Final report B.PAS.0006: Resistant grass varieties and endophytes

Pasture dieback causes unhealthy growth and death in a range of introduced and native grasses across Queensland and into northern New South Wales, resulting in large losses in beef production areas. Pasture mealybug, Heliococcus summervillei Brookes, has been identified as the leading cause of pasture dieback.
Possible management strategies include the identification of resistant or tolerant pasture varieties, and the use of endophytic fungi (both seed born and facultative) to protect pasture grasses possibly in association with pasture mixes including legumes. In New Zealand, pasture dieback in rye grass caused by the mealybug Balanococcus poae has been effectively managed by selection of endophytic fungi and incorporation into commercial pasture varieties. Fungal endophytes may be transferred in seed, while others are acquired from healthy soils, primarily by colonising the roots.
Assay methods were developed and used. The relative susceptibility of 15 selected grass varieties to pasture mealybug was determined in screenhouse assays by infestation with the mealybug H. summervillei. Significant differences in susceptibility were detected. Mekong brizantha, Argentina bahia, curly Mitchel grass, Biloela buffel and Gatton panic were all less susceptible to mealybug than American buffel. Callide and Katambora Rhodes, Paspalum nicorae, green panic, and Gayndah buffel were all similar in susceptibility to American buffel. In sensitive, rapid and statistically robust laboratory assays, mealybugs on Gatton panic and Mekong brizantha died more rapidly and in higher proportions, and were less able to develop to maturity than on American Buffel.
Assays demonstrated that the seed-born fungal endophyte Epichloë festucae reduced the susceptibility of perennial ryegrass to the Australian pasture mealybug H. summervillei, and reduced the severity of dieback symptoms. This result provides ‘proof of concept’ of the use of endophytes, as used commercially in New Zealand against the pasture mealybug Balanococcus poae.
This work demonstrates a direct link between ‘pasture dieback’, pasture diversity and soil health. Analysis of soils sampled systematically across Queensland and New South Wales showed that severity of dieback was reduced in pastures with greater diversity of species in 5 key genera of beneficial rhizospheric-competent fungal endophytes. Abundance of these fungi was shown to be higher in trial plots planted with legumes in pasture mixes than in plots sown with pasture grasses alone. Management practices that increase diversity of beneficial rhizospheric fungi, such as inclusion of legumes, and impacts of management that reduces abundance and diversity of these fungi (such as, possibly, hot burning) requires investigation.
Further research to screen varieties for tolerance/ resistance and to determine tolerance / resistance mechanisms using a combination of rapid laboratory assay method and transcriptome analysis (B.PAS 0003) are strongly recommended. Further assays and detailed analysis of plant/endophyte/mealybug interactions in grasses to should be conducted in combination with trials on pasture legume mixes is strongly recommended.
The project has significantly increased options to manage pasture dieback. Tolerant and resistant pasture varieties have been identified. The benefit of both seed-born and rhizospheric fungal endophytes in reducing mealybug and pasture dieback in the laboratory and in the field has been demonstrated. Rapid assays have been developed to reduce cost and increase speed and statistical power of future screening for development of new pasture varieties and endophytic symbionts for protection of tropical and subtropical pastures.