Virus yellows complex (VYC) describes a disease caused by members of the Polerovirus genus. This includes beet chlorosis virus (BChV), beet mild yellowing virus (BMYV) and potentially beet western yellows virus. Other viruses within this group may also be involved and mixed infections are common. Virus yellows complex commonly occurs on beet in NZ. Secondary fungal infections particularly by Alternaria and Stemphylium are often associated with these viruses.

 

Identification

Yellowing of the leaves between veins spreads within foliage. Leaves become thicker and brittle leading to death. Note that symptoms can be confused with nutrient deficiencies. Secondary interveinal necrosis also occurs as the plants age.

 

Importance

Virus yellows complex can cause major losses in fodder beet and in sugar beet. Large patches of plants in crops can be affected. These virus diseases cut leaf quality, palatability and yield.

 

Spread

Virus yellows complex is primarily spread by aphids, mainly the green peach aphid. Aphids can acquire the virus after feeding on sap for 5–10 mins on infected plants but usually take an hour or more to become highly infectious. Most transmission takes place within 6–12 hrs after feeding starts, although aphids can retain the virus for up to 17 days.

 

Management

Because of the time it takes for virus infection to be transmitted by aphids, insecticide can be effective. Insecticidal seed dressings effectively control aphids in the plant establishment phase. This should be followed by close crop monitoring to detect any build-up in aphid numbers, and timely application of insecticide if needed.

Controlling weeds that can harbour aphids helps reduce the risk of infestations, together with aphid control in neighbouring crops, such as brassicas. Aphids from these sources can colonise beet crops and spread infection through the stand.

No good source of resistance is reported to be available for virus yellows complex in fodder beet.

Aphid flights mainly occur in late spring (Nov to early Dec) and autumn from mid-April to late May. Thus delayed sowing and monitoring of crops in autumn can help manage this disease.