Gröngödsling mot jordburna svampar 2004–2009

Conclusion

Sugar beet can be attacked by a range of different soil-dwelling pathogenic fungi. In Sweden it is Aphanomyces cochlioides that causes the highest incidence of root rot, but various species of Pythium and Fusarium can also cause problems with root rots on certain soils. Incorporating green biomass of Brassica species and oats into the soil, a process known as bio¬fumigation, leads to the release of sulphur-containing compounds that appear to have an inhibitory effect on many fungi.
The main aim of this project was to evaluate the effect of biofumigation in the management of soil-borne fungal diseases of sugar beet in the Nordic climate. A further aim was to examine how to include these crops as intercrops in existing sugar beet crop rotations, while at the same time meeting the demands placed on a nitrogen-trapping catch crop. This meant that the crops could not be incorporated into the soil before 20 October. In the series we tested the high glucosinolate sarepta mustard cultivar Fumus developed in Australia and two other cultivars (Architect and Doublet), but also oil radish (cvs. Colonel, Adagio), white mustard (cvs. Emergo, Maxi) and oats (cvs. Gunhild, Ivory). During the period 2004-2009, a total of 10 trials were established on soils experiencing recurring problems with root rot.
The effect on soil-borne fungi, determined as disease index on sugar beet plants in the field, was generally small. A significant decrease in disease index was only recorded in one trial. The effect was most obvious at autumn ploughing, where all crops had a lower disease index than the control. The effect was also apparent at spring ploughing for sarepta mustard, white mustard and oats, but not for oil radish.
When the amount of soil infection was measured after the different green manure crops and compared against the control (stubble), it was found that all crops had decreased the amount of soil infection, although to a very low degree. The greatest decrease occurred with the high glucosinolate sarepta mustard cultivars Fumus, Architect and Doublet, which gave an average reduction of just under 7%. This is probably too low a level to be of any practical significance. A decrease of up to 90% would be needed to have any effect in practice. The effects measured in this project were one-year effects and it is possible that repeated use of oil crops on several occasions in the crop rotation could decrease the soil infection or keep it at a low level. Such investigations have recently begun in a crop rotation series at NBR.
There may be several reasons why only small disease control effects were observed. A number of conditions must be met if high concentrations of sulphur-containing compounds are to be produced. The amount of fresh green biomass must be around 50 tons per hectare. This must be finely chopped and incorporated immediately, preferably within half an hour, in order to prevent the volatile compounds being lost to the atmosphere.
A further condition for good control of soil-borne fungi by green manure crops is that they are established as early as possible so that they have time to develop a sufficient quantity of biomass. According to our experiences, this means that the crops must be sown by 20 August at the latest. With sowing in September the growth is generally too poor. One method that works well, particularly for white mustard, is to insow it around two weeks before the main cereal crop is harvested.
Oil radish and white mustard were recently approved as catch crops in Sweden. A number of studies have shown that oil radish has a great capacity to take up nitrogen from soil layers even down to below 1 m depth. The roots of oil radish grow very rapidly in the autumn and after only one month they extend to over 1 m depth. After a further month the roots can reach down to 2.5 m. It is not entirely clear when the nitrogen taken up by the green manure crop in the autumn is released, but a large proportion is probably released early in the following year. Measurements of sugar beet yield in this project showed that the sugar content decreased in a number of the trials and that the blue number increased for the green manure crops compared with the stubble control. This indicates that some nitrogen is released later and taken up by the sugar beet.