Ergot in cereals

Pathogen

Claviceps purpurea

Hosts

In order of decreasing susceptibility, the disease affects rye, triticale, wheat, barley and oats (infection of oats is rare). It also affects a wide range of grasses, particularly black-grass.

Symptoms

The causal fungus only attacks the ear at flowering, replacing the grain in a few spikelets by a hard, purple-black sclerotium, known as an ergot. Ergots are formed of a mass of fungal tissue and have a white interior. Ergots can be very large, up to 2 cm in length, and are very obvious in the standing crop and in contaminated grain samples.

Infection can be observed before these ergots form as droplets of honeydew that contain fungal spores may form during anthesis. This honeydew can lead to the development of saprophytic moulds.

Life cycle

Ergot is not a true seed-borne disease, as it is not carried on or in seed. However, it can be spread by ergots in contaminated seed. It is also one of only two diseases included in the UK Seed Certification Scheme for Cereals (the other being loose smut).

At or near to harvest, ergots fall to the ground where they remain dormant. In the following spring, they germinate and produce mushroom-shaped spore bearing structures (stroma). The spores are spread by the wind to nearby open flowers of grasses and cereals. The spores germinate in the flower, infecting the ovaries. This infection leads to the production of secondary spores (conidia) encased in a sweet-smelling, sticky secretion – commonly referred to as honeydew. This attracts insects that carry the spores to other flowers, where further infection can occur. Rain splash and physical contact can also spread the spores. The fungus then grows in place of the grain to form the visible ergot, completing the life cycle.

Ergots are either harvested with the grain or fall to the ground where they remain as a source of inoculum for the following year. Generally, ergots only remain viable on the soil surface for one year in the absence of host crops, but they may survive longer in stored grain.

Importance

The disease has very little direct effect on yield but the ergots contain large amounts of toxic alkaloids (mycotoxins). Contaminated grain, fed to stock or used to make flour, can pose a risk to animal and human health. Such grain may be rejected, require cleaning or demand a reduced price. Standards for the number of ergot pieces exist for certified seed.

There are a several strains of the fungus: some infect grasses and cereals, whereas others are restricted to specific hosts. Wheat and other cereals are less commonly affected than rye, although more open-flowered wheat varieties can be badly affected. The disease is favoured by cool, wet conditions during flowering which facilitate spore production and prolong the flowering period, making infection more likely. Late tillering crops and crops with secondary tillers are more susceptible.

The increased scientific understanding and improvements in agricultural practices and milling techniques (grading, sieving and sorting) have eliminated the severe epidemic outbreaks of ergotism in the UK. There are year-to-year fluctuations in ergot severity that can be explained, to some extent, by variations in weather during flowering.

There is currently no EU legislation for maximum levels of ergot alkaloids. However in 2019, the EC proposed the introduction of limits for total ergot alkaloid content of: 75 ppb in processed barley, wheat, spelt and oats products; 250 ppb in processed rye products; and 20 ppb in processed cereal based food for infants and young children.

The approved AIC Contract for Grains/Pulses No.2/16 states that: “grain shall not contain more than 0.001% ergot by weight for feed grain and zero tolerance for all other grain.”

The maximum levels for ergot sclerotia are currently defined within the Codex standards at a level of 0.05%. The EC have proposed the lowering of this level to 0.02%. This is likely to apply from July 2021 (affecting the 2021 harvest).

Ergot alkaloids – Avoiding a bad trip on new regulations

Joe Brennan – Policy & Research Officer, nabim

Milling Wheat Conference 2020 presentation

High-risk factors

  • Open pollinated wheat varieties
  • Varieties with a long flowering period
  • Cool and wet conditions during flowering, which facilitates spore production and prolongs the flowering period
  • Grassweeds, particularly black-grass
  • Grass margins containing early flowering grass species
  • Late and secondary tillering

Management

Variety

There are no current cereal varieties that have resistance to ergot infection. Varieties that have a longer or more open flowering habit will be more susceptible to infection, due to easier access of spores able to infect the floret. Florets that remain closed during pollination and for a few days afterwards provide a mechanical barrier to the entrance of spores and are more likely to escape infection. Susceptibility to ergot infection persists for only a few days after fertilisation, after which point, the ear becomes resistant to further infection.

