Yellowed leaves are not uncommon in some species of Magnolias. However, they do occur less frequently than other types of Magnoliaceae (Magnolia Family). These particular plants are susceptible to various diseases such as leaf spot disease, powdery mildew and others.
The most common cause of yellowing leaves is insect damage. Insects like aphids, scale insects and whiteflies will eat the plant’s foliage causing it to turn yellow or even blackish-brown. You may see these pests attacking your magnolia before you notice any signs of yellowing leaves. If you have seen them, then they probably attacked your plant during the summer months when it was dormant.
They usually don’t attack at all until late fall or winter when they come out from their hibernation.
If you haven’t noticed any insect activity, then chances are that the problem isn’t caused by insects. The root system of your plant could be damaged by disease. For example, if there is a fungus growing in the soil around your plant, then it could severely affect its growth.
Another possible cause of yellowing leaves is a bacterial infection called leaf spot disease. Leaf spot disease affects certain varieties of Magnolia and causes them to lose their leaves. There are several different strains of this disease that can affect different parts of the plant’s life cycle, but the main symptom is yellowing leaves.
These are only a few of the many factors that can cause your plant to lose its leaves. If you have ruled out all the more common causes, then you can try contacting a professional in your area. They will be able to tell you specifically what is wrong with the plant and how to fix it.
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Sources & references used in this article:
Study on the Content of Chlorophyll and Physiological Characteristics of Magnolia grandiflora Yellow Leaf by S Xiaoan, Y GuoJun – Journal of Shandong Forestry Science and …, 2012 – en.cnki.com.cn
Etiology of cercospora leaf spot of Magnolia by CS Hodges, FA Haasis – Mycologia, 1962 – Taylor & Francis
Management of Pests, Plant Diseases and Abiotic Disorders of Magnolia Species in the Southeastern United States: A Review by GW Knox, WE Klingeman, M Paret… – Journal of …, 2012 – meridian.allenpress.com
A spectrophotometric study of reflection of light from leaf surfaces by CA Shull – Botanical Gazette, 1929 – journals.uchicago.edu