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Fungi – affecting grazing animals
Facial eczema is a disease that occurs in warm, moist areas of New Zealand. It is caused by the fungus Pithomyces chartarum that grows as a saprophyte on the dead grass debris – usually the outer leaves which form a litter layer at the base of plants. The spores produced by this fungus contain a toxin called sporidesmin. Spores of the fungus are dispersed onto green grass nearby; the animals ingest the spores and the toxin which is in or on the spore coat.
The toxin causes liver damage in affected animals, and as a result, compounds that are normally degraded by the liver and which photosensitise their skins, accumulate in the animals. The animals suffer from light damage on parts of their skin not covered by wool or hair (thenon-pigmented exposed areas). Lips, ears, noses, eyelids, udders become inflamed, swollen and scabby. The animals seek shade and stops feeding.
Because fungal spores are the cause of the disease and severe symptoms do not usually occur until spore numbers are high, farmers can monitor weather conditions. Spores are produced when there are two days or more with 2-6mm rain and grass temperature is above 12ºC. If spores are above 100,000 per gram of pasture then outbreaks are likely.
Once these levels are reached then the farmer can:
- Spray with fungicides,
- Move animals to brassica crops
- Decrease the stocking rates (open the gates)
- Dose animals with zinc compounds
- Select breeding stocks with lower levels of disease.
Another disease of grazing animals, namely ryegrass staggers, is caused by the fungus, Neotyphodium lolii, which cannot live outside the grass and can never leave one grass to infect another. It produces toxins in the grass but causes the grass no obvious harm. It is transmitted exclusively via seeds of the host.This fungus produces two types of toxins:
- Lolitrem B and ergovaline.
These toxins cause the ryegrass staggers, a neurological disorder. Symptoms include wobbling and shaking of the animal, inability to stand properly and loss of coordination especially when animals are disturbed. It can reduce lamb weight by 20-30% and effected animals also produce more faeces and therefore dags, which increases the chances of fly strike. On the whole animals recover quite quickly when removed from the infected pasture. The disorder is more severe in dry warm conditions.
- The second type of toxin is peramine.
This toxin is toxic to insects that feed on grasses, such as the Argentine stem weevil. The toxin prevents feeding and egg laying of the insects. The pasture lasts longer with increased growth by 20-50% in spring and summer if this fungus is present.
Agriseeds produce endophyte infected ryegrass seeds for sale.
- Explain the similarities and differences between these two terms: SAPROPHYTE and ENDOPHYTE
- What are the main factors that determine when plant fungal diseases will occur?
- Why is facial eczema more common in the North Island of New Zealand?
- Discuss and evaluate the five management strategies the farmer can employ to minimise the effects of the fungus that causes facial eczema. (Consider for each the financial implications, the animal’s disease resistance, the effect on growth of the fungus and the change in feeding conditions for the animals).
- If zinc is added to the drinking water why must you ensure that fresh water is not also available to the grazing animals.
- Explain how peramine toxin might result in an increase in grass growth and drought tolerance.
- Discuss why AgriSeeds market ryegrass seed infected with a fungus.
- Explain how and why this ryegrass produced by Agriseeds might have been developed.
- Explain how dags might result in an increase in fly strike.
- Explain how flies damage the grazing animal’s flesh.
Check the answers.