Identification and management of mosaic viruses and secondary pathogens in buffalo turf (TU19000)

During 2020 this investment helped the turf industry better understand the distribution and frequency of mosaic viruses in buffalo grass paddocks across the country. Buffalo grass yellowing is causing great concern to turf farmers in Australia, particularly in New South Wales and Queensland. Many disease control methods have been tested by farmers, but no industry-wide solution has been found.

The project team looked at whether there is a correlation between the severity of infection and farm management practices such as the use of pesticides and fertilisers, irrigation methods and farm hygiene, and how seasonal and environmental factors can impact yellowing, which is the key symptom of the viruses. Anecdotal information from turf growers was also collected on how seasonal and environmental factors impact the severity of the disease, as well as indications for how significant this issue is for industry.

The team investigated whether panicum mosaic virus (PMV) and sugarcane mosaic virus (SCMV), were responsible for at least some of the yellowing symptoms. Experiments were also done to search for any previously unrecognised viruses.

Disease surveys were conducted in buffalo grass on 27 farms in New South Wales, Queensland and Western Australia. SCMV was the most damaging and widespread of the viral pathogens detected, being present in all three States with all major buffalo grass cultivars affected.

Studies showed the SCMV isolates were related to an ‘East African clade’, containing maize virus isolates from Kenya and Tanzania, as well blue couch (Digitaria didactyla) and Sabi grass (Urochloa mosambicensis) virus isolates. Analysis suggested multiple introductions of SCMV from some of these alternative hosts, rather than a single introduction and dispersal through trade of infected turf.

Diagnostic tests were also undertaken for Bermuda grass latent virus (BGLV), which causes viral lethal necrosis of buffalo grass in Florida when present as a mixed infection with SCMV. It was not detected in buffalo grass but was found in a green couch (Cynodon dactylon) plant collected from a turf farm in south east Queensland. This Is the first time BGLV has been recorded outside the USA and demonstrates that the two agents responsible for lethal necrosis are present in Australia and pose a threat to the buffalo grass turf industry.

PMV had much more restricted distribution than SCMV, as it was only found in one cultivar in the Hawkesbury Valley. Panicum papanivirus 1, a satellite virus commonly associated with PMV in the USA, was not detected. Analysis suggests a single introduction of PMV to Australia, with localised dispersal. Given the restricted distribution, eradication of PMV from Australian turf farms would be feasible and desirable to prevent St Augustine grass decline.

Virtually all farmers were unaware of the presence of plant viruses in their turf. Although some farmers had implemented crop hygiene practices to prevent cross-contamination of turf varieties, none of these were adequate to prevent virus spread. As a consequence of positive identification of SCMV on a property, the farmer took immediate action to eradicate it with an herbicide application.

Many cases of buffalo grass yellowing could not be attributed to viral infection and plants affected by this type of yellowing had poor root health. Fungi were isolated from decaying roots with Curvularia most common species. A causal relationship between its presence and symptoms was not demonstrated, although it should be considered a prime candidate for causing the poor root health.

Interestingly, environmental factors previously demonstrated to favour epidemics of Curvularia spp. Included overuse of nitrogen fertilisers and build-up of thatch and grass clippings. Many of the farmers said they thought these factors were associated with the onset of buffalo grass yellows.

Yellowing is one of the most common disease symptoms observed in grasses. It can be caused by a range of pathogens, as well as abiotic factors such as plant nutrition and water stress. It is therefore not surprising that buffalo grass yellowing syndrome in Australia appears to have multiple independent causes, including plant viral and fungal pathogens and trace element deficiencies. Recommendations for future research to develop integrated disease management strategies were made by the project team.