Bermudagrass decline was the first identified root decline, or rot, in warm-season turfgrass caused by the pathogen Gaeumannomyces graminis var. graminis. This same pathogen was later found to cause similar root rots in other grass species. The disease in these grasses has been collectively termed “take-all root rot”. Further studies and research have refined our understanding of the Gaeumannomyces genus and unveiled new species associated with take-all root rot. The species of Magnaporthiopsis and Candidacolonium have also been found to contribute to Bermudagrass decline. These diseases, despite differences in specific pathogens, are grouped together due to similarities in the root and shoot symptoms they cause in the host plants.

Causal agents

Gaeumannomyces spp., Magnaporthiopsis spp., and Candidacolonium spp.

Primary Hosts

• Cynodon species
• Eremochloa ophiurodiedes
• Paspalum vaginatum
• Pennisetum clandestinum
• Stenotaphrum secundatum
• Zoysia japonica

Disease Cycle

Gaeumannomyces graminis, along with other ectotrophic fungi, are known to colonize warm-season turfgrasses, with G. graminis being universally found in grasslands. The disease cycle and survival strategies of these fungi may be comparable to that of G. avenae, but with an enhanced tolerance for temperatures above 25°C (77°F).

To withstand adverse environmental conditions, these ectotrophic fungi colonize and survive within the roots, rhizomes, and stolons of the host plants. The fungi spread from plant to plant by growing along the surfaces of these roots or stolons.

Moreover, the fungi can also be spread by the transportation of infected roots, rhizomes, and stolons, often due to human activities such as aeration and vertical mowing. Similarly, the transport of infected vegetative material like turfgrass sod and sprigs can disseminate the fungi.

Sod that’s severely weakened by root rot may be challenging to harvest.

Epidemiology

No information is available on this new complex of pathogens.

Symptoms

Bermudagrass decline of Cynodon spp. hybrids and take-all root rot share similar symptomology. The diseases first appear in irregularly shaped, chlorotic patches up to 1 m (3 ft) in diameter. The lower leaves are the first to become chlorotic, progressing into the upper canopy. Roots, stolons, and rhizomes turn off-white to dark brown, and isolated black lesions may be present. The entire root system may eventually turn black and become completely rotted. Less severely infected roots may bear dark strands of hyphae oriented parallel to the root axes. Affected patches may become thinned or void of living turf, with dead areas that may exceed 5 m (16 ft) in diameter.

Credit: Maria Tomaso-Peterson/Mississippi State University

Signs

Signs of Bermudagrass decline and take-all root rot include the presence of dark brown to black runner hyphae and growth cessation structures on the surfaces of roots, stolons, and rhizomes. The most common sign is the unique, lobed hyphopodia structures. These structures are formed by Gaeumannomyces spp., the principal causal agent of the diseases. In addition, flask-shaped, beaked, ostiolate perithecia with curved necks may occasionally be observed protruding through the lower leaf sheaths, although they are rarely seen on affected plants unless incubated in a laboratory. It is also possible to isolate ectotrophic root-infecting fungi associated with rotted roots from asymptomatic plants of the same species, such as the turf on aprons, fairways, and sports fields surrounding affected areas.

Credit: Maria Tomaso-Peterson/Mississippi State University

Management

Cultural practices include stress-relieving measures such as adjusted mowing height, frequent core aeration, and maintaining adequate plant nutrition. Chemical approaches encompass the use of slow-release nitrogen sources or acidifying fertilizers, avoiding nitrate-based ones, and preventive use of specific fungicides. Together, these tactics can help mitigate the incidence of diseases like take-all root rot and Bermudagrass decline.

Cultural

All cultivars and hybrids of warm-season grasses appear to be susceptible to infection by these ectotrophic fungi. Practices that relieve plant stress are effective in reducing symptom expression. Raising the mowing height during periods of high rainfall can help. Core aeration should be performed frequently to relieve or prevent soil compaction, and cores should be removed. Solid tines should be used to reduce root injury.

Adequate plant nutrition is crucial, particularly nitrogen, potassium, and micronutrients. Slow-release nitrogen sources or acidifying fertilizers should be used, while fertilizers containing nitrate sources should be avoided.

Chemical

Fungicides may help reduce the incidence of root decline when used preventively. Acropetal penetrant fungicides (e.g., benzimidazoles, demethylation inhibitors, and strobilurins) may help reduce the incidence of root decline in Stenotaphrum spp. when used preventively. Demethylation inhibitor fungicides that are safe on Cynodon spp. have been effective when applied in late summer and irrigated in immediately after application.