While it has been previously investigated (see AHDB-funded research final project reports: PR456 PR457 and PR603), there is no system within the AHDB Recommended Lists to reliably score the openness of flowering and link this positively to reduced infection risk. The development of true tissue resistance continues to be an area for further research by the plant breeding community.

Fungicides

There are currently no fungicide sprays approved for use on cereals to control ergot infection. Previous AHDB-funded work (final report PR254), using radio-labelled fungicides, detected negligible movement of foliar-applied fungicides to the point source of infection, the ovary. This is not surprising, as it would be undesirable to have products that could migrate into the grain.

Some azole-based seed treatments are recommended for ergot control and act by reducing fungal growth and development of the ergot. They do not provide complete control of germinating ergots and should be used in conjunction with other management options to reduce the risk of infection.

Grass weeds and margins

Controlling grassweeds is important to minimise the risk of fungal spores available for secondary spread. Black-grass control is especially important as it flowers earlier than the main cereal crop, allowing a build-up of inoculum (honeydew phase) that can be readily transferred during the cereal flowering period.

While it was thought that the increase in field margins could lead to an increase in inoculum build-up and subsequent infection of cereal crops, AHDB research found no significant impact (final report PR456). However, grass margins do still pose a small risk by providing a reservoir of secondary inoculum that could infect wheat, particularly late tillers around the edge of the crop. This risk can be minimised by sowing later-flowering grass species. Some grass species such as, cocksfoot, couch grass, timothy, tall fescue and tall oat grass pose a greater threat to cereal crops due to their ease of infection and flowering time.

Husbandry

Good crop husbandry continues to be the most reliable method of reducing the risk of ergot infection. In heavily infected crops, harvesting the field headlands and tramlines (where later tillers prevail) separately from the bulk of the crop will reduce contamination of the main crop.

As ergots only remain viable for one year in soil, sowing a non-cereal crop or ploughing (to at least 5 cm) to bury the ergot will reduce the amount of inoculum available in the next cereal crop. Any susceptible grass weeds should also be controlled to get the full benefit of these practices.

Keeping an accurate record of where ergot infection has been most prevalent on farm will help assist in future decisions on rotations. Sowing clean seed will prevent planting inoculum in the new crop.

Cleaning procedures

In severe years and where there has been a poor level of control, the harvested grain can be cleaned. This can either be by a mobile cleaner on the farm or by the trader/processor, by prior agreement after delivery. The latter may result in a reduction of the price paid.

Several 'cleaning' methods may be used, including gravity separation (with or without an air screen cleaner) and mechanical sieves that remove foreign bodies on the basis of size. Sieves may be less effective where whole ergots or ergot fragments are the same size as the grain. More recently, effective grain colour sorting systems have become available but are used mainly by processors and within central stores.

Summary

  • In heavily infected areas, harvest field headlands and tramlines separately from the bulk of the crop
  • Plant a non-cereal crop or plough to ensure ergots are buried to at least 5 cm depth
  • Control grassweeds, especially black-grass
  • Avoid open flowering varieties and varieties with a long flowering period
  • Sow later-flowering grass species in grass margins
  • Avoid sowing contaminated seed – clean farm-saved seed thoroughly to remove ergot
  • There are currently no fungicide sprays approved for use on cereals to control ergot infection, but some seed treatments may have a small effect by preventing ergot germination
Cereal disease management homepage

Ergot sclerotia in wheat grains

Ergot alkaloids under the spotlight

Ergot alkaloids under the spotlight

An article from Crop Production Magazine

Research

How ergot alkaloids can move to apparently ‘clean’ grain has been revealed by recent research. Published by AHDB in 2019, the work found that the mycotoxins appear to move between flowers and can be detected in grain that appears clear of infection. It was also found that broken ergot sclerotia in harvested grain can be a potentially significant contamination source.

Determining the routes of transmission of ergot alkaloids in cereal grains (Project Report 603)

The causal fungus only attacks the ear at flowering, replacing the grain in a few spikelets by a hard, purple-black sclerotium, known as an ergot.

Secondary spores (conidia) encased in a sweet-smelling, sticky secretion – commonly referred to as honeydew – are produced on infected grains.


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