Verticillium wilt is a common plant disease caused by soil-borne fungi, primarily Verticillium dahliae and Verticillium albo-atrum. The fungus invades the plant through the root system, often entering through natural openings or small wounds. Once inside, the fungus colonizes the plant’s vascular system. The plant responds by producing gum-like substances that, along with the fungal growth, clog the vessels. This internal blockage prevents water and nutrients from reaching the leaves and stems, leading to the characteristic symptoms of wilting and dieback.
External Symptoms on Leaves and Branches
The most visible sign of Verticillium wilt is the sudden, often localized, wilting of foliage and branches. In woody plants, this symptom frequently appears on only one side of the plant or on a single branch, known as unilateral or sectorial wilting. This uneven distribution occurs because the fungus may only be active in the vascular bundles supplying that specific section.
Leaves on affected branches typically display a distinct sequence of color changes, beginning with chlorosis, or yellowing, which often starts at the margins and advances inward. This yellowing progresses to necrosis, appearing as brown or scorched tissue, before the leaves eventually shrivel and die. In some plants, especially herbaceous ones, V-shaped chlorotic lesions may form on the leaf edges.
The wilting is often most pronounced during the hottest parts of the day when the plant’s water demands are highest, and may appear to lessen or recover overnight. Over time, the affected branches experience dieback, where the tips and entire limbs die from the lack of water. Infected plants also show signs of chronic stress, such as stunted growth, sparse or abnormally small foliage, and premature defoliation.
Internal Diagnostic Signs
To confirm a suspicion of Verticillium wilt, an examination of the plant’s internal structure is necessary, focusing on the water-conducting xylem tissue. By cutting a cross-section of a symptomatic stem or branch, one can look for characteristic vascular discoloration. This staining is a direct result of the plant’s reaction to the fungal invasion, which includes the deposition of dark pigments within the xylem.
The discoloration usually appears as a ring or series of dark streaks, contrasting sharply with the healthy, lighter-colored wood. The color of the streaking can vary depending on the host species; in most woody plants, it is dark brown or greenish-black, but in some, like maple, it may appear olive green. It is helpful to make cuts on branches that are showing early stages of wilting, as older, dead wood may show general decay that obscures the specific streaking.
Host Range and Disease Progression
Verticillium wilt has a broad host range, affecting over 350 species of eudicot plants, including trees, shrubs, vegetables, and ornamental flowers. Common susceptible hosts include maple, catalpa, smoke-tree, tomato, potato, and strawberry. Notably, all monocots, such as grasses, and conifers are generally immune to the disease. The progression of the disease varies significantly depending on the host and the strain of the fungus.
In herbaceous crops like vegetables, the infection is often acute, leading to rapid wilting and death within a single growing season. Woody plants, such as mature trees, typically exhibit a chronic, slower decline, where symptoms may worsen over several years before the plant succumbs.
The fungus survives in the soil for extended periods, sometimes for over a decade, in the form of resilient resting structures called microsclerotia. These structures withstand harsh conditions and germinate when a susceptible plant’s roots grow nearby. Once an area is infected, replanting with susceptible species will likely result in reinfection.
Differentiation from Similar Plant Stressors
Identifying Verticillium wilt requires careful observation, as its symptoms can mimic those caused by other issues like drought or nutrient deficiencies. Drought stress typically causes uniform wilting across the entire plant, unlike the localized, unilateral wilting often seen with Verticillium wilt. Nutrient deficiency usually results in uniform chlorosis, whereas Verticillium wilt often causes yellowing that progresses to necrosis from the leaf edge inward.
It is important to distinguish Verticillium wilt from other vascular diseases, particularly Fusarium wilt, which affects many of the same plant types. Fusarium wilt is favored by warmer soil temperatures, while Verticillium wilt is more common in temperate and cooler climates.
While both diseases cause dark streaking in the stem’s vascular tissue, Fusarium tends to cause discoloration lower in the stem, sometimes restricted to the base of the plant. The presence of unilateral wilting combined with the characteristic dark, often greenish, streaking in the xylem provides strong visual evidence for Verticillium wilt. However, definitive confirmation often requires laboratory analysis to isolate and identify the specific fungal pathogen.