BREAKING BACK OF
Do we need a break every three years to control
take-all? Scientists at Rothamsted are exploring the options, writes Lucy Stephenson.
IT COULD be a serious take-all year. Warmer weather with plenty of moisture will encourage it, warns Dr John Jenkyn of IACR Rothamsted.
Monitoring the level of disease will be crucial. But with take-all, you cant always see what youve got. Indeed a thriving crop may support higher disease levels than a poor one.
Whiteheads at flowering are the obvious symptoms, but its more informative to look for early signs, he advises (see box).
Unfortunately the take-all showing in one years crop is no help in estimating the chances of severe disease next year. This is because its the amount of inoculum in the soil and the weather that determine the risk to following crops.
Field testing for inoculum is limited by the costs, but known field history can give some indication of risk.
The potential for a severe year increases as the inoculum accumulates in successive crops. Breaks take the inoculum level down because the fungus survives in root residues which largely decompose during the break year.
The number of consecutive cereals can be prolonged if the build-up of inoculum can be slowed, says Dr Jenkyn. The scientists highlight:
• Later sowing.
The time available for root residues to decompose before a new host is sown means that sowing date makes a difference. Earlier-sown crops are exposed to more inoculum. Earlier sowing in one year can increase the amount of inoculum, and hence disease, in the next. Warmer autumn temperatures also favour the fungus, adds Dr Jenkyn.
• Control volunteers, in break crops and in set-aside.
Volunteers, and grass weeds help the fungus bridge the gap between cereal crops. They also eradicate the benefits of later sowing, warns consultant plant pathologist Dr David Hornby.
• Alternate spring and winter cereals.
Dr Hornbys new research shows that alternating spring and winter cereals delays the onset of severe take-all.
• Good husbandry.
Preventing the disease from getting a grip also reduces the amount of inoculum it will leave for following crops.
Where P levels are low the impact of take-all is higher. Its important that P is applied right from the first wheat, says Dr Jenkyn.
Nitrogen applications promote tillering and root growth and so also tend to reduce disease. But nitrogen on a badly diseased crop is wasteful because damaged roots are less able to take it up and so more is leached.
There is evidence that some varieties are more prone to yield loss, points out Dr Jenkyn. He draws attention to work by ADAS which shows that wheats such as Rialto accumulate more stem carbohydrates and may use these to maintain yield.
However, varieties such as Spark dont accumulate so much in the stem. These may not yield as well – particularly as non-first wheats, which are likely to be more severely attacked by take-all.
• Bridge crops.
Using less susceptible cereals to interrupt sequences of winter wheats to try to induce take-all decline, without the penalty of severe disease, will delay the take-all maximum. Unfortunately when winter wheat is resumed, disease may be higher than it would have been in continuous wheat.
Dr Hornby also cautions that winter barley isnt always less susceptible than winter wheat. Winter triticale however is a candidate to replace winter wheat when the risk of take-all is high.
Most of the inoculum is present near the surface of the soil. So inverting the soil by ploughing can delay contact with the roots. But its inevitable that inoculum will accumulate eventually. "These strategies only work in the short term; take-all will still become severe in the fourth year," says Dr Jenkyn.
If all else fails, the silver lining to a badly-hit crop is take-all decline. A severe year is a requirement for this natural control to become established. It prevents severe take-all in subsequent years and cereals can be grown continuously. Its believed that take-all decline results when antagonistic microflora, including bacteria called fluorescent pseudomonads as well as fungi, become more prevalent. The virulence of the take-all fungus, and the way its colonies grow and reproduce may also change.
Take-all decline occurs in winter wheat and spring barley. Dr Hornbys work has shown that take-all decline can also take place in winter barley and winter triticale. However, the decline is not sufficient to protect subsequent wheat crops.
So what if the agents for take-all decline could be applied to the soil at drilling? Pseudomonas seed dressings are currently being developed in the US, but they havent worked at Rothamsted.
Another contender for natural control is a fungus called Phialophora. It competes for space on roots and enhances the plants own immune system, explains Dr Hornby. Although a root fungus, it is more benign than take-all.
Phialophora builds up under grassland, so can delay the onset of take-all in cereals which follow grass. One year in set-aside wouldnt be enough for the fungus to build up though, says Dr Hornby.
Biological controls are unlikely to match chemical controls such as Monsantos seed treatment, MON65500 and AgrEvos triazole Jockey (fluquinconazole).
The new chemistry will make continuous winter wheat more viable, and more wheat will certainly be the more profitable route post-2000. But of course there are other good reasons to have a break crop.
• For second and subsequent crops sown in September, early symptoms can show in 3-4 weeks. Look for symptoms from Christmas onwards in later sown crops.
• Look for black lesions on the roots; place in water in a white dish to see best contrast.
• Size of lesion: from a pin-prick to several centimetres. Lesions over 1cm long are almost certainly take-all.
• Smaller lesions could be take-all, but may be confused with several other fungi that have similar symptoms, such as Pythium which can also cause dark brown/black discolourations.
Source: Richard Gutteridge, IACR Rothamsted.