Plum trees can decline quickly, but the process leaves distinct clues for the attentive owner. Successful recovery depends entirely on accurately diagnosing the underlying problem. The tree signals distress through its leaves, bark, and overall structure, making a methodical inspection the first step toward saving it. Recognizing the specific signs of environmental stress, insect activity, and disease allows for targeted measures to restore the tree’s health.
How to Inspect a Declining Plum Tree
The initial inspection should follow a systematic approach, beginning with the foliage, which often provides the earliest visible warnings. Examine the leaves for discoloration, such as yellowing (chlorosis) or browning, and check for unusual spots, holes, or a sticky residue known as honeydew. Note whether the leaves are wilting or curling, as these signs can indicate problems ranging from water stress to a severe root issue.
Next, inspect the trunk and branches, looking closely at the bark for physical irregularities. Plum trees naturally produce a sticky, amber-colored sap called gummosis. However, excessive oozing, especially when accompanied by cracks, cankers, or small holes, suggests a deeper problem. The location of the oozing sap is important, as damage near the base often points to different pests than damage higher up on the limbs.
Finally, inspect the root collar, the flared area where the trunk meets the soil line. The root flare should be visible and the bark should not be constantly wet or buried under soil or mulch. Look for signs of rodent feeding, which can girdle the trunk, or roots that are circling the trunk (girdling roots). This area is a frequent entry point for disease and a common location for planting issues.
Non-Biological Causes of Stress
Many plum tree problems stem from the growing environment rather than from living pathogens or pests. Water issues are a frequent culprit; plum trees require consistent moisture but are susceptible to root rot in poorly draining soil. Overwatering starves the roots of oxygen, while drought stress causes premature leaf drop and wilting, often leading to susceptibility to secondary pests.
Soil chemistry also plays a significant role in tree health, with nutrient deficiencies causing visible symptoms in the foliage. A lack of nitrogen results in pale or yellowish-green leaves. In contrast, interveinal chlorosis, where the veins remain green but the tissue between them turns yellow, often indicates a deficiency in iron or magnesium, which can be tied to improper soil pH.
Other site-related issues can stress the tree, including mechanical damage from lawn equipment or improper planting depth. Planting a tree too deeply, burying the root collar, restricts the exchange of gases and can lead to slow decline. Furthermore, unsealed or poorly made pruning cuts create open wounds that are entry points for fungal spores and bacterial infections.
Identifying Common Plum Tree Pests
Specific insects cause distinct types of damage that help narrow down the diagnosis, often requiring a close look at the bark and foliage.
Peach Tree Borer
The peach tree borer is a destructive pest whose larvae tunnel into the cambial layer just below the bark. The most telling sign of their presence is a mass of gummy sap mixed with sawdust-like material, known as frass. This is usually found near the base of the trunk or at the soil line.
Aphids
Aphids are small, soft-bodied insects that cluster on new growth and the undersides of leaves, sucking out plant sap. Their feeding causes leaves to curl, distort, and sometimes turn yellow. They secrete a sticky substance called honeydew, which encourages the growth of sooty mold, appearing as a black coating on the leaves and branches.
Scale Insects
Scale insects present as small, immobile bumps on the bark of branches and twigs, often appearing like tiny helmets. These pests are protected by a waxy coating and feed by piercing the plant tissue to extract fluids. Heavy infestations can lead to branch dieback and reduced tree vigor.
Recognizing Major Plum Tree Diseases
Fungal and bacterial diseases often present with unique symptoms, allowing for differentiation from environmental or pest damage.
Brown Rot
Brown rot, caused by Monilinia fungi, is a common issue that affects blossoms, twigs, and fruit. Infected blossoms turn brown and water-soaked. The disease causes the fruit to rot, eventually shriveling into hard, spore-producing structures known as mummies that cling to the tree.
Black Knot
Black knot is a fungal issue caused by Dibotryon morbosum, which creates hard, tumor-like swellings on the branches and twigs. These galls start velvety green but mature into rough, black, elongated knots that can eventually girdle and kill the affected limb. The fungus gains entry through wounds, typically during wet spring weather.
Bacterial Canker
Bacterial canker, caused by Pseudomonas syringae, results in sunken, dark areas on the bark of the trunk and limbs. These cankers often ooze a gum, especially in late winter and early spring. The bacteria can also cause the death of smaller twigs and buds.
Steps for Tree Recovery and Long-Term Health
Addressing non-biological stress involves correcting the underlying cultural practices to restore the tree’s strength. If a watering issue is suspected, check the soil moisture four to six inches deep before providing water, aiming for deep, infrequent irrigation to encourage robust root growth. Use a soil test to determine if nutrient deficiencies exist, then apply a balanced fertilizer with nitrogen, phosphorus, and potassium as needed, or amend the soil with aged compost.
For pest management, targeted treatments are often necessary to protect the tree without harming beneficial insects. Dormant oil applied in late winter before bud break is effective for controlling overwintering scale insects and some aphid eggs. Aphid infestations during the growing season can often be managed with a strong spray of water or an application of insecticidal soap, while peach tree borers may require a wire to physically remove the larvae from the trunk or the application of a trunk spray.
Disease management relies heavily on cultural control and sanitation to reduce the source of the infection. For brown rot and black knot, the removal of all infected material is paramount, which means pruning out black knots several inches below the visible swelling and removing all mummified fruit from the tree. It is important to sterilize pruning tools with a solution of bleach or rubbing alcohol between each cut to prevent spreading pathogens to healthy tissue.