Stem rot is a common plant affliction resulting from infection by various fungal or bacterial pathogens, which primarily target the plant’s main support structure. This disease compromises the integrity of the stem, often beginning near the soil line where moisture is retained and pathogens thrive. Identifying the specific visual changes on the stem and the subsequent effects on the upper plant structure is the first step in understanding the progression of this disease.
Visual Signs on the Stem Surface
The first observable signs of stem rot typically manifest as changes in the color and texture of the stem’s exterior tissue, often near the crown of the plant. Healthy tissue is replaced by areas of discoloration, commonly appearing as dark brown, black, or reddish-brown lesions. These spots may also present as reddish or purplish streaks that run parallel to the stem, indicating initial infection of the outer cell layers.
The texture of the infected area provides clues about the type of pathogen involved, with two general appearances being common. A “wet rot,” often caused by water molds or bacteria, presents as a water-soaked or slimy lesion that is soft to the touch. Conversely, a “dry rot,” often caused by fungi like Rhizoctonia, results in a dry, firm, and sunken lesion known as a canker.
These lesions are localized areas of dead tissue that may start small but tend to expand over time. As the infection progresses, the lesions grow larger and deeper, potentially forming elongated, slightly depressed areas on the stem surface. The visibility of these distinct, often firm cankers against the surrounding healthy tissue is a clear indicator that the disease is actively consuming the structural material of the stem.
Structural Consequences of Stem Rot
The failure of the plant’s physical support system and internal plumbing is a key consequence of stem rot. As the lesions expand to completely encircle the stem, a process known as girdling occurs, which severs the connection between the roots and the upper foliage. This action destroys the mechanical strength of the stem, causing the plant to suddenly snap or collapse, frequently at the point of infection just above the soil line.
Internal damage is revealed when an infected stem is cross-sectioned, showing discoloration within the vascular tissue. The normally white or pale green interior may appear brown or black, a sign that the pathogen has destroyed the xylem and phloem vessels responsible for transport. In advanced bacterial soft rots, the internal tissue may become mushy, soft, and even hollowed out, sometimes accompanied by a foul odor.
In cases of fungal infection, the visual signs can include the presence of the pathogen’s reproductive structures on the stem’s surface. A white, fluffy, or cottony fungal growth, known as mycelium, may be visible on the wet, decaying tissue. Additionally, certain pathogens produce small, hard, black resting bodies called sclerotia, which are embedded within the diseased tissue.
Secondary Symptoms on Leaves and Foliage
The rot’s impact on the stem leads to secondary signs that become visible on the leaves and upper parts of the plant. The most common symptom is wilting, which occurs even when the soil has sufficient moisture because the damaged stem cannot transport water from the roots to the leaves. This wilt is often sudden and irreversible once the stem is significantly compromised, indicating a failure of the plant’s hydraulic system.
The blockage of water and nutrients also causes distinct color changes in the foliage, such as rapid yellowing, known as chlorosis. This yellowing typically begins with the oldest or lowest leaves and progresses upward as the vascular disruption starves the plant of resources. In severe cases, the leaves quickly turn brown and die (necrosis).
Beyond wilting and discoloration, the plant’s overall growth rate will be severely impacted, leading to stunting compared to healthy specimens. Over time, the tips of branches and stems above the point of infection may also begin to die back, a symptom called dieback. These upper-plant signs are indirect consequences of the primary infection in the stem.