150 Butler. — A Study on Gummosis of Primus and Citrus , with 
In the arid regions where the trees depend on irrigation for their supply 
of water, and the appearance of the disease is traceable to this source, there 
will be again two methods of combating the malady: (1) in very permeable 
soils the periods between irrigation should be shortened and the humus 
content of the soils increased by green manure crops ; ( 2 ) in shallow clay 
soils underlaid by an impermeable subsoil the periods between irrigation 
should be shortened and the amount of water given at any one time reduced as 
much as possible ; in these soils drainage would undoubtedly be beneficial. 
I have already mentioned that exanthema develops more readily in 
highly cultivated groves than in those which are less perfectly tilled. 
Swingle and Webber 1 observe that groves planted in soils underlaid by an 
impermeable subsoil, and which succumb to the disease when cultivated, 
may be successfully maintained under a regime of no cultivation, the soil 
around the tree being simply mulched with fine straw, oak leaves, or other 
suitable material. 
To conclude: Any method of cultivation which tends to promote 
regular instead of fluctuating growth may be considered a preventive or 
a remedial measure for exanthema. 
1 Loc. cit., p. 20. 
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