Brown Patch, caused by the fungus Rhizoctonia solani, is one of the most common and damaging turfgrass diseases, particularly in areas with warm, humid summers. This soil-borne pathogen leads to a rapid decline in lawn quality. Addressing this disease effectively requires a targeted approach, combining an understanding of the pathogen’s biology with the strategic use of chemical control methods. The optimal fungicide strategy involves selecting products based on their mechanism of action and employing application timing and rotation to ensure long-term efficacy.
Identifying Brown Patch Symptoms and Causes
The disease first manifests as roughly circular patches of thinned, discolored grass that can range from a few inches to several feet in diameter. On closely mowed turf, such as bentgrass, a distinct dark purplish-gray band, often called a “smoke ring,” may appear around the patch’s periphery, indicating actively spreading fungal growth. Within the affected areas, the individual leaf blades often display tan or light brown lesions with dark brown or reddish borders, a characteristic sign on wider-bladed grasses like tall fescue.
Brown Patch outbreaks are triggered by specific environmental conditions that allow the R. solani mycelium to spread from the soil and thatch layer to the grass blades. The fungus becomes most aggressive when night temperatures remain consistently above 68°F and daytime temperatures exceed 80°F. Prolonged leaf wetness is another significant factor, requiring the grass blades to be continuously wet for 10 to 12 hours or more for infection to occur. Turf fertilized with excessive nitrogen during the summer months is also highly susceptible to disease severity.
Systemic vs. Contact Fungicides
Fungicides are categorized by how they interact with the plant: contact and systemic. Contact fungicides, also known as protectants, work exclusively on the surface of the grass blade. They create a chemical barrier that prevents fungal spores from germinating and penetrating the plant tissue. Since they do not move into the plant, contact products must be reapplied frequently to protect new growth.
Systemic fungicides, or penetrants, are absorbed by the plant and move internally through its vascular system. This internal movement allows the product to protect tissue that was not directly sprayed. Systemic products are preferred for their longer residual protection and capacity to act both preventatively and curatively. They can stop an infection that has already begun, a capability contact fungicides lack.
A comprehensive disease control program often integrates both types, leveraging the immediate surface protection of a contact fungicide with the internal, longer-lasting protection of a systemic product. Systemic fungicides often target a single biochemical pathway, increasing the risk of the pathogen developing resistance over time. Therefore, their use requires careful management, particularly when treating persistent diseases like Brown Patch.
Essential Active Ingredients for Control
The “best” fungicide for Brown Patch is not a single product but a group of highly effective active ingredients used in rotation. These ingredients are classified into chemical groups based on their mode of action, standardized by the Fungicide Resistance Action Committee (FRAC). The most effective classes for controlling Rhizoctonia solani are the Demethylation Inhibitors (DMIs) and the Quinone Outside Inhibitors (QoIs), which are frequently used together.
DMIs (FRAC Group 3) include active ingredients such as propiconazole and triticonazole. These systemic fungicides inhibit a specific enzyme in the fungal sterol biosynthesis pathway, disrupting the formation of the fungal cell membrane. Propiconazole provides strong curative properties, offering a fast knockdown of existing infections. Its residual activity is often shorter than other classes, and it can sometimes cause temporary growth regulation or stunting if applied at excessive rates.
QoIs, also known as Strobilurins (FRAC Group 11), are considered the backbone of a successful Brown Patch program. Active ingredients like azoxystrobin, fluoxastrobin, and pyraclostrobin work by inhibiting fungal respiration, effectively blocking the fungus’s energy production. Azoxystrobin is frequently cited as a long-lasting active ingredient that offers excellent preventative control, sometimes protecting the turf for up to four weeks. Fluoxastrobin is recognized for its strong solo activity and systemic movement, providing both curative and preventative action.
Another important class is the Benzimidazoles (FRAC Group 1), exemplified by thiophanate-methyl, which serves as a valuable rotational product. This group interferes with cell division in the fungus, offering both protective and curative action. For homeowners, combination products that pair a QoI and a DMI, such as azoxystrobin and propiconazole, are popular because they offer two different modes of action in a single application. This provides a broader spectrum of control and a degree of built-in resistance management.
Application Timing and Rotation Strategies
Controlling Brown Patch is most successful when fungicides are applied preventatively, before the disease becomes visually apparent. The ideal timing for the first application is determined by soil temperature, which signals the beginning of the fungus’s active growth cycle. Preventative treatments should begin when nighttime low temperatures consistently reach 60°F to 70°F, or when the 24-hour average soil temperature reaches 65°F.
Once the disease is active, a curative application is necessary, though it will only stop the spread and not “cure” the tissue that is already damaged. Application intervals typically range from 14 to 28 days, depending on the specific active ingredient, the product concentration, and the severity of the disease pressure. The product label must always be consulted to determine the precise reapplication rate and safety precautions.
The most effective strategy is Fungicide Resistance Management, which involves rotating products based on their FRAC code. Relying solely on a single mode of action encourages the fungal population to develop resistance, rendering the product useless over time. A sound rotation involves alternating between different FRAC groups, such as applying a Group 11 QoI first, followed by a Group 3 DMI in the next application window.
Proper application technique is also important for optimizing efficacy. For contact fungicides, it is recommended not to water the turf immediately after treatment, allowing the product to dry and form its protective surface barrier. Conversely, systemic fungicides absorbed by the roots should be watered in shortly after application to facilitate their movement into the plant’s system. Additionally, irrigation should be scheduled for the early morning hours, as this allows the leaf blades to dry quickly, shortening the critical leaf wetness duration required for infection.