Brown Patch

Brown patch is a foliar disease that affects cool-season grasses. It does not harm the crowns, stolons, or rhizomes of turfgrass plants. The disease was first observed in the late 1920s and has been a persistent issue in turfgrass management. Even moderate outbreaks of brown patch can lead to thin and poor-quality turf, making it susceptible to weed, moss, or algae invasion.

 

Causal agents

Brown patch is commonly associated with the fungus Rhizoctonia solani, but recent research has revealed that multiple Rhizoctonia-like fungi can be isolated from typical brown patch symptoms, especially in tall fescue. Two additional species, Rhizoctonia cerealis and Waitea zeae (formerly known as Rhizoctonia zeae), have also been found to cause brown patch, particularly during the months of June, July, and August. As these different species induce similar symptoms, visual diagnosis alone is not sufficient to determine the specific causal agent of brown patch.

 

Primary Hosts

All cool-season grasses. Lolium perenne, Festuca arundinacea, and Agrostis capillaris are most affected. 

 

Disease Cycle

Rhizoctonia spp., including R. solani, W. zeae, and W. oryzae, can survive in the soil and plant debris as sclerotia, monilioid cells, or thick-walled mycelia during unfavorable periods. These structures are highly resistant to environmental conditions and pesticides. Sclerotia can germinate within a wide temperature range, with an optimal temperature of 31°C (88°F). The fungus spreads radially in the soil or thatch, forming circular colonies. Under warm and humid conditions, the hyphae infect the moist tissues of turfgrass leaves and sheaths. Infected tissues become waterlogged and dark, but when exposed to sunlight or drying conditions, they turn tan or brown, shrivel, and die. Sclerotia may form on or within infected tissues and are released into the thatch when the tissues decompose.

 

Epidemiology

Brown patch caused by R. solani in cool-season grasses is most severe when there is high leaf wetness or humidity above 95% and night temperatures above 20°C (68°F). Hot, rainy, or humid weather can quickly lead to extensive damage in large areas of turf. Prolonged periods of leaf wetness, around 10 to 12 hours, are necessary for significant brown patch damage to occur. Smaller patches or thinning of the turf are often the result of infections under warm, wet conditions. Factors such as very humid or cloudy weather, poorly drained soil or thatch, and poor air circulation can increase the duration of leaf wetness, especially during the early morning dew. Dense, highly fertilized, and frequently watered turf is more prone to Rhizoctonia spp. infections. In tall fescue, brown patch is most severe when growth is limited by low fertility and heat stress. Inadequate levels of phosphorous and potassium can contribute to the disease’s impact. Models have been developed to help predict brown patch outbreaks in cool-season turfgrasses.

 

Symptoms

Brown patch symptoms on cool-season grasses vary depending on the mowing height. Circular patches ranging from 15 cm to over 90 cm (7 to 35 in) in diameter appear, with colors ranging from brown and tan to yellow. Irregularly shaped lesions with tan centers and dark brown borders are visible on the affected leaves. In humid conditions, mycelium may be present among the border of affected patches leading to the symptom known as “smoke rings.”

 

 

Credit: John Kaminski/Penn State University

Figure 13.6.1: Brown patch symptoms include (a) circular patches or rings of brown, discolored turf. (b) Often a “smoke ring” symptom appears when the disease is active. (c) Turf turns tan or brown and remains blighted when the pathogen is not active until growing conditions are more suitable for plant growth. (d) On taller turf, tan lesions with a chocolate-brown border often appear at the margins of the individual grass blades.

 

 

Signs

Signs of brown patch include the presence of mycelium and, in some cases, sclerotia. Mycelium appears as white to grayish strands on the leaf surface, particularly during periods of high humidity. Sclerotia are compact, dark-brown to black structures that may form within the affected plant tissues or on the leaf surface. These signs can aid in the identification of brown patch disease.

 

Credit: John Kaminski/Penn State University

Figure 13.6.2: Signs of the Rhizoctonia solani include mycelium (a) along the outer edge of individual patches or (b) within the affected area. (c) Puffs of mycelium may also be seen. (d) Individual hyphae are septate, branched at right angles, and often have a constriction where the hyphae branch off. 

 

 

Management

Brown patch can be controlled both preventively and curatively. The disease is heavily influenced by cultural practices including irrigation and fertilization. Fungicides can be highly effective at controlling the disease, and no resistance has been documented in Rhizoctonia pathogens in turfgrass.

 

Cultural

Environmental conditions play a crucial role in the severity of brown patch disease. High levels of readily available nitrogen can enhance the disease, while insufficient nitrogen that limits turf growth can also intensify it. Slow-release nitrogen sources are less likely to enhance brown patch, and low applications of quick-release nitrogen sources have minimal impact. Irrigation practices also influence the severity of brown patch, with early-morning irrigation reducing its severity and evening irrigation intensifying it. Good surface and subsurface water drainage are essential to prevent excessive soil wetness and humidity. Techniques such as manual removal of dew, promoting air circulation, and controlling thatch depth can help mitigate brown patch development. Selecting less susceptible species of turfgrass or cultivars with known resistance can help reduce disease severity.

 

Credit: John Kaminski/Penn State University

Figure 13.6.3: Tall fescue is highly susceptible to brown patch, but differences in disease severity among various cultivars can be seen.

 

 

Chemical

Curative control can be achieved using QoI fungicides (e.g., azoxystrobin or pyraclostrobin), penthiopyrad, or fluxapyroxad. It’s important to note that symptoms may increase for a few days after application as previously infected tissues continue to develop symptoms.

Avoid high rates of DMI fungicides on putting greens during hot summer weather to prevent undesirable growth-regulator effects. Putting greens treated with growth-regulating DMI fungicides may suffer greater infestations of algae during summer. Additionally, applications of Cutless (flurprimidol), a plant growth regulator, have been shown to reduce the efficacy of several DMI fungicides against brown patch.

For high-maintenance tall fescue lawns, ready-to-use fungicide formulations available to homeowners may not provide satisfactory control, based on research trials. However, azoxystrobin was an exception. QoI fungicides have generally performed well over a four-week application interval. Granular formulations of fungicides can provide adequate disease suppression, but sprayable formulations often offer better control in lawns.

Lastly, be aware that some fungicides like chlorothalonil, iprodione, PCNB, and vinclozolin are no longer labeled for use on home lawns. Residential-use fungicide products have shown poor performance in field experiments.