59 
Conditions favouring Disease .—The conditions which favour the 
diseaso are often confounded with the cause, but wo must distin¬ 
guish between them. There are what may be called predisposing 
causes, conditions which affect the plant injuriously aud render it 
liable to the attack of the fungus parasite. They may be guilty 
of aiding and abetting and preparing the way for the fungus, but 
of themselves do not induce the special diseaso known as “Collar 
rot.” As far as known, it is necessary that this particular 
fungus invade the weakened tissue in order to produce the charac¬ 
teristic symptoms which we speak of as the disease, and whatever 
is antagonistic to these conditions or destructive of this fungus 
will tend in the direction of preventing or remedying the disease. 
Unsuitable soil is said by some to produce the disease, because it 
always occurs towards the base of the stem, near to or beneath 
the surface of the soil ; but transplanting the trees to similar soil 
may lead to their recovery. Sudden changes in the temperature 
of the soil surrounding the stem are also supposed to induce it, 
the sudden and extreme variations affecting the regular flow of 
the sap and producing contraction and expansion of the bark. 
The want of proper drainage is likewise a contributing condi¬ 
tion, because it will prevent the perfect aeration of the roots, and 
the water may be allowed to lodge round the stems. 
Close and deep planting, producing an excess of shade and a 
deficiency of feeding roots ; excessive irrigation, keeping the soil 
soaked with water ; excessive cultivation, which may tend to 
injure and disturb the roots ; and the continuous use of organic 
fertilizers are all said to encourage the disease. 
Propagation by grafting is also foifud to render the tree sus¬ 
ceptible at the junction of the scion and stock, and, as will be 
shown more fully later, certain varieties and certain stocks are 
found to be highly resistant to the diseaso. 
It is very generally stated that the use of Lemon stocks renders 
the treo specially liable to this disease, but Mr. Tryon finds that 
trees on Oniugo stocks are equally liable as far as the Toowoomba 
district is concerned. 
Cause .—The cause of this disease is still in dispute, but the 
contagious nature of it seems to point to some parasitic organism. 
In 1878 Briosi described a fungus which be always found accom¬ 
panying the diseaso, and named it Fusarium limonis . He was 
inclined to regard it as the cause of the disease, but had not 
sufficient ovidence to prove it. He adds, however—“I do not 
believe there can bo any doubt that its presence accelerates the 
disorganization of the tissues and aids in extending the diseaso.” 
I have invariably found the same fungus penetrating the 
diseased tissues with its long slender wandering filaments, and 
being found here as well as in Italy strengthens the belief that 
it causes the disease. 
