Microbe of the Month: Friendly fungus to grass, but poison to grazing animals

Anyone driving around Kentucky and adjoining states is bound to notice the lush green of much of the turf and pasture land. It is the colour of tall fescue, a grass that grows vigorously to produce stunning green swards and is exceptionally resistant to attack by insects and nematode worms. It also survives well in areas subject to drought and in soil depleted of nutrients.

These qualities, which have made tall fescue particularly attractive to farmers in the upper South and lower Midwest of the United States, come from a fungus, Acremonium coenophialum, which grows inside the plant. The infection does not harm the grass, which shows no visible signs of the fungus within. Seeds carry Acremonium from generation to generation. Soon after germination, thread-like filaments of the fungus begin to spread through the plant, particularly in the leaf sheaths. As flowering approaches, they concentrate in the tissues where the flower heads are to form.

But there is a paradox behind the agricultural success of tall fescue. The microbe responsible for the grass's robust characteristics also has toxic effects, the chemical basis of which is becoming clear, on grazing animals. Because some less hardy varieties of tall fescue do not carry Acremonium, there are hopes that they can be infected with novel strains, genetically engineered to confer benefits but not disadvantages. Meanwhile, as Donald Ball of Auburn University, Alabama, and colleagues report in the current issue of American Scientist, farmers must accept the harm to their livestock or switch to a grass with much less robust characteristics.

The earliest indications of problems came to light after tall fescue first began to be planted in the US during the Forties. There were reports of declining health and productivity in cattle, sheep and horses grazing on the grass. But only in 1973 was the fungus incriminated, when Department of Agriculture scientists investigated a farm near Mansfield in Georgia, where cattle in one field of tall fescue were perfectly healthy while those on apparently identical pasture nearby were sick. They found that 10 per cent of the grass in the first field was infected with fungus, compared with virtually 100 per cent in the second.

We can now discern a symbiosis between the grass and the fungus. The plant provides nourishment for Acremonium, which confers benefits in return. One example is the 'leaf roll' of infected fescue when, during periods of drought, the blades of grass curl inwards to minimise water loss. Uninfected plants, unable to retain water in this way, may die under identical conditions. Acremonium also makes the fescue toxic to both insects and grazing livestock. Analysis indicates that this is attributable to substances known as peptide ergot alkaloids, produced by the fungus. Although Acremonium does not grow into the leaf blades, the toxic alkaloids travel there through the tissues from elsewhere in the plant.

One of the most striking effects of the infected grass in cattle is 'summer slump'. In high temperatures, the animals become lethargic, salivate excessively and spend much of their time seeking water and shade. Recent studies have also shown that cows and sheep grazing on infected fescue produce less milk and have lower pregnancy rates. A conservative estimate of the total losses among livestock in the US is dollars 500m to dollars 1bn a year.

But considerable difficulties face a farmer who decides to plant uninfected fescues or totally different grass. Not only does this mean relinquishing the benefits of pest resistance, a long growing season, the capacity to grow in land impoverished in nutrients, and tolerance of drought and a wide range of acidity and alkalinity. There is also the considerable expense of destroying the existing grass, sowing new seed, sustaining livestock while the pasture is being replaced and, almost inevitably, encouraging soil erosion.

Whether science can create a fungus that benefits the plant without accompanying drawbacks remains to be seen. Given that the natural association between Acremonium and tall fescue is so intimate, efforts to modify it for human benefit may prove more difficult than the genetic engineers imagine.

(Photograph omitted)