17 May 2002

More min-till, more toxins?

By Andrew Blake

MORE minimum tillage could make it harder for wheat growers to meet new EU limits on grain toxins.

That is the possible downside to min-till being investigated by scientists at Harper Adams University College.

Reduced tillage systems leave more straw residues on the soil surface, which can be a substantial source of fusarium inoculum, says senior plant pathologist Peter Jenkinson.

Before the late 1990s, fusarium ear blight epidemics occurred only once every decade or so. But CSL figures show the disease has been a significant problem, reducing yields and quality, every year since the particularly bad 1998 season, notes Dr Jenkinson.

His belief that the trend to min-till may be responsible is backed by a BASF study in Germany. It showed that the important mycotoxin, deoxynivalenol or DON, in fusarium infected grain was cut to only 17% after the land was ploughed and an ear-spray applied.

That compared with 100% infected grain from a no-plough, no-spray approach, 37% for no-plough plus spray and 29% for plough and no spray.

The importance of removing residues before drilling is also highlighted by early results from a five-year HGCA/Food Standards Agency study in the UK.

Molecular diagnostics specialist Simon Edwards has found that wheat from crops after maize has significantly more mycotoxins than when following other crops.

"Maize leaves behind a lot of plant material that can be heavily laced with fusarium and is a big inoculum source for a following wheat," he explains.

"Minimal cultivation can also be a big risk factor because it leaves potentially infected stubble and straw on the surface."

But too few of his 300 wheat samples in the first year of the study were from crops off minimally cultivated land to establish a definite link between min-till and fusarium or mycotoxin incidence. "There is no evidence yet, but the German work suggests there is a correlation," he says.

As well as last years maize stubble effect, he found mycotoxin incidence and levels were worst in southern and eastern England. That was probably due to warm weather and heavy rain during flowering, says Dr Jenkinson.

Exposed straw residues are colonised by ear blight fungi, notably Fusarium culmorum and F graminearum, and Microdochium nivale, he says. Those produce spores that given the right conditions can infect the next cereal causing foot rot. Spores released at this stage can be splashed on to ears by heavy rain.

Unless that ear infection is controlled by a T3 spray fusarium species produce mycotoxins and typical pink grain symptoms. They also reduce thousand grain weight and yield.

M nivale, although not a mycotoxin producer, is the main cause of seedling blight.

Crops with 10% infected spikelets can lose up to 0.5t/ha (0.2t/acre), but only the worst affected in 1998 would have been hit that hard, he says. Typical loss in a bad year is about half that.

"There is no relationship in the field between the severity of infection and the level of mycotoxins, possibly due to the presence of antagonistic fungi. But there is in the laboratory when ears are inoculated with fusarium alone," he says.

&#8226 Min-till encouraging the disease?

&#8226 Yield and quality reduced.

&#8226 Tighter controls on mycotoxins.

&#8226 Miller assessments to be tested.

New evidence suggests a link between min-till cultivations and ear blight in cereals. An appropriate ear spray can protect grain quality and yield and keep crops on the right side of proposed mycotoxin limits (see panel).

Voluntary limits

EU mycotoxin advisory limits may be introduced within two years, believes Dr Jenkinson. They will be very low: 0.5ppm for processed products like breakfast cereals and pasta and 0.75ppm for grain and flour.

Legislation could then require routine testing of grain for all six main mycotoxins, adds Dr Edwards.

Ideally, if fusarium infected wheat could be spotted visually, testing might be unnecessary, saving time and money.

To that end several millers this season will be asked to take part in a key exercise. The aim is to see whether their visual assessments of a range of samples with known mycotoxin contents can be used to make accurate predictions of acceptability, he explains.

"You could call it a blind visual assessment."