There is still a misunderstanding as to why fodder beet crops in intense cropping areas turn yellow in late summer. In the past, extra Nitrogen (N) and/or large amounts of Potassium (K) have been recommended, but these costly treatments do not improve yield. While late N may green the crop briefly, it does so by promoting new leaf growth at the expense of bulb growth and increases the risk of nitrate poisoning in stock.
The main cause is early infestation by aphids carrying virus yellows, a disease complex affecting fodder beet and brassicas. A recent project, run in conjunction with Corteva Agriscience and Plant Diagnostics Ltd, investigated this in Canterbury beet crops. PCR testing identified several strains: Beet Chlorosis Virus (BChV), Beet Mild Yellow Virus (BMYV), Soyabean Dwarf Virus (SbDV), Beet Mosaic Virus (BtMV), and Turnip Yellow Virus (TuYV). TuYV is mostly linked to brassicas but also found in fodder beet. These viruses are spread by aphids in either a semi-persistent or persistent way, except BtMV, which is non-persistent. In a collaborative trial between PGG Wrightson and Corteva, BChV appears to be the most common in New Zealand fodder beet crops. Overseas, yield losses from BChV in sugar beet have been recorded at 20 to 30 percent.
There are no New Zealand trials showing the impact of this disease complex in beet crops locally. However, applying extra nutrients is ineffective and, at current fertiliser prices, unwise.
Virus yellows are not simple and needs an Integrated Pest Management (IPM) approach. Monitoring this year shows aphids begin flying at end of October and peak around the second week of November. As timing varies by season, crops should be inspected at least weekly.
- Use threshold-based spraying: one green wingless aphid found per four plants up to 12 leaves. (1)
- Use registered insecticides that protect beneficial insects.
- Alternate insecticide modes of action for resistance management. Avoid pyrethroids, as they kill beneficial insects and green peach aphid often show resistance overseas.
Cultural controls and good crop hygiene are essential because insecticides alone are not enough. Ploughing buries plant residues, reducing the ‘green bridge’ that help aphids survive winter, and lowers virus sources by removing weed hosts and infected volunteers, such as Weed beet or bolters) Crop Rotation is also critical. Beet after brassicas, or brassicas after beet, is not an
adequate rotation as both can carry virus yellows. Early drilling with well-timed N applications help crops reach 12 leaves before the peak aphid migration.
Exploration into companion crops, under sowing with barley to disrupt aphids, endophytes, and virus-resistant sugar beet varieties is being conducted overseas, and we are paying close attention.
Contact your PGG Wrightson Technical Field Representative for help in adopting an IMP approach in your fodder beet crops.
