Brown leaves on a plum tree signal that the tree is under stress. This discoloration is a symptom, not a disease itself, requiring careful observation for successful diagnosis. Causes range from microscopic fungal and bacterial infections to insect damage and environmental stressors. Identifying the precise cause is the first step toward ensuring the tree’s health and productivity.
Diseases That Cause Browning
Brown or necrotic tissue often points to a fungal or bacterial pathogen thriving in moist conditions. Brown Rot, caused by Monilinia fructicola, is a common fungal disease. It typically infects blossoms, causing them to brown and wither. The infection can spread from the flower stem into the twig, resulting in a canker that girdles the branch. Leaves on the affected twig suddenly turn brown, shrivel, and die, remaining attached to the dead wood in a distinct symptom known as “flagging.”
Shot Hole Disease, caused by Wilsonomyces carpophilus, is another frequent fungal issue. The infection starts as small, purplish-red spots that expand, developing tan or brown centers with a reddish margin. The plum tree isolates the damaged tissue, which dries up and falls out, leaving a characteristic round hole. Severe cases cause extensive leaf browning and premature defoliation, weakening the tree over time.
Bacterial Canker, caused by Pseudomonas syringae, also contributes to leaf browning, often alongside trunk and branch damage. The bacteria cause small, dark, water-soaked spots on the leaves during cool, wet spring weather. These spots turn brown or black, and the dead tissue may fall out, creating a shot-hole appearance similar to the fungal disease. A tell-tale sign is the presence of sunken, dead areas on the bark, frequently accompanied by an amber-colored, gummy ooze.
Insect Pests and Damage
Spider Mites, particularly tiny webspinning species, feed by piercing leaf cells and sucking out the contents, a process called stippling. This feeding initially causes fine, light-colored speckling on the leaves. Heavy infestations lead to an overall bronzing or brown discoloration often described as “scorching,” which is particularly noticeable on the undersides of the leaves.
Borers, such as the Peach Twig Borer or Flatheaded Borers, primarily attack the wood, but the damage translates to the foliage. Peach Twig Borer larvae mine new shoots in the spring, causing the terminal growth to wilt and die, known as a “shoot strike.” Flatheaded Borers target stressed trees, tunneling beneath the bark. If they girdle a branch, all leaves distal to the damage will brown and die suddenly.
Aphids indirectly contribute to browning and decline, though they are known for causing leaves to curl and distort. As they feed on plant sap, they excrete honeydew, a sticky, sugary substance. Honeydew serves as a growth medium for Sooty Mold fungus, which covers the leaf surface in a dark, crusty layer. This black coating reduces the leaf’s ability to photosynthesize, accelerating decline and sometimes leading to leaf drop.
Environmental Stressors
Water stress is a primary culprit, as both too much and too little water can result in similar symptoms. Drought conditions cause the tree to restrict water flow, resulting in crispy, brown edges and tips on the leaves, known as marginal scorch. Conversely, over-watering leads to waterlogged soil, starving the roots of oxygen. The roots die and fail to supply water to the canopy, resulting in a similar leaf scorch pattern.
Nutrient imbalances can also produce distinct browning patterns that mimic disease or drought. For instance, a deficiency in potassium causes symptoms to appear first on older leaves as a characteristic marginal scorch. This occurs because the tree pulls the limited potassium from older leaves to support new growth.
Chemical injury, either from herbicide drift or over-fertilization, causes sudden and often uniform browning. Herbicide drift, such as from a lawn weed killer, can cause leaves to turn yellow and then brown rapidly, sometimes showing abnormal or distorted new growth. Over-application of fertilizer acts as a salt, which can “burn” the roots. This leads to desiccation and a marginal leaf scorch identical to drought stress.
Corrective Actions and Future Prevention
Addressing plum tree leaf browning requires immediate sanitation and better cultural practices for long-term health. The first action is to remove and destroy all affected plant material, including brown leaves, infected twigs, and mummified fruit. Pruning out dead or diseased wood should be done by cutting several inches below the visible sign of infection to ensure the entire pathogen-affected area is removed.
Proper water management is fundamental to preventing common environmental stresses. Check the soil moisture several inches below the surface before watering. Aim for deep, infrequent irrigation that encourages deep root growth rather than shallow, frequent sprinkles. Maintaining a layer of mulch around the base, kept several inches away from the trunk, helps moderate soil temperature and retain moisture.
For future prevention, promote good air circulation within the canopy through regular, strategic pruning. This helps leaves dry quickly and reduces the risk of fungal and bacterial infections. If a disease or pest is suspected, targeted treatment may be needed, such as a dormant season oil application or a fungicide application timed with the tree’s bloom period. Consulting a local arborist or extension office can confirm the diagnosis and recommend effective treatment options.