Zoysia grass is a popular choice for homeowners due to its dense growth habit and tolerance for heat and drought. This warm-season turfgrass forms a thick, carpet-like lawn, making its sudden decline, often appearing as brown patches or thinning areas, confusing for property owners. To understand why a Zoysia lawn may be dying, one must systematically investigate the most common threats, including specific pathogens, damaging pests, and issues related to cultural maintenance.
Fungal Diseases Specific to Zoysia
Fungal diseases are a primary cause of decline in Zoysia lawns, thriving when environmental conditions favor pathogen growth. The most common disease is Zoysia Patch, also known as Large Patch, caused by the fungus Rhizoctonia solani. This disease is most active during the transition seasons of spring and fall when nights are cool (below 70°F) and moisture is high.
Zoysia Patch manifests as roughly circular or irregularly shaped patches of brown or straw-colored turf. The outer edge of the patch, where the fungus is actively spreading, may display a distinct reddish-orange or smoky-brown border, sometimes called a smoke ring. The fungus attacks the leaf sheath and the crown, disrupting the flow of water and nutrients. Management involves reducing nitrogen fertilizer application during active disease periods and improving air circulation.
Dollar Spot is characterized by small, round spots of bleached or straw-colored grass. This fungus is often observed when the lawn is underfed with nitrogen or experiences prolonged leaf wetness, such as heavy morning dew. If untreated, these small spots can merge to form large, irregular areas of dead turf. Proper irrigation that allows grass blades to dry quickly is a key cultural practice for prevention.
Brown Patch, also caused by Rhizoctonia solani, affects Zoysia during warm, humid summer months when temperatures are consistently high. Unlike Zoysia Patch, which prefers cooler seasons, Brown Patch thrives in conditions of poor air circulation or overwatering. It appears as circular patches of yellow or brown turf, sometimes with a dark ring around the edges. Controlling these outbreaks often requires addressing underlying issues like excessive thatch buildup and poor drainage.
Insect Pests and Subsurface Damage
Damage caused by insect pests often targets the root system and crowns of Zoysia plants. White grubs, the larval stage of various scarab beetles, are destructive subsurface pests. These C-shaped larvae feed on grass roots, causing the turf to weaken, yellow, and die in irregular patches.
Severe grub damage causes the affected grass to lose its anchorage, allowing the turf to be rolled back easily like carpet. A population of ten or more grubs per square foot warrants intervention. Chinch bugs cause damage above ground by piercing grass blades and sucking out nutrients. Their feeding introduces a toxin that results in rapidly spreading, irregular patches of yellowing and browning turf, often starting in the hottest, sunniest areas of the lawn.
Other pests include Sod Webworms, caterpillars that feed on grass blades at the crown level, causing a ragged appearance. Hunting billbugs are also a problem; their larvae bore into the stems and rhizomes, causing the stems to break easily. The combined damage from these pests—root consumption, stem damage, and nutrient extraction—leads to dead Zoysia patches that mimic drought stress, making accurate identification essential.
Environmental and Maintenance Stressors
Many Zoysia decline issues stem from cultural practices or environmental extremes rather than living organisms. Water stress is a frequent culprit, as both overwatering and underwatering lead to similar symptoms of browning and wilting. Although Zoysia is drought-tolerant, it requires about one inch of water per week, including rainfall, for optimal health.
Overwatering, especially shallow and frequent watering, saturates the soil, promoting root rot and creating humid conditions favored by fungal diseases. Conversely, chronic underwatering forces the grass into drought-induced dormancy, turning it tan or brown, which leads to death if the stress is prolonged.
Mowing errors also compromise lawn health. Scalping, or cutting the grass too short, removes too much leaf blade, reducing the plant’s ability to photosynthesize and recover. Zoysia should be maintained at a height between 0.5 and 2 inches, and no more than one-third of the blade should be removed in a single session. The dense growth habit of Zoysia also produces thatch, a buildup of organic material that prevents water and nutrients from reaching the roots if it exceeds a half-inch thickness.
Soil compaction is another abiotic stressor, often found in high-traffic areas or heavy clay soils. Compacted soil impedes the movement of air and water, restricts root growth, and increases susceptibility to disease and drought. Other factors include chemical damage from applying herbicides when the grass is stressed, and over-fertilization, which stimulates excessive growth, contributing to thatch and increasing vulnerability to diseases like Brown Patch.
Diagnosing the Culprit and Immediate Recovery
Determining the exact cause of Zoysia decline requires a methodical approach using simple, hands-on diagnostic tests. To check for subsurface root damage from pests like grubs, homeowners can perform the “tug test.” If the grass pulls up easily with little resistance, it suggests the roots have been severed by feeding pests.
To evaluate soil conditions, the “screwdriver test” identifies water issues or compaction. A screwdriver should easily push into the soil four to six inches; difficulty inserting it indicates severe compaction or extreme dryness. Visual inspection helps distinguish between fungal patches and drought stress. Fungal diseases often present with distinct circular patterns or lesions on the blades, while drought stress appears as a uniform, widespread browning of the turf.
Immediate recovery steps should prioritize triage and stress reduction. If a pathogen is suspected, temporarily stop fertilizing, as nitrogen can fuel fungal growth. Reducing the frequency of watering is a common initial step, favoring deep, infrequent watering over saturation. Simple actions that reduce stress on the recovering turf include improving drainage and ensuring the mower blade is sharp and set to the correct height.