Yellowing accompanied by Wilting is a predominant symptom in most cases where transpiration is more active than root-absorption beyond a certain limit, as is well known in cases of prolonged drought. It may also be caused in evergreens by the foliage transpiring actively in bright January weather, for instance, while the ground is frozen and the chilled root-hairs cannot absorb.
In other cases similar appearances are traceable to insects devouring the roots, e.g. wireworms, and the malady is sometimes enhanced by their accumulations so fouling the wet soil that the roots die off, owing to want of oxygen and to the excess of carbon-dioxide and poisonous matters.
Yellowing may also result from the presence of poisonous or acid gases in the atmosphere or soil, such as chlorine, hydrochloric acid, sulphurous acid, etc., in the neighbourhood of chemical works, or from the escape of coal-gas in streets, etc., points of importance in connection with the use of fungicides and insecticides.
Yellowness is the prevailing symptom in many cases of fungus attack of the roots or collar of the plant, the resulting stoppage of transpiration being also sometimes supplemented by rotting of the roots, and the consequent deprival of oxygen and accumulation of foul gases. In other cases Fungi, and even Bacteria, have been found to have made their way into the principal vessels, the lumina of which they stop up, thus reducing the transpiration current.
Certain insects may also induce a general yellowing and wilting of plants by entering or destroying the tissues concerned in the transpiration-e.g. Oscinis, the Frit Fly, and Cecidomya, the Hessian Fly, which attack young winter wheat within the sheaths and cause the plants to turn yellow and wilt.