The most common symptom of drought injury is dying around margins of leaves with the dead leaf tissue between veins toward the midrib. Areas 1/4 to 1/2 inch wide along veins are the last to become desiccated.
Drought injury can occur naturally or it may be induced by man. Natural drought injury occurs when there is an inadequate supply of soil moisture available to the plant. A deficiency of water affects the marginal leaf tissue more than other parts of the plant which results in a partial or complete collapse of the cells. Prolongation of this state results in the death of feeder roots and, therefore, recovery of the plant to the normal condition is slow.
Man-made drought injury occurs when roots have been damaged mechanically or when there is an excessive accumulation of salt in the soil. Salt accumulation can develop by using irrigation water containing salts or by the use of excessive rates of inorganic fertilizers. High salt concentrations in the soil solution reduces water absorption by root hairs. In time, the osmotic pressure (pull) in the soil becomes greater than that in the cells of the roots and under such conditions the net movement of water is from the roots into the soil. This can occur even though there is abundant moisture in the soil. This phenomenon is frequently referred to as “reverse osmosis.”
Root pruning will also cause drought symptoms to occur. Cuts or fills made while installing curbs or other masonry structures may cause root pruning. The construction of driveways, patios, and other masonry or asphalt structures near or around trees will interfere with the exchange of gases around the root system and result in root pruning. Excessive amounts of fill soil around a tree or other plants causes this same type of root pruning due to poor aeration. When it becomes necessary to build patios or driveways near shade trees, care should be taken to prevent mechanical root pruning and also provisions should be taken to incorporate a tile system in the structure to permit proper aeration.