19 April 1996

Hope in sight for end to take-all?

A new cure for take-all may be on the horizon. If it works crop rotations could change radically. John Allan looks at the scope for such control and considers more immediate work on the role of set-aside in take-all decline

TAKE-ALL has been widely researched but it is only now that chemical control is looming.

The first disease signs growers usually see are the so-called whiteheads in late season, often associated with poor soil conditions. But the real damage is done much earlier below ground.

A new benzamide fungicide, MON 41100, is being evaluated in several European countries. But it is unlikely to be on the market before the year 2000, according to Patrick OReilly of Monsanto.

He is keen not to "over-hype" the chemicals potential. "We have had some good data from the past two years of trials." But several seasons work are needed to show its performance is consistent, he stresses. "But we are very committed to it. Where take-all was severe, in other words where we had 20% whiteheads in untreated plots, we have seen yield responses in wheat of up to 2.5t/ha." All the trials received a sound programme of normal inputs to eliminate other variables, he notes.

Even on "sub-clinical" infestations, where there were no visible symptoms in untreated plots, the new chemical has given an extra 0.25t/ha (2cwt/acre), he adds.

Yield loss from take-all is aggravated by wet autumns, mild winters, and warm, moist conditions in late spring, and dry summers. But during cool weather in early spring the root disease can develop faster than the cereal plant. "It is a race against the clock," says Mr OReilly.

Winter cereals at risk display root symptoms in the autumn. "If you lift plants from high risk fields and root wash them you will see the first signs of attack as a blackening, not browning, of the young roots. This may later develop and extend to the stem base," says Mr OReilly. "It is important to monitor for infection early in the season on these smaller roots. It is often a good indicator of a later attack," he explains.

To protect these early roots MON 41100 is likely to be formulated as a seed dressing.

Work at IACR-Rothamsted has shown that very low numbers of infected roots a plant in the autumn can lead to an epidemic if the weather is favourable.

The degree of root attack is not readily visible, so it may be that grain quality and yield suffers because root damage interferes with nutrient flow. "We will be looking at specific weight and thousand grain weight this year," says Mr OReilly.

Another bonus of tackling take-all has been identified in work at ITCF in France. Researchers have found control may boost efficiency of nitrogen use by up to 30%.

TAKE-ALL decreases quite rapidly if it cannot find a suitable host. These include not only wheat and barley but couch and some other grass weeds. A different strain of the disease attacks oats.

Historically, rotational and cultural methods have been used to keep it under control. But set-aside creates a potential problem for growers using conventional (non-continuous cereal) rotations.

The best set-aside control of the disease will be from a non-host crop, such as industrial rape, in which grass weeds and volunteer cereals are completely controlled, says Rothamsteds John Jenkyn. Naturally regenerated set-aside is another matter. Varying levels of volunteer cereals and grass weeds confuse the picture.

Permitted set-aside cultivation or ploughing creates a fallow until the next cereal is sown. Such fallows are new to agricultural practice, says Dr Jenkyn. In addition, set-aside green cover may be mown or sprayed off. Trials at Rothamsted are focusing on the impact of such variable set-aside cover and management practices on take-all in following cereals.

As well as testing different cultural and spraying techniques, the researchers have established plots with wheat at less than four plants a sq m up to some with well over the normal seed rate.

Soil assessments after one year show that the density of cereals affects the level of take-all – the more plants, the more inoculum. They also show inoculum is higher after ploughing in August rather than May. The implications of that will be examined in a first wheat crop to be harvested this year.

Growers with continuous cereals going into set-aside with natural regeneration will not lose benefit after this one-year "break", says Dr Jenkyn. "The take-all decline will be maintained." &#42

Is take-all destined to become a disease of the past? If a new fungicide lives up to expectations big changes to crop rotations could follow.