Chlorosis in Macadamia

Chlorosis in Macadamia

Arthur Wallace


Chlorosis caused by faulty iron nutrition may be one of the most serious problems of the Macadamia industry in southern California. The factors causing chlorosis in Macadamia are not well enough understood to serve as a guide to practices designed to avoid or control it. Resistant rootstocks, if available, would be the most promising solution. Suggestions for avoiding chlorosis in macadamia include not planting macadamia on highly calcareous soils and not over irrigating.
The use of the new iron chelates that maintain a soluble supply of iron in the soil may lead to a practical method of controlling iron chlorosis in macadamia and other crops, especially since the chemical industry is continually improving the formulations. Cost has been and still is a deterrent to their use in commercial plantings.

At the first annual meeting of the California Macadamia Society the author was asked--even before the talk was much more than started--"What is chlorosis?" Farmers and homeowners in the irrigated areas of the West are well acquainted with a typical bright yellow or yellowing with greenish veins coloring of leaves of plants that should be green. This yellowing is known as chlorosis. It comes in various degrees from slightly chlorotic to severely chlorotic. Chlorotic plants are unthrifty--some eventually die.

Most of the chlorosis is what scientists call "lime-induced chlorosis." This is an iron deficiency of plants growing on soils containing free or excessive amounts of calcium carbonate. If a soil bubbles on addition of acid (lemon juice is a good test), it contains free calcium carbonate. This "induced" condition is greatly exaggerated by excessive soil mixture and by low temperature. Not all plant species are susceptible to chlorosis,. Some plants, notably azaleas, gardenia, blueberries, and rhododendron prefer to grow in acid soils. In neutral or slightly alkaline soils these plants tend to become chlorotic. There are other plant species that are green in neutral and slightly alkaline soils but become chlorotic in calcareous soils, particularly with excess soil moisture and the presence of bicarbonates in the irrigation water. These include avocado, peach, pear, apples, raspberry, some citrus, and many other plant species. It is the opinion of the author that Macadamia belongs in the first classification, that is, it grows best in an acid soil. A good rule is not to plant them on highly calcareous soil if possible. It would be best to put them on slightly acid soils. Of eight macadamia seedlings planted into each of five different soil treatments as follows:
(a) slightly acid soil, (b) acidified with H2SO4, (c) added 1/4% CaCO3, (d) added 5% CaCO3, and (e) added sodium bicarbonate; 6, 6, 3, 3, and 0, respectively, remained in good condition after two months. These data indicate that generalization is difficult. Some varieties of Macadamia are more sensitive to chlorosis than are others. This is true also for other plant species. Some grape varieties are extremely sensitive to chlorosis ,but these same varieties can be successfully grown on calcareous soils by grafting them to chlorosis-resistant rootstocks (6). Likewise deciduous fruit trees on almond rootstock do better than those on peach rootstocks when on high-lime soil (2). Among citrus, sweet orange, trifoliate, and grapefruit rootstocks are more susceptible to lime induced chlorosis than are either rough lemon or sour orange rootstocks (I). Can similarly resistant rootstocks do the same thing for Macadamia? It is hoped that they can.
Continued investigation in this area is important.

In the June-July, 1955, issue of What's New in Crops and Soils, D. M. Thorne of the Utah State Agricultural College, in an article entitled "Lime induced chlorosis--new challenge to science," wrote that only recently differences have been found in the respiratory systems of
chlorosis-resistant and chlorosis-susceptible plants. He suggested that if plant breeders can alter plant color, height, and quality, why can't the respiratory system also be altered? This is a large, unexplored field where plant breeding can help solve plant nutritional problems. Can such be done for Macadamia? Perhaps sufficiently resistant material already exists and must merely be classified.

The chlorosis of Macadamia appears to be an extremely complicated phenomenon. Seedlings have become chlorotic in acid soils where there should be little likelihood of such occurring. Rootstock varieties thought to have some resistance to chlorosis have on occasion become chlorotic even on non-alkaline soils Balance among the micronutrients, iron, zinc, manganese, and copper seems to be involved, as is over irrigation and also bicarbonates in the irrigation water.

Attempts to control iron chlorosis bv the application of iron compounds to either the soil or the plant have until recently usually resulted in failure. In 1952 Stewart and Leonard (4) in Florida found that certain organic chemicals called chelates, that hold iron tightly in rings, could keep iron sufficiently soluble when applied to the soil to make chlorotic plants turn green. In 1953 it was learned that in the calcareous soils of the western United States these chelates were not sufficiently effective and were too expensive for use in commercial orchards. In each year since then significant advancements have been made in this field. In 1955 a new compound was tested that proved to be very effective in extremely small amounts. It was very stable in calcareous soils and was less toxic to plants than any chelating agent previously used (5). In 1956 many new chelates will be tested both in soil and in spray application. It seems very likely that at least one of these
chemicals will be effective at a cost, which will warrant its use in orchards. They will be tested on Macadamia.

Mr. E. Frolich, of the Department of Subtropical Horticulture, has made a number of studies of chlorosis in Macadamia. He has propagated relatively large populations and observed their growth under nursery conditions. He has made many observations on varieties that are more susceptible to chlorosis than are others. He has studied many graft combinations to determine whether the scion or the rootstock is more responsible for the chlorosis. His studies continue, as do those in the physiology of lime induced chlorosis.

*Ph.D., Assistant Professor of Subtropical Horticulture, UCLA, Los Angeles. s


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young Red Blush grapefruit trees. Amer. Soc. Hort. Sci. Proc.

57: 125-132. 1951.

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Bull. 736. 1953.

3. Haas, A. R. C. Lime-induced chlorosis of citrus in relation to soil

factors. Plant Physiol. 17: 27-51. 1942.

4. Stewart, I., and Leonard, C. D. Iron chlorosis--its possible causes and

control. Citrus Magazine, June 1952.

5. Wallace, A., et al. Comparisons of five chelating agents in soils, in

nutrient solutions, and in plant resources. Soil Sci. 80: I01-

108. 1955.

6. Wann, F. B. Chlorosis yellowing of plants, causes and control. Utah

Agric. Exp. Sta. Circ. 85. 1930.