A