South Africa Wine Technical Yearbook 2025

AUGUST

Grapevine fanleaf virus By Hennie Visser

Introduction Fanleaf virus in vineyards refers to a group of viruses characterised by their distinctive symptoms, such as fanleaf formation, yellow mosaic and vein banding. As a plant pathogenic virus belonging to the Secoviridae family, all species from the genus Vitis , including Vitis vinifera, Vitis rupestris and various hybrids, are regarded as hosts. The virus occurs worldwide in all wine-producing countries where the nematode Xiphinema index serves as the primary vector for the virus. It is considered one of the damaging grapevine viruses and has a notable impact on the lifespan, yield and fruit quality of the vine. Consequently, the fanleaf virus has a major economic impact on the wine industry. Symptoms The fanleaf virus typically causes deformed leaves with a characteristic fan-like shape and prominent, sharp-toothed margins (Photo 1). Chlorotic spots, like yellow mosaic patterns or yellow colouration of the veins, often occur (Photos 2 and 3). Infected vines often exhibit reduced vigour. Other common symptoms include millerandage (incomplete/hens-and-chicks fruit set), smaller and/or fewer grape bunches, which can lead to significant crop losses – in some cultivars up to 80% (Photos 4 and 5). Transmission of the virus The primary vector is the dagger nematode, Xiphinema index , which transmits the virus from one plant to another. Given the possibility of nematode transmission with contaminated irrigation water, it poses a particular danger to intensively irrigated vineyards near contaminated

vineyards. Although mechanical transmission via, for example, pruning shears is possible, the probability is low. Worldwide, the virus is mainly spread through the propagation and replanting of infected plant material. In South Africa, the fanleaf virus is mainly found where the Hex and Breede Rivers meet, as well as in all the wine grape cultivation areas downstream from this point. This includes areas like Robertson and Montagu. Additionally, the fanleaf virus also occurs in the Olifants and Orange River regions, although it is limited to small areas. Xiphinema index can survive in a wide range of soil types, but prefers light- to medium-textured soils like sandy loam and loam soils. Xiphinema index is well adapted to well drained soils and is not often found in high numbers in heavy clay soils with periodic wetness. Soils with a high lime content can also create favourable conditions for Xiphinema index. The lighter, well-drained soils enable greater mobility for the nematodes, which can then move between hosts with ease and spread the virus quickly. This, coupled with a higher concentration of oxygen in these soils, leads to more rapid proliferation of Xiphinema index in comparison with wetter and heavier (higher clay content) soil conditions. Sensitivity of cultivars and rootstocks A wide range of cultivars is susceptible to infection with the fanleaf virus. Some cultivars do become infected, but show minimal symptoms and are therefore considered more tolerant. To date, no Vitis vinifera cultivars exist that offer resistance to the virus, although there are varying levels of tolerance.

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Deformed leaf with a fan-like appear ance and prominent toothed margins – characteristic of infected vines.

Chlorotic spots on leaves – typical of the fanleaf virus group.

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TECHNICAL YEARBOOK 2025