Observations on Squamosis and Exanthema of the Citrus. 119 
The conditions favourable to the development of gummosis (inducive 
in the autogenous form, conducive in other cases) are: (1) Growing trees in 
heavy, retentive, poorly drained soils; (2) Growing trees in an otherwise 
suitable soil, but underlaid by an impermeable subsoil, the situation not 
being such as to afford ample drainage ; (3) Irrigation methods allowing an 
excessive accumulation of water around the trees for a period of time;. 
(4) High fertility combined with soil and drainage conditions as mentioned. 
under I, 2, and irrigation methods as mentioned under 3; (5) High 
fertility and excessive irrigation: (6) Excessive fertilization, especially 
with nitrogenous manures ; (7) Continuous wet weather in spring. 
Too much stress, however, should not be laid on any one of the above 
factors alone. For instance, if we take soil texture in its relation to auto- 
genous gummosis we find that in the Cz¢rws, where this form of the malady 
has been best studied, there is no xe varietur relationship whatsoever. 
With these few remarks on the development of gummosis in the 
orchard, I pass to the study of the symptoms of the disease. 
Minute. Gummosis begins and attains even a high state of develop- 
ment within the plant before any symptoms appear externally. Only after 
the gum has accumulated to such an extent that sufficient pressure has: 
been produced to raise the epidermis, or to drive the flux pearling through 
the crevasses in the same, when it is no longer intact, does the disease 
become apparent. This is generally considered a young stage of gummo- 
sis, though in reality the disease may have developed very considerably by. 
this time: With the further development of the malady the distended 
epidermis bursts and the gum flows out in greater or less amount, or, if the: 
first external symptom was a pearling through epidermal crevasses, simply’ 
shows as an increase in the outflow. 
Gummosis may affect a tree locally or generally; the fruit (Saucer 
Peach, almond, orange, and citron), the twigs and smaller branches, one or 
more limbs, the trunk, or the entire tree. In Prunus the disease appears 
usually in the larger limbs and trunk (PI. VIII), and is generally more frequent. 
in the neighbourhood of the crotches. In the Cztvws gum flow appears 
more generally, one might almost say exclusively, upon the trunk towards 
the base, frequently immediately above the union of stock and scion 
(Plate VII). Gummosis may, however, when extremely intense, affect the 
entire tree in both Prunus and Citrus, gum oozing out from both the. 
trunk and the limbs. Such generalized gummosis, though rare in other 
species of Prunus, is not infrequently observed in young cherries; in the 
Citrus, on the other hand, it is quite exceptional and has only been observed. 
on the lemon. 
When gummosis progresses. sufficiently for gum to appear on the 
surface of the bark, the cortex in the neighbourhood of the exudate will 
