The wiles of the wicked web blight

Everyone is used to the idea of Rhizoctonia solani causing damping off or cankers on plant stems at the soil line — we’ve seen that on vinca, impatiens, New Guinea impatiens, dahlia and many other plants. But are you aware that some strains of Rhizoctonia can also venture up into the canopy of the plant, devouring leaves as they go? This results in a symptom called “web blight” — and it can be a killer. An incorrect diagnosis is easy to imagine, as plants with extensively infected foliage appear to be dying from Botrytis, Phytophthora blight or Fusarium wilt.

1. Rhizoctonia solani is a common fungus. It forms cobweb-like growth on infected leaves or stems. When seen under the microscope, the filaments have angular and right-angle branching.
2. Certain strains of Rhizoctonia may blight foliage directly. Some ornamental crops particularly vulnerable to this web blight are azalea, rosemary and chrysanthemum. Beans are also prone to this disease, and turfgrass as well. The pathogen is moved on blown or splashed soil particles, or on infected plants. Unlike most other fungal pathogens, it does not routinely form spores. (More research is needed to understand the role of basidiospores formed by the sexual state of R. solani). Rhizoctonia moves well without spores because it can form mycelial bridges between nearby plants, literally growing from one plant to another.
3. Rhizoctonia web blight is found in hot, humid environments — and thrives when plants are kept tightly together. Rain and relative humidity of 80-90 percent or more favors disease development. The ideal temperature for disease is 68-86 degrees Fahrenheit. Climate differences explain why the disease is found in the southern United States more regularly than in the north.
4. Plants will appear healthy from above even as the fungus is spreading through the lower foliage of densely packed mums, azaleas or rosemary. Last year, when fall crops of garden mums in the northern United States were spaced, the injury from Rhizoctonia web blight was suddenly apparent. Research by Warren Copes at USDA-ARS has shown that crowding in azaleas does not actually make blight worse if environmental conditions are ideal for the disease. In normal production conditions, however, keeping foliage from touching could be important for reducing plant-to-plant spread.

Monitoring

Watch for brown leaves and webbing on rosemary, mums, azaleas or other crops. Inner foliage may appear browned and matted together, and some stems may be coated white or tan by the fungus.

Monitor foliage quality at the center of a block, not just at the outer edges.

Use a microscope to distinguish between cobwebs and Rhizoctonia, or send a suspicious sample to a diagnostic lab.

Prevention

• Handle cuttings and plants with careful attention to sanitation.
• If plants are started pot-to-pot, make an effort to space them when needed to provide air movement between plants.
• Never reuse pots after an outbreak without a concerted effort at cleaning and disinfecting them: Rhizoctonia forms sclerotia, hard knots of fungus tissue that glom onto pots and resist a simple rinse.
• Irrigate early in the day.
• Gather up all leaf debris after an outbreak; use a clean growing surface.

Treatment

• Preventive fungicide treatment may be essential during summer heat in very humid climates. Treat on a calendar basis if web blight is an annual problem; monitor crops if you are a northern grower and treat if symptoms appear.
• Flutolanil (e.g. ProStar, Contrast) and strobilurins (e.g. Heritage, Compass) are examples of effective Rhizoctonia materials for use on ornamentals (see labels for restrictions).
• We know relatively little about the ability of biocontrols to inhibit web blight, but competitors on the leaf surface might help to reduce this disease; biocontrols would be the only option for rosemary.

Margery is a plant pathologist specializing in ornamentals at Cornell’s Long Island Horticultural Research & Extension Center. She aims to help growers outwit diseases.