The sudden yellowing of foliage on your plum tree is a symptom of stress known as chlorosis, which means the leaves are losing their green pigment, chlorophyll. Chlorophyll is necessary for the tree to perform photosynthesis and create energy, making this discoloration a serious warning sign. Diagnosing the exact cause requires a systematic approach, as the issue could stem from environmental factors, chemical imbalances in the soil, or the presence of pests and diseases. Understanding the specific pattern of yellowing and the conditions surrounding the tree will help pinpoint the problem and guide the appropriate action.
Cultural and Environmental Stressors
Improper watering is among the most frequent causes of leaf chlorosis because it directly impacts the tree’s ability to absorb nutrients and oxygen. Excessive water saturates the soil, suffocating the roots by displacing the oxygen they need, which leads to general yellowing and decline. Conversely, insufficient water causes drought stress, prompting the tree to shut off the water supply to the leaves, which then scorch, turn yellow, and drop prematurely. To avoid both extremes, check the soil moisture four to six inches deep before providing more water.
External conditions like temperature extremes can also induce temporary yellowing or leaf shed. A sudden late spring frost can shock new, tender foliage, causing it to yellow and potentially die back. Similarly, intense heat waves, especially when paired with dry soil, can mimic drought stress, resulting in scorched and yellowed leaves. Newly planted plum trees are susceptible to yellowing due to transplant shock, a temporary condition where the roots struggle to establish themselves.
Soil Chemistry and Specific Nutrient Deficiencies
The availability of nutrients is highly dependent on soil chemistry, particularly the pH level, which measures acidity or alkalinity. Plum trees thrive in slightly acidic to neutral soil, but in alkaline soil with a high pH (above 7.0), essential micronutrients become chemically locked up or insoluble. This condition, known as nutrient lockout, means the tree cannot absorb the nutrients even if they are present.
Iron deficiency, a common result of high pH, produces a distinctive interveinal chlorosis where the tissue between the veins turns bright yellow while the veins themselves remain sharply green. Since iron is immobile within the tree, these symptoms appear first on the youngest leaves at the tips of the shoots. Nitrogen deficiency, by contrast, causes a uniform pale yellowing across the entire leaf, beginning with the older, lower leaves because nitrogen is mobile and the tree moves it to new growth.
Magnesium deficiency also presents on older leaves first, typically causing yellowing along the leaf edges, a symptom called marginal chlorosis. The yellowing often progresses inward, leaving a distinct green inverted ‘V’ shape at the base of the leaf. Correcting these issues starts with a professional soil test to confirm the pH and deficiency, followed by appropriate amendments like sulfur to gradually lower high pH or the application of chelated iron compounds for immediate relief.
Insect Pests Causing Leaf Damage
Certain insect pests cause yellowing by physically damaging the leaves or by depleting the tree’s resources. Aphids are small, sap-sucking insects that cluster on new growth, causing leaves to curl, distort, and turn yellow. They excrete a sticky substance called honeydew, which can lead to the growth of sooty mold, compounding the stress on the foliage.
Spider mites thrive in hot, dry conditions and cause damage that results in a stippled, yellowish appearance on the leaves due to the destruction of individual plant cells. A fine webbing may be visible on the underside of the leaves or between the branch tips, confirming their presence. Larger pests like plum borers attack the wood under the bark. Their feeding can girdle branches or the trunk, disrupting the flow of water and nutrients, leading to yellowing and dieback of the canopy above the damage.
Pathogens and Fungal Infections
Systemic and localized infections by pathogens are a major cause of yellowing, often indicating a deeper health problem. Root rot is a fungal condition frequently exacerbated by prolonged overwatering or poor drainage, which causes the roots to decay. When the roots cannot absorb water and nutrients, the entire tree exhibits symptoms of severe stress, including widespread yellowing, wilting, and eventual decline.
Fungal leaf spot diseases, such as Shot Hole Fungus (Coryneum Blight), initially appear as small, circular purple or brown spots on the leaves that are often surrounded by a yellow halo. The center of these spots typically dries out and falls away, creating a characteristic “shot hole” appearance. Widespread infection causes the entire leaf to yellow before dropping prematurely. Bacterial canker is a serious disease that targets the branches, causing oozing lesions and the subsequent yellowing and dieback of the foliage on the affected limbs.
Managing these infections often involves cultural practices like improving air circulation through pruning and ensuring good sanitation by removing fallen, infected leaves. For fungal issues, copper-based sprays or fungicides may be necessary. However, once a tree is extensively infected with a severe disease like root rot or advanced canker, recovery can be difficult or impossible.