40 
the circumstances into consideration, I am inclined to agree with 
Mr. Try on that the scabs are primarily caused by the Mites, and 
then the fungi follow in their wake. 
The Orange leaf figured in the same Plate (Fig. 3) is from 
South Australia, but the appearance presented by it is common 
enough with us. The under surface has numerous brown 
irregularly-shaped pustular bodies upon it, sometimes becoming 
almost black in colour. When a section of the leaf is made this 
scab-like or warty formation is found to he duo to an excessive 
development of corky tissue at these particular spots. It is not 
easy to explain why corky tissue should be developed in this 
irregular and erratic fashion on the under surface of the leaf; 
but since cork prevents excess of evaporation and is most de¬ 
veloped on leaves (of the eucalypfcs, for instance) in desert country, 
the opinion may be hazarded that it is an effort of the tree to 
retain as much moisture as possible in periods of drought or in 
dry situations. 
Conditions favouring disease .—Where the air is moist and the 
land low the scab-producing fungus flourishes most abundantly. 
And it is found that in high and dry situations, especially where 
the atmosphere is comparatively dry during the growing season, 
there the disease is almost unknown. 
Since the germinating conidia can only penetrate young and 
growing tissue, it is when the leaves and fruits are growing 
rapidly that the most danger is to be apprehended. If the fungus 
can be kept in check at this season, then littlo damage is usually 
done later on. 
Cause .—As already indicated, the causes of scabbing may he 
numerous and varied, and each case must be determined on its 
merits. In the Potato scab, for instance, the explanations to 
account for it are nearly as numerous as the investigators who 
have studied the disease, and many consider that a wound of some 
sort precedes the appearance of fungi. In the Apple and Pear 
scab, there is a definite fungus known to produce it; but in other 
cases fungi maybe associated with “scabbing” without being 
the primary cause of the disease. Infection experiments would 
be required to settle the point. 
Where fungi are concerned they mostly belong to the group of 
Moulds, or what is technically known as the Hyphomycetes , but in 
some instances given, the fungi belong to the group in which 
thero aro perithecia or spore cases. It is interesting to note that 
in every such case there was a mould associated with it which 
was probably the couidial stage of the higher fungus. The 
mycelium or threads of the fungus may only be found penetrating 
the tissues, or they may appear on the surface as a delicate net¬ 
work. There is generally a scarcity of fructification, so that it is 
not always easy to tell the exact species of fungus concerned, but 
