More than half the Scottish fields tested in a recent HGCA project contained club-root at some level, indicating that the disease is more widespread than previously thought.
Many of the fields were infected with low levels of the disease, but this could lead to more significant problems, said Fiona Burnett, a researcher for the Scottish Agricultural College.
Levels were as low as 1%-2% of the biomass, which was below the level that would be detected commercially, but this could quickly escalate to damaging levels, she told visitors to a recent HGCA/SAC knowledge transfer event in Perth.
Clean land was diminishing and tight rotations were increasing the problem, she explained. “Many people are on three year rotations and sometimes they are worse than that.”
Climate change studies predicted that rising temperatures would speed club-root development. “We know that club-root development in the soil stops below about 15ºC, but modelling suggests the risk in 2020 will be greater.”
Growers could keep growing the crop until it was no longer viable, try using soil amendments or look at using resistant varieties.
Extending rotations was the most consistent way to reduce disease pressure, but the difference between a one-in-three and one-in-four rotation was minimal, she said. “One-in-five is slightly better, but you have to go up to one-in-eight to achieve 80% of the yield potential in the field.”
Ideally, growers should add an additional break crop into rotations, but a lack of profitable alternatives made that a difficult decision, she said.
Trials with soil amendments which raised pH and calcium content identified two products with potential. In three years of trials, LimeX and Perlka showed some promise early in the season, but yield benefits were minimal. “The advantages are far less clear than they are in vegetable crops.”
Clubroot-resistant varieties Mendel and Cracker were effective in some club-root infected soils, but because they shared the same resistance mechanism there was a risk the club-root could overcome them.
Growers should test their soils, look out for clubroot and come up with a long term control strategy, she added.
Low disease season
Disease levels might have been low last season, but growers shouldn’t be tempted to cut back on fungicide inputs this year, said Dr Burnett.
High cereal prices meant fungicide inputs would be a good investment this season. “We had a very low disease season last year and there is always a temptation among growers to manage the current crop on the previous year’s experience.”
But with current high prices there was a strong argument for implementing robust fungicide programmes, she said. “Each extra input can give you around half a tonne extra yield on a responsive variety.”
Scottish growers should consider kicking off their programmes with a T0 application to give flexibility later, even though foliar disease risk might be lowered by cold temperatures, said Dr Burnett.