NON-VARASITIC, OR PHYSIOLOGIC PLANT DISEASES 565 



sis, Collar-blight, Coulure, Court-noue, Curly-top, Die-back, Exan- 

 thema, Foot-rot, Fruit-spot, Gummosis, Intumescence, Leaf-curl, 

 Leaf-scorch, Mai di gomma, Melanose, Mosaic, (Edema, Pithiness, 

 Pourriture, Roncet, Rosette, Scald, Stippen, Sunburn, Tipburn, 

 Tomosis, Tumor, Water-core, Yellows, Zopal. 



The following diseases, selected because of their interest and im- 

 portance to plant growers, maybe looked upon as belonging to this class. 



Stag-head, or Top-dry. The disease so designated frequently re- 

 sults from lack of proper food in the soil. The gradual death of the 

 top of the tree is an indication of the malady, as well as the loss of 

 active growth in the lower part of the tree. It is found in forested 

 areas where by burning, or by denudation, the conditions have been 

 changed. Stag-head is frequently seen in park trees where the 

 natural undergrowth has been removed and where the covering of turf 

 prevents the access of rain to the roots of the trees, or where the stock 

 of humus has become depleted in the soil. The soil tends to dry out 

 in summer and in some of the parks in Philadelphia its surface for 

 several inches becomes baked hard. This is assisted by the constant 

 tramping of many feet beneath the trees. The soil becomes impover- 

 ished, especially in nitrogen and starvation of the tree becomes evident 

 with the slow death of its terminal branches. As a preventive measure 

 a constant supply of food should be provided. Wherever practicable 

 the ground beneath the tree should not be sodded completely, but 

 should be planted to low-growing shade-enduring plants, and if pos- 

 sible, the soil should be top-worked and dressed each year with manure, 

 or other plant food. Along streets and walks this is rendered difficult 

 by the proximity of paving material, but as in Paris each tree should 

 have around its base an unpaved area through which the water can 

 seep into the soil and by which plant food can be added. An open 

 grating can be placed so as to protect the surface soil about the tree 

 from the tramping of passersby. 



Root Asphyxiation (Suffocation). — The health of trees and other 

 plants depends on the proper aeration of the soil. This is conditioned 

 on the size and proximity of the soil particles or the amount of water 

 present, and on the proximity of pavements, fills or grading materials, 

 etc. The lack of air is of far-reaching importance. The organisms 

 of nitrification cannot carry on the process of nitrogen fixation in soils 

 poor in oxygen, and this is true of wet soils or those which are poorly 



