58 
progress of the disease further mischief is prevented, but usually 
the disease runs the following course:—The cambium layer, 
which lies between the wood and the hark, is the formative por¬ 
tion of the stem, giving rise to new wood on the inside and fresh 
bark on the outside, and containing the necessary elements for 
building up. The object of the parasite seems to be to reach 
this treasure-house, and accordingly the decay reaches the cam¬ 
bium layer and even extends into the wood. When this happens 
there is no possibility of the renewal of the bark, and that portion 
is dead. Then the disease spreads further in all directions, prin¬ 
cipally around the base of the stem and down to the main roots. 
Gradually the diseased area is extended, the tree is completely 
girdled, and death ensues. While this is taking place near or 
beneath the ground, the foliage is affected and becomes yellow 
and sickly from the insufficient supply of nourishment from the 
roots. 
The exudation of gum at the collar, the decay of the bark 
there in patches, and the consequent disagreeable odour, the un¬ 
healthy appearance of the foliage, and the death of the small 
shoots are all symptoms of this disease. 
Mr. Williams, of Doncaster, who has been growing Oranges and 
Lemons for a number of years, says that—“ Generally, the first 
visible intimation of the disease is the fruit setting abnormally 
thick, and the foliage turning a sickly yellow colour.” This 
fruit-setting is an evident effort of the tree to provide successors 
before it finally perishes. 
Effects. —This is regarded as the most destructive of Citrus 
diseases, as it is also one of the most widespread. 1 have no 
means of estimating the loss caused by it in Australia, but in 
Florida the annual damage is calculated to be about £20,000 ; 
and in Italy, taking an average of sixteen years, there is a loss of 
£25,000. Under ordinary circumstances affected trees gradually 
die off, but sometimes the wound heals of its own accord. When 
trees are attacked, they may produce au extra large crop of fruit 
the first season, but this is usually the expiring effort, and little 
or none is produced afterwards. In a few months the <c ring- 
barking of the stem may be almost completed, and as the 
disease spreads from tree to tree, in the course of a few years 
entire orchards may be devastated. The progress of the disease 
will be rapid or slow, according as the conditions favour it or 
otherwise. 
The healthy root action is interfered with, and there is a defi¬ 
ciency of nourishment, the foliage is affected and is unable to 
assimilate properly, the girdling of the stem prevents that free 
communication between root and leaf which is essential to plant 
growth, and, finally, with the destruction of the cambium layer, 
the centre of vital activity, the tree is doomed. 
