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does not appear. First page follows. Introduction The term “mottle-leaf“ designates a functional disease of certain species and varieties of the genus Citrus. The most evident symptom of this disease is the absence of chlorophyll from certain areas between the veins of the leaf and, in more advanced stages, dwarfing of the leaves. The exact cause, or causes, of mottle-leaf have not been determined, but the evidence at hand supports the belief that the disease is independent of parasitic microorganisms. Climatic conditions often influence the occurrence and intensity of the disease, but it cannot be said that they have been shown to be the determining factor. Soil conditions have been found to be more directly related to the occurrence of mottle-leaf. The application of nitrates (Vaile(20) or of excessive amounts of urea (Haas(7)) may produce mottling of orange-tree foliage. The ratio of calcium to potassium also has certain pertinent relations to the disease (Kelley and Cummins,(10) Reed and Haas(17)), yet applications of calcium to the soil are by no means a corrective. Frequent applications of organic manures have been found to be one of the most satisfactory means in California for holding in check this disease, or for ameliorating the condition of trees badly affected with mottle-leaf. The use of iron and zinc salts for the control of functional diseases of fruit trees has received much attention in California in recent years. Peach trees affected with little-leaf (Chandler, Hoagland, and Hibbard(5)) and orange trees affected with mottle-leaf (Johnston(9)) have shown striking benefits from applications of zinc sulfate singly or in combination with iron sulfate. Maze(12) pointed out a significant relation between zinc and sulfur metabolism in maize. He found that roots which were grown in a solution deficient in zinc were soon coated with an ocherous deposit.

The effects of zinc and iron salts on the cell structure of mottled orange leaves