14 BULLETIN 1286, U. S. DEPARTMENT OF AGRICULTURE 
doned, to await the return of prices which will make further tapping 
under such conditions profitable. 
Where the disease occurs on small plants or in the nursery the 
trials made by Bancroft (2), Stahel (22, 23), and others indicate 
that it can be held in check by frequent spraying with Bordeaux 
mixture. On the plantation, however, spraying as an annual pro- 
gram is for obvious reasons impracticable. In this respect the South 
American leaf disease offers analogy to chestnut blight, in that the 
value of the individual tree is usually not sufficient to compensate for 
a schedule of treatment necessary to keep it free from the disease. 
The method of control recommended by Stahel (23, p. 107), 
based on his studies of the fungus but apparently not as yet ex- 
perimentally demonstrated, aims at eradication of the disease. Ac- 
cording to him, since the disease is perpetuated entirely by the 
conidia! stage, which can only infect the very young growth, by 
keeping all the trees free of this for a short period the conidia 
would not fall upon susceptible parts and would die. He therefore 
prescribes the cutting out of all the new shoots which appear dur- 
ing a period of three or four weeks over the entire plantation. 
To prevent reinfection adjacent estates would of course need to 
cooperate. 
From preliminary trials Stahel estimates that no more labor 
would be required by this method than for spraying. He admits, 
however, that complete eradication is impossible, because of the 
wild Heveas and the long distances over which the wind could 
carry the conidia from them to the plantations. It is further- 
more questionable whether the measure would be successful even 
when applied locally in view of our inadequate knowledge of the 
fungus. 
The failure to find thus far a practicable method of control for 
the leaf disease is naturally discouraging, but emphasizes the need 
for further study of the problem. One important direction which 
such a study might take is toward the development of disease- 
resistant types. For this purpose attention is called to the prom- 
ising possibilities which exist in the abandoned Guiana plantings. 
In a few severely diseased plantings inspected during this survey 
occasional trees were observed to be unaffected. Furthermore, after 
several years' observation of the disease in British Guiana, Bodkin 
(Harrison 9, p. 16) states that in every cultivation attacked by 
the disease a few trees stand out prominently, having made ex- 
cellent growth and having large heads clothed with healthy dark- 
green foliage, and that such trees appear to be immune to the dis- 
ease. It is conceivable that within such a variable species as Hevea 
hrasiliensis substantial differences in susceptibility occur, just as 
were demonstrated by the writer (18) in the case of the brown 
bast disease. The recent assurances of successfully propagating 
the rubber tree vegetatively make possible the preservation and 
rapid multiplication of any resistant type. The selection of such, 
however, will not be so simple as one might at first suppose, because 
characteristics other than resistance must be taken into account. 
The ideal would be, of course, to find types which combine a superior 
quality of rubber and a high yield with resistance. Careful study 
will be necessary, since the supposed resistance in many cases will 
prove to be only apparent and due to escape through coming into 
