The Burning Bush (Euonymus alatus) is a popular landscape shrub, prized for the vibrant crimson color its foliage takes on each autumn. While known for its hardiness, this woody plant is susceptible to various pests and pathogens. Understanding the specific nature of these problems, whether caused by fungi, bacteria, or insects, is the first step toward effective management. Issues commonly affect the plant’s foliage, though more serious problems can originate at the stem and root level.
Fungal Diseases of the Leaves
Fungal infections often affect Burning Bush leaves, triggered by high humidity and poor air circulation. Powdery mildew, caused by fungi like Oidium euonymi-japonici, manifests as a distinctive white or grayish, dusty coating on the upper surfaces of the leaves and young shoots. This growth thrives when warm days are paired with cool, humid nights. While it rarely kills the shrub, it can cause slight yellowing and premature leaf drop.
Management begins with cultural controls, such as pruning dense inner branches to increase airflow and avoiding overhead watering. For persistent infections, apply a fungicide containing active ingredients like myclobutanil or propiconazole for systemic protection. Horticultural oils, applied as a 2% solution, can also smother the fungus on contact.
Leaf spot, which can be caused by fungi such as Cercospora or Colletotrichum, often results in Anthracnose. These diseases are characterized by small, dark, irregular spots or lesions that may develop grayish-tan centers. Severe infestations can lead to significant defoliation, though the disease is generally not life-threatening. Non-chemical control involves promptly raking up and destroying fallen leaf debris to remove overwintering fungal spores. Fungicides such as thiophanate-methyl protect new foliage during periods of high moisture.
Insect Pests Often Confused with Disease
The Euonymus Scale (Unaspis euonymi) is one of the most damaging problems for the Burning Bush, often mistaken for a disease due to the resulting decline in plant health. These insects are armored scales, protected by a hard, waxy covering that makes them difficult to control. Female scales are small, oyster-shaped, and brownish-gray, while the smaller males are white and oblong, often clustered in dense masses on the stems and twigs.
Scale insects damage the shrub by inserting piercing-sucking mouthparts to extract sap. This feeding causes leaves to develop yellow spots and can lead to branch dieback or death in heavy infestations. The most susceptible stage for treatment is the “crawler” stage, the mobile, newly hatched nymph that emerges in late spring and early summer.
Targeting crawlers with horticultural oil or insecticidal soap is a less toxic approach that smothers the insects before they develop their protective armor. During the dormant season, a dormant oil spray can be applied to the twigs to control overwintering females. For severe cases, a systemic insecticide like dinotefuran is recommended because it is absorbed by the plant and moves through the vascular system to kill the feeding insects. It is important to note that the common systemic insecticide imidacloprid is generally not effective against this specific pest.
Root and Stem Pathogens
Problems originating at or below the soil line are typically more severe and difficult to manage, as they disrupt the plant’s vascular system. Root rot is a common issue, caused by soil-borne pathogens like Phytophthora species when the plant is exposed to excessively wet conditions. Symptoms appear above ground as wilting and yellowing leaves, mimicking drought stress because damaged roots cannot absorb water efficiently.
Upon inspection, the roots of an affected plant will appear dark, mushy, and decayed, confirming root rot. The primary treatment is cultural, requiring immediate action to improve soil drainage and modify watering habits. Amending heavy clay soil with organic matter can increase porosity and mitigate future risk.
Another serious, though less common, systemic problem is Crown Gall, caused by the soil-inhabiting bacterium Agrobacterium tumefaciens. This bacterium enters the plant through wounds near the soil line and stimulates the host cells to grow in a disorganized way, producing abnormal, tumor-like growths called galls. These galls are initially soft and light-colored but become hard, woody, and dark brown as they age.
Galls typically form at the root crown, disrupting the flow of water and nutrients throughout the shrub. Chemical treatments are of limited value for the home gardener; removal and destruction is the recommended action for a severely infected plant to prevent bacterial spread. If removed, avoid replanting a susceptible species in the same location for at least two years, as the bacteria can persist in the soil.