Feb. 1, 1902.] THE TROPICAL AGRICULTURIST. 
519 
though they should not be entirely stopped when 
the rainy seasons are on. In dry weather the 
disease on the bark is comparatively easy to detect, 
while when the stems and branches are wet the 
slightly darker appearance on the outside of ; the 
bark is masked ; and further, the san and dry air 
being most important agents in the destrnction of 
the fungus, any work done while the air is highly 
charged with moisture is less likely to be effective. 
Other Remediks. 
V-arions other remedial and preventive means have 
been tried by myself and otheis, which should be 
mentioned here. The desire of the planter is gener- 
ally for au application eithet to the tree or to the 
soil which shall cure or drive away the evil he 
fears, This is natural, and the wish is shared by cul- 
tivators in all parts of the world. The object of cure 
however is, in the first place, to be as far as pos- 
sible absolutely effective; and in the second place, 
practicable in regard to its cost and probability of 
carrying it out faithfully. In dealing with cures and 
preventions in Ceylon for cacao canker we must 
attempt to get some method which, while effective, 
is simple, and requires a minimum of skill and 
app3.ratus. 
The vaule of Copper Sulphate either by itself or 
mixed with lime as in the Bordeaux mixture has 
long been recognized by all who are interested in 
sanitary work for plants. This substance is the 
best deterrent of the growth of spores, and if a 
covering could be continuously kept on the surface 
of stems, branches, and pods, no new patches would 
occur. But in the dry weather (when the blue 
stone if applied will stay on) is a time when spores 
can seldom germinate, as there is little or no mois- 
ture which is necessHry for them. If the applica- 
tion is made in rainy weather it is washed off by 
the next shower, and it would be impossible to 
keep the tree covered with it. For these reasons, 
while blue stone is an excellent preventative, the 
beueiits gained in the case of cacao canker would 
not be commensurate with the cost of application. 
Another favourite remedy is the application of tar, 
either to the outside of the tree before cutting and 
shaving or to the cut surfaces. This a most treacher- 
ous method, as it deludes the planter into a false 
security. The fungus not being entirely removed, 
the mycelium grows vigorously underneath (in some 
cases I have seen spores produced and breaking 
through the tar), so that it prevents second exami- 
nation to see if the fungus has been exterminated. 
It is often therefore a coverer of bad work. In some 
cases cow dung has been applied to the cut sur- 
faces. This also is harmful, as it keeps the part 
damp, harbours spores, and conceals the place, when 
it should be again examined. 
Many cultivators favour the application of lime 
— thrown on dry — to the trees and on the ground 
beneath them. The drawbacks to the application 
of blue stone apply also to lime as regards the part 
of the plant above ground. lu regard to its effects 
on the soil, while in some land it has beneficial 
manurial qualities, its effect as a fungicide is limited. 
Spores do not grow on dead leaves or on the ground. 
1 have tried to cultivate them on both, but though 
if kept moist they germinq ted, they did not persist. 
It is therefore almost superfluous to take any measures 
against spores lodged on the ground or on dead 
leaves and other debris. We have seen that the 
fungus does not affect the roots, and the eff jct of 
lime in the soil does not help the tree in its battle 
with the canker fungus or make it any less liable 
- to attack. 
Predisposition of Trees to Disease. 
The question of liability of cacao trees to infec- 
tion by the canker fungus has caused a good deal 
of misunderstanding with regard to the question of 
the disease. 
In the case of the cacao canker, I stated in my 
second report of 1898, and it seems well to repeat 
it, that " !^fo special prediapoaition of the tree is 
necessary for the attack of the fungus. " There is 
a belief, which is very general both in Ceylon and 
other lands, that all diseases of plants can be pre- 
vented by keeping the plants in a high state of 
health and vigour. This is true of many diseases, 
and the opposite is also true in some cases that 
excessive quantities of nutrition taken up by the 
root predisposes to certain diseases. Every speciflo 
disease, i.e., disease in which an animal, iuugus, 
or bacterium is the prime factor, must be considered 
by itself, and no general law as to lack of health 
predisposing to disease is accepted by plant patho- 
logists. 
In the case of cacao canker, my observations over- 
thousands of acres of cacao in all parts of Ceylon, 
from Rambnkkana to Moneragalla, at all elevations 
and aspects and on all soils, show that cacao in 
all conditions of vigour contracts the canker, provided 
the spores are there and conditions present to allow 
them to germinate. There are many instances of 
"shuck" eaoao adjacent to more vigorous trees with 
the disease equally prevalent on both, and there 
are cases of the best cacao suffering while some 
adjacent poorer trees for some reason have escaped. 
Mr. Marshall Ward, now Professor of Botany at 
Cambridge, found this equal liability to be the case 
in the coffee leaf disease, and it has probably since 
come under the observation of many readers of 
this Circular that this leaf fungus attacks equaUy 
both healthy and " shuck " coffee. In his report on 
the leaf disease (1882,) Professor Ward says: "No 
special predisposition on the part of the coffee is 
required for its infection, and no other conditions 
are necessary to the spore than moisture and the 
presence of air, &c., as with any germinating seed. " 
In order to further test the truth of my obser- 
vations in regard to this question of predisposition, 
I sprayed four trees of old red cacao which were 
showing all signs of abundant health, and four 
others shuck and ragged. Only seven out of the 
eight acquired the disease, and one^of the healthy 
ones was not affected, .for |the reason, I believe, 
Iha-t it was more exposed to air and sunshine and 
tli9 spores were dried up. 
It is important to grasp this fact of the vigour of 
the caoao tree being no safegurard against the can- 
ker, as, if the planter does not realize it, he may 
be neglecting to take any precautions or keep any 
look out for the enemy which does not spare the 
best trees. 
The question of predisposition of unhealthy trees 
leads us to the consideration of the possibility of 
obtaining a variety of cacao which would be immune 
or less liable to canker than those now grown. 
When canker first appeared it was held by some 
that the Forestero variety was to a great extent 
immune. It is a hardier breed than the old red, 
and stands wind, drought, and all other inimical 
conditions better than the red variety, but it has 
not been attacked markedly less than the red by 
the canker, and experiments of inoculation show that 
it is equally liable. 
Whether a variety will be found that will be 
" disease resisting " or not cannot be said, but it is 
a search well worth the while of the cacao planter, 
and in prosecuting it some facts should be remem- 
bered. The pod disease is a most important factor 
in the spread of canker, and wnen pods are pro- 
duced in the rainy season, they are more liable to 
acquire canker. A variety, therefore, which produces 
and ripens its fruit during the dry months will 
have a certain protection from disease owing to ita 
time for bearing fruit "dodging" the dangerous 
season. The typical Forastero green pods have a 
thicker epidermis, and this gives a partial protection 
against canker spores. 
In selecting seed for breeding a variety of cacao 
which will be " disease resisting," the characters to use 
are thick skinned pod, smooth surface of periderm, 
i.e., outside of bark, production of fruit at dry seasons, 
