CITRUS DISEASES. 63 



studied this problem. The Italian botanists have favored the theory 

 of fungus origin, assigning the blame to a white mold-like fungus 

 (Fiisarium limonis), eommonly found in connection with foot-rot 

 cases. This fungus, or one practically identical with it, has also been 

 found in Porto Rico, ])ut it has not been evident through inoculations 

 that it has any causal relation with the disease in question, acting 

 rather as a follower or saprophyte only. 



Of recent years, the most widely accei)te(l idea lias l)een tluit 

 of a non-parasitic, or physiological disease, due to certain environ- 

 mental factors. Briefly these have been considered to be alternating 

 l)eriods of drought and excessive moisture, close planting, poor drain- 

 age and excessive use of organic fertilizers. Observations have quite 

 clearly shown that the disease is more prevalent in lowlands, or 

 where drainage is poor, and that there is undoubtedly a relation 

 ])etween it and close planting. In spite of this, however, it is the 

 writer's opinion, based on observations, that foot-rot is due to a 

 definite fungus (not Fusarium), although studies to date have failed 

 to locate it. The progress of the disease from one locality to another, 

 and from tree to tree as well as the result of recent work in Florida, 

 would seem to confirm this theory. The fungus now held responsible 

 in Florida, also occurs in Porto Rico as the cause of a disease of 

 beans and tomatoes, but preliminary inoculation tests have given 

 negative results here. 



Control, 



It is interesting to note, that the method of handling affected trees 

 is exactly that which would be followed if a fungus were known for 

 certainty to be the cause. In brief this is tree surgery. All diseased 

 bark should be cut away, well back into healthy tissue, using sharp 

 instruments to insure smooth cuts. Care must be exercised that 

 narrow points or bands of diseased tissue running out into normal 

 areas are cleaned out, since otherwise these will remain as infection 

 centers from which the disease will continue to spread. This cutting- 

 out process must be performed not only on the trunk, but out along 

 the main roots as well, in fact wherever diseased tissue exists. It 

 is failure to observe this precaution that has negatived so many 

 attempts at control. The practice has very commonly been to work 

 down to the surface of the soil and there stop. The soil must be 

 dug away from the crown roots, so as to expose them to light and 

 air, and make possil)le a thorough search for all infected bark. Fol- 



