Control strategies for leaf fungi in sugar beets – timing of sprayings in relation to different harvest times

Conclusion

This experimental series comprised trials with sugar beet at three sites in 2010–2011, with harvest in the middle of each month from August to the following February. On each harvesting occasion, four levels of spraying (0, 1, 2 and 3 per season) to control leaf fungi were compared. NBF took over the series of trials in 2006 and during the period 2006-2010 also carried out two trials per year in Denmark using the same treatment regime. More detailed reports and documentation on the entire project can be found on the NBR website at www.nordicbeet.nu.

Spraying was carried out during the period 10 August to 23 September. Fungal attack progressed slowly. Significant levels of attack only occurred at the Lönnslätt and Granhill sites. All treatments gave good effect.

Corresponding trials in the period 2006–2009 with earlier and later attack will be used to study the effect level more closely and the need for repeated sprayings to ensure green foliage and maximum yields during the entire harvest period.

One, two and three sprayings carried out at three-week intervals starting around 10 August kept the beet foliage completely or almost completely free from attack by leaf fungi with harvest in September, October or November.

Yield increased from around 6.5 ton sugar per hectare in August to a peak of 14.6 ton in November. The trend over time was similar at all three sites. The highest sugar yield was recorded at the Granhill site, with a peak of 15.5 ton per hectare.

Growth from the middle of August to the middle of September was a record high, with 5.1 ton sugar per hectare compared with 3.4 ton as a mean for nine trials in the period 2007-2009. Warmth and an adequate supply of water were the reasons. The continued growth from the middle of September to the middle of October resulted in 3.0 ton sugar per hectare, compared with 3.7 ton for 2007-2009. The values reported for November are with 2-3 sprayings against leaf fungi.

As a consequence of the low levels of fungal attack, the yield gain was at best moderate. As a mean for the three trials with harvesting in November, the sugar yield increased and the sugar content increased:

• For one spraying by 0.2 ton per hectare and 0.2 percentage units
• For two sprayings by 0.5 ton per hectare and 0.2 percentage units
• For three sprayings by 0.1 ton per hectare and 0.2 percentage units

On 24 November 2010 the temperature fell below zero and this heralded the start of a period of continuous frost that lasted until the beginning of January. Therefore the harvest scheduled for 15 December could not be carried out because of frozen soil.  Around 6 January the daytime temperature rose above zero once again and on a few days in the middle of January it was possible to harvest the sugar beet. However the beet had frozen around the neck and often even more, depending on where and how much snow had provided insulation against the frost. The temperature sank to around 20 degrees below freezing a few nights before Christmas. All three trials were harvested on 20-21 January 2011. All plot samples were inspected by the regular staff at the sample wash together with the grower representative. All damaged material was removed and counted as contamination.

There were considerable differences in the amount of sugar lost between the sites, which were attributable to differences in snow cover, minimum temperature and the cumulative degrees of frost to which the beet were subjected. The same beet variety, Rosalinda KWS, was used at all sites.

The loss in sugar content of 2 percentage units from November to January was the same for all sites and all treatments.

Did treatment against leaf fungi limit the sugar losses? There was no unequivocal yes to this question, but the plots that received three sprayings against leaf fungi, with the final spraying being carried out on 22 or 23 September, gave the highest root yield and the highest sugar yield at all three sites. Root yield for the triple spraying treatment at harvest in November was 400 kg below that for the untreated plots, but at harvest in January the root yield in the same plot was 6.1 ton above the untreated level. This difference cannot be explained by growth during the period, so it must be attributed to a larger part of the beet material being kept sufficiently healthy to reduce losses during sample washing.