35 
“SCABBING” OF FRUIT AND LEAVES. 
The term “ Scab ” is so loosely applied and is susceptible of 
such a variety of meanings that it is advisable to give some 
general definition before discussing the agencies by which it may 
be caused. If the normal smoothness of the surface tissue of a 
fruit or leaf is studded with elevations or depressions which are 
partly or entirely rough and broken, with a dead or discoloured 
appearance, then it will be here spoken of as “ Scabbed.” The 
active factors in the production of these symptoms are so various 
and so commonly associated with other possible agencies, that it 
is often no easy task to determine the primary cause. Some 
mechanical agent, such as a thorn, or more frequently the punc- 
turo of some mite or insect, may so irritate the skin of the fruit 
as to induce a flow of the essential oil, which spreading over the 
skin becomes oxidized and converted into a resinous material 
which interferes with physiological processes and produces scab- 
like elevations. To the disturbing effect upon the cells of the 
surface tissue, due to the growth of certain fungi, may also be 
traced the production of unsightly warts or depressions, accom¬ 
panied or not by the exudation and oxidation of the essential oil. 
On leaves the presence of certain animal or vegetable parasites 
may stimulate the formation of corky tissue, thus giving them a 
scabby appearance. Hence any reference to “ scab,” as if it were 
a specific disease, duo to a definite organism, is not in accordance 
with the facts, and is to be deprecated. Only those forms of scab 
which are produced by or associated with fungi will be dealt with 
here, with the soli I ary exception of the corky formation on the 
under surface of the leaf. One great reason why it is often so 
difficult to determine the exact species of fungus causing the scab 
is that generally the vegetative phase only of the fungus can be 
found, though it may bo that the detached flakes afterwards pro¬ 
duce the fructification. 
It appears that scab has been noticed mostly on the Lemon, 
and that Australia has a comparatively early record of it, owing 
to the favorable nature of the climate. At present it has rather 
a limited geographical distribution, being absent from Europe and 
Africa, as far as known, but occurring in Japan, America, and 
Australia. It first appeared in Florida about 1884, and, according 
to Swingle and Webber, it was undoubtedly introduced from 
Japan. There is a scab disease recorded on the Lemon-leaf in 
Queensland as early as 1876, and it would be interesting to trace 
its origin, but probably it might be found on some of the native 
Citrus trees. 
c 2 
