566 SPECIAL PLANT PATHOLOGY 



drained. Flooding of tree roots is frequently the cause of the death 

 of the tree. This is seen in low places underlaid by a hard pan, where 

 the groundwater comes close to the surface, or in stiff soils, which 

 become saturated and hold their water for a long time. Bad aeration 

 of the soil coupled with the presence of noxious gases is frequently the 

 cause of disease and death in street planted trees. As preventive meas- 

 ures the ground should be kept stirred about the bases of the trees, or 

 where the ground has been filled in around the tree, small patches of 

 bark should be removed to induce the formation of adventitious roots 

 from the wounded areas beneath the new soil surface. 



Desiccation. — This phenomenon is noticeable in plants exposed to 

 bright sunlight following a spell of cold or cloudy moist weather. The 

 young leaves and tender shoots of such plants frequently wither and die 

 under such conditions. This is sometimes called sun-scald, but evi- 

 dently it is due to a too rapid loss of water, so that the tender parts 

 wither. The excessive loss of water is due to the fact that the leaves 

 produced in very moist air are not adapted to resist excessive transpira- 

 tion even where there is an abundant supply of water in the soil. In 

 other words, the leaves and tender shoots have not been sun hardened. 

 The writer has noticed such a state in the spring when a dry hot spell 

 of weather succeeds a moist cool spell. This disease is produced in the 

 West and Southwest by hot dry winds which sweep over the country, 

 or in South Florida by what are called dry hurricanes. The "Sirocco" 

 on the African coast of the Mediterranean Sea, in Malta and Italy is a 

 hot dry desiccating wind, and so is the " Khamsin," a hot wind from the 

 desert, which blows across Egypt. The leaves of plants are literally 

 cooked, or parched, with such dry winds. The cold dry winds of 

 winter may produce the same effects as the warm dry ones.^ 



Remedial measures under such climatic conditions would be difficult 

 to operate. Frequently in dry regions the formation of a dust mulch 

 by cultivating the soil surface is a method of conserving soil moisture, as 

 is also the application of litter of various kinds. Top pruning in dry 

 seasons will often check the excessive demand for water and thus pre- 

 vent injuries to the rest of the tree. Copious watering of the soil 

 under such dry conditions may save the destruction of the orchard 

 trees or cultivated plants. Winter blighting, or dry-out of coniferous 



' Hartley, Carl and Merrill, T. C: Storm and Drouth Injury to Foliage 

 of Ornamental Trees. Phytopathology, V, 20-29, Feb., 1915. 



