the primary cause. He says, however: “I do not believe 
_ there can be any doubt that its presence accelerates the disorganization 
of the tissues and aids in extending the disease.” 
The manner in which the disease spreads, appearing at different dates 
first in one country and then in another, and extending gradually from 
grove to grove, strongly indicates that itis contagious, being caused or 
at least greatly aggravated by some parasitic organism. Some investi- 
gators also think that the disease is due to physiological derangements, 
for instance, imperfect aeration of the roots. The most effective treat- 
ment known is based largely on this supposition. 
Observation has shown that the disease is most common where any 
of the following conditions exist: Improper drainage; planting the 
trees so close together that the ground is continually shaded and kept 
moist; continuous use of organic fertilizers; excessive cultivation; con- 
tinuous excessive irrigation, which keeps the soil water-soaked; and 
| deep planting. 
Trees badly affected with the disease frequently recover if trans- 
planted to some other location, on apparently similar soil, and given 
1 Briosi, loc. cit., 495. 
