Yellow tuft disease is caused by a water mold fungus, and is also known as “downy mildew”. The affected areas may have a water-soaked appearance and can range in size from a few centimeters to several meters in diameter. As the disease progresses, the affected turfgrass may develop a thinning or wilted appearance, and in severe cases, the grass can die, leaving behind bare patches.

Causal agents

Sclerophthora macrospora. This organism Is classified within the kingdom Chromista.

Primary Hosts

Yellow tuft occurs in most turfgrass species. In Stenotaphrum secundatum and Zoysia japonica, the disease is also known as downy mildew

Disease Cycle

The disease cycle of Sclerophthora macrospora involves the systemic spread of its mycelium into various parts of the plant, including stems, sheaths, leaves, culms, and inflorescences. It can also invade seeds and become seedborne. The pathogen persists as mycelium in the intercellular spaces for extended periods. During moist weather, it produces sporangiophores that protrude through leaf stomata, bearing pearly white, pyriform sporangia. These sporangia are only turgid when the leaf surface is wet and collapse when the leaves dry, leaving behind a residue. Sporangia germinate to release numerous zoospores that actively swim in moisture on leaves and water surfaces. Zoospores encyst and germinate to produce hyphal strands, which can infect the juvenile meristem of another plant. Oospores, although more common in annuals like Digitaria spp., can also be produced in infected leaves of perennial grasses. They can persist for several months and germinate under suitable conditions to produce sporangia or germ tubes.

Epidemiology

The epidemiology of yellow tuft involves its dependence on free water for the movement of infectious spores. As a result, the disease is commonly found in low-lying areas with poor drainage. The pathogen has a close association with the infected plant and thrives under conditions that are favorable for the growth of the grass.

Symptoms

Yellow tuft initially manifests as slightly stunted growth and slightly thickened or broadened leaf blades without discoloration. As the disease progresses, small yellow spots or patches develop, representing excessively tillered plants with dense clusters of yellow shoots and shortened roots. The affected plants can be easily detached from the turf.  The symptoms are most prominent in late spring and autumn, particularly in poorly drained areas. In irrigated turf, yellow tuft symptoms can persist throughout the summer, with infected plants sometimes dying during hot, dry periods.

In S. secundatum, the infected plants do not show adverse effects but exhibit symptoms resembling viral diseases, including linear white streaks parallel to leaf veins and slightly raised epidermis over the streaks. In Z. japonica, the primary symptom is the yellow tuft, but the disease may also cause a diffuse yellowing.

Credit: John Kaminski/Penn State University

Signs

Signs of the pathogen include the formation of white, downy growth on the leaf surfaces during cool and wet periods. These growths consist of sporangia, which can be observed microscopically. Additionally, microscopic examination of the leaves may reveal the presence of pyriform sporangia, oospores, which are larger than those of Pythium spp., as well as coenocytic mycelium and zoospores.

Credit: Peter Dernoeden/University of Maryland

Management

Managing yellow tuft involves reducing standing water on turfgrass surfaces and the application of select fungicides.

Cultural

To manage yellow tuft, it is important to maintain adequate drainage to prevent water accumulation on the turf surface during irrigation and rainfall. The turf should be managed to promote active but not overly lush growth. Yellow-tufted plants can be detached by vertical cutting. Unfortunately, no cultivars are known to be resistant to yellow tuft.

Chemical

When it comes to fungicide treatment, mefenoxam is the most effective when applied preventively. For curative applications, fungicides must be applied 2-3 times on a 14-d interval to be effective.