Observations on Squamosis and Exanthema of the Citrus . 149 
growth in the various rings ; and it is quite conceivable that the widening 
of this range could be sufficient to produce exanthema. The inorganic 
manures are reputed to be less favourable to the development of exanthema 
than organic manures, and it is altogether probable that this is due to the 
solubility of the former. Nitrates are soon carried into the drainage waters 
and lost, whereas the organic nitrogen must first be nitrified, and this only 
occurs under suitable conditions of soil moisture, conditions which would be 
realized when the available water in the soil became sufficient to promote 
vigorous growth in the trees. Hence the bad effects following the use of 
organic manures. 
Swingle and Webber 1 observe that cultivation increases the suscepti- 
bility of the Citrus to exanthema, and even causes a more virulent outbreak 
of the disease in affected trees. They explain this effect of cultivation on 
the ground that the surface roots are destroyed and the trees forced to 
grow down into an unfavourable subsoil, but this supposition is not well 
founded : cultivation simply increases nitrification and, when deep enough, 
prevents excessive evaporation of water from the soil. We must, therefore, 
consider in humid climates nitrification due to cultivation as a possible 
source of danger and one not to be neglected if no cultivation is as effective 
a prophylactic measure as it appears to be. 
5. Preventive and Remedial Measures. 
All the preventive measures used in combating exanthema are also 
remedial ; similarly, the remedial measures are all prophylactic. It is 
unnecessary, therefore, to distinguish between the two. 
To successfully combat exanthema one must prevent the irregularity 
of growth which is such a marked feature of the disease. The methods 
employed for attaining this end are quite various, and sometimes even 
apparently conflicting. I shall not attempt to exhaust the list. 
When exanthema is traceable to the excessive use of organic manures 
we know that for these manures to be inducive sufficient available water 
must be present in the soil, and we have two cases to consider: (1) exanthema 
does not appear when the groves are not fertilized, and (2) the malady is 
present in a mild form when manures are not employed. In the first case 
drainage would be beneficial though not absolutely essential ; in the second 
drainage would be indispensable, and in both the amount of nitrogen fur- 
nished the trees should not exceed their immediate requirements. The 
nitrogen would probably be most advantageously obtained from green 
manure crops, though the various other sources are not precluded. Heavy 
fertilization with potassic and phosphatic manures, which salts act somewhat 
as growth restrainers, is, in many cases, desirable, and has proved effective. 
1 Loc. cit, p. 20. 
