Aphid-Borne Viruses

Symptoms of Beet yellows virus, Beet western yellow virus and Beet chlorosis virus are very similar and typically first observed on older leaves that begin to yellow in the area between the veins where small reddish brown spots often appear, giving the leaves a distinct bronze cast. Eventually leaves become thick, leathery, and brittle. Severe strains of Beet yellows virus first cause a vein etching of the heart leaves, followed by yellowing of entire leaf blades or sectors of older leaves. The vein-etching symptom is only apparent for a brief period, but its presence is a strong indicator of Beet yellows virus infection, because the other aphid-transmitted viruses do not produce this symptom. When leaves are infected with Beet mosaic virus, young leaves are infected first and show a mosaic or mottled pattern that may disappear or fade as the leaves mature. Infections involving more than one aphid-transmitted virus have been observed in the field, and co-infections by Beet yellow virus and Beet mosaic virus can lead to increased disease severity if plants are infected as seedlings.

These viruses were common problems when sugarbeets were produced in the Central Valley, but they are not as common in the Imperial Valley, which is the sole remaining area of sugarbeet production in California. These viruses are vectored primarily by the green peach aphid, Myzus persicae, and the black bean aphid, Aphis fabae. Other aphids, including the bird cherry-oat aphid and blue alfalfa aphid, have been shown to vector Beet yellows virus, but their significance in the spread of the disease is still unclear.

The aphids obtain Beet yellows virus and Beet mosaic virus primarily from overwintering beets; Beet western yellows virus and Beet chlorosis virus have a very wide host range, however, including plants in the crucifer and composite families. Disease potential is greatest in years when aphids are able to colonize beets early in spring and multiply rapidly; crop losses can be considerable, ranging up to 2% or more per week of infection when plants are infected with Beet yellows virus. Plants infected at early stages of development suffer the heaviest losses; late infections (4–6 weeks before harvest) may not cause significant yield loss.

To control this disease, eliminate overwintering hosts (beet-free periods) and plant to avoid migrating aphids (vector-free period generally in May and June). Fields planted 10 to 20 miles from old plantings generally avoid economic losses, and a barrier of even 5 miles significantly reduces infection. This is especially true for Beet yellows virus, which has the most severe effect on yield when it infects the crop during the seedling stage. For additional information see the section on green peach aphid.

Tolerant and resistant varieties are being developed and may be commercially available for areas where aphid vectors and serious virus infections are endemic; check with your field representative or farm advisor for the most up-to-date information.

Comments on Control

Because of the closure of sugarbeet factories in Woodland and Tracy, the threat of beet yellowing viruses is limited to an area immediately south of the Delta where crops are planted in spring and then overwintered. The source of the virus in this area may be naturalized populations of wild beet in the Delta region. The disease-free program, postponing planting in spring until after the most significant danger from aphid flights, still applies in this area. Monitor overwintered fields in spring, and analyze samples for yellowing viruses before planting beets on nearby land. If Beet yellows virus is not detected, earlier planting dates are allowed.