1 August 1998

WARNING BELLS RING OUT

Wheat growers face a double whammy from late season disease as they prepare for harvest. Our reports highlight old and new threats to next seasons crops.

FUSARIUM in the wheat crop about to be harvested is at unprecedented levels with crops in many areas facing complete ear infection.

The result will not only be shrivelled and sooty ears dragging harvest quality down dramatically but a legacy of inoculum which will threaten to explode in 1999 crops pre-harvest. Of more immediate concern, and notwithstanding the expected lower yields, is the further threat of mycotoxin development in affected crops which could lead to rejection by millers and compounders.

Interim results from the 1998 survey of winter wheat diseases carried out by the Central Science Laboratory and ADAS at GS73-75 indicate that all wet weather leaf diseases are at very high levels. However, fusarium ear blight is significantly higher than is usual, particularly ear infection caused by Fusarium culmorum, Microdochium nivale and possibly other fusarium species, says the CSL.

Survey data indicate the disease is currently affecting an average of 1% of the total ear area but Dr Judith Turner, a senior plant pathologist with the York-based CSL, warns that in suitable weather conditions a 10-fold increase in ear infection is feasible by the time crops are harvested.

Fusarium culmorum accounts for about 50% of the infection in the CSLs early analysis of 450 samples from across the cereal-growing counties in England and Wales. This type of fusarium is capable of producing significant levels of mycotoxins under UK conditions.

Dr Turner adds that more than 50 of the crops examined are showing more than 30% of plants affected by fusarium. On average, in the middle of July there was about 1% infection, compared with 0.1% in 1996. The CSL had picked up on an increase in inoculum carry-over last year but the main finger of blame must be pointed at the wet June weather which provided just the right conditions to splash fungal spores around.

Unfortunately there is nothing cereal growers can do about ear blight this year as sprays to control the disease should have already been applied. However, there are potential risks to next years crops from high levels of infected seed and increased inoculum in the soil, she points out. Worst affected regions this year are in the south and south-west of England but there are scattered reports of fusarium in East Anglia.

The key time for preventive action was during the first two weeks of June when the wet weather was encouraging infection and anthesis was taking place in wheat plants. Dr David Jones, of ADAS Rosemaund, points out that most ear wash sprays applied by growers would have been insufficient to deal with fusarium infection.

He says that this year the only way to get a respectable treatment was to have gone on with a full rate of Folicur (tebuconazole) shortly after ear emergence but before infection occurred at early flowering.

* See also Crops Advice Line on Farmers Weekly Interactive (FWi) at www.fwi.co.uk

KEEPING AN EYE ON SMUT

A SEARCH by Government plant health and seed inspectors has so far failed to spot any further outbreaks of flag smut, the seed- and soil-borne fungal disease that has appeared in UK wheat for the first time this year.

The disease is raising alarm – and not only on account of potentially devastating yield loss. The major grain trading nations Canada and the US consider it dangerous enough to warrant quarantine restrictions.

The flag smut first came to light when a keen-eyed crop consultant sent off blackened and stunted samples from an Essex field of Riband to NIAB in early June.

The grower is not identified, but it is clear the Riband was grown from C2 seed produced in the UK. Other fields on the farm grown from the same seed stock are also showing symptoms – possibly indicating contaminated seed.

The source of the seed infection remains a mystery, but Government specialists are tracking back through the seed production pipeline. So far no other symptoms have been seen in crops grown from the same seed supply.

One suspicion is that fungal spores may have blown in from elsewhere. The disease does affect grasses in the UK, but it is thought cross infection is unlikely because the wheat strain may be specific to wheat alone.

First sign of infection is black/grey striping of the flag leaf. Stunting becomes increasingly obvious as the season progresses – affected plants could be half normal height, showing excessive tillering and often without a visible ear. Leaves become frayed and twisted, covered in black soot-like powder.

Flag smut is normally associated with winter-sown wheat grown in climates with hot, dry summers and mild winters. It is known in the Middle East, Australia, and some parts of Asia, Chile and the north-western US.

Plant pathologist Dr Claire Sansford of the Central Science Laboratory, York, believes the mild winter and dry spring encouraged fungal development this year in Essex. It raises the question whether the UK climate is becoming more conducive to the disease.

Transmit Infection

Cultivation practices which leave plant debris near the surface help transmit infection, she says. But spring-sown wheat escapes – even with highly susceptible varieties.

Once in the soil, the fungus can survive for up to four years, and could spread rapidly to other fields through soil contamination of machinery, and through wind blow.

The seed treatment Baytan (fuberidazole and triadimenol) gives full control of flag smut. Resistant varieties are used in countries where the disease is more common, but the susceptibility of UK cultivars is unknown. Possible cultural strategies would include late or spring drilling when the soil temperature is below 10¡C, shallow drilling to speed emergence, and non-wheat break crops.

Anyone who finds such symptoms in wheat this year is asked to contact the CSL on 01904 462000.