Nerium oleander is a robust and beautiful evergreen shrub, but like all plants, it is susceptible to a range of diseases. Understanding these ailments from a plant's physiological and pathological perspective is key to effective treatment and maintaining a healthy specimen.
This is a severe, systemic bacterial disease. The bacterium Xylella fastidiosa invades the xylem vessels, the plant's water-conducting tissues. The bacteria multiply and form a biofilm that physically blocks the flow of water and nutrients from the roots to the leaves. From the plant's perspective, this is a drought stress induced from within. The initial symptom is a marginal leaf scorch, where the edges of leaves turn yellow and then brown and brittle, while the leaf center may remain green. This progresses inward, leading to branch dieback, and ultimately, the death of the entire plant. There is no cure. Infected plants should be removed and destroyed to prevent transmission by leafhopper insects to healthy oleanders.
This disease is caused by the bacterium Pseudomonas savastanoi, which enters the plant through wounds or natural openings. The bacteria produce compounds that interfere with the plant's normal growth hormone regulation, specifically auxins. This hormonal disruption triggers the rapid and disorganized division of plant cells, leading to the formation of large, rough, tumor-like galls or knots, primarily on stems, branches, and sometimes roots. These galls disrupt the vascular system, weakening the plant and causing stunted growth and dieback. Pruning out and destroying infected branches well below the visible gall is necessary. Disinfecting pruning tools between cuts is critical to prevent spreading the bacteria.
Sooty mold is not a direct pathogen but a clear indicator of another problem: a piercing-sucking insect infestation, such as aphids, scale, or whiteflies. These insects feed on the plant's phloem sap and excrete a sticky, sugar-rich waste called honeydew. This substance coats the leaf surface, providing a perfect substrate for various non-parasitic black fungi to grow. From the plant's viewpoint, the main issue is not the fungus itself, which does not penetrate the leaf tissue, but the thick layer of mold that blocks sunlight. This significantly reduces the plant's ability to perform photosynthesis, leading to reduced vigor and stunted growth. The treatment focuses on controlling the underlying insect pest population with horticultural oils or insecticidal soaps. Once the insects are gone, the sooty mold can be washed off the leaves with a strong jet of water.
Root rot is a common issue often triggered by cultural conditions, specifically poorly draining, waterlogged soil. In saturated soil, oxygen is depleted, causing root cells to suffocate and die. This creates an opportunity for soil-borne fungal pathogens like Phytophthora and Pythium to attack the weakened and damaged root system. These fungi are especially destructive as they are water molds, thriving in wet conditions. They infect and destroy the root cortex and vascular tissues, preventing the plant from absorbing water and minerals. Above-ground symptoms mirror this, appearing as wilting, yellowing leaves, and dieback despite wet soil. Improving soil drainage is the primary preventative measure. For mildly affected plants, reducing watering and applying a appropriate fungicide may help, but advanced cases often require plant removal.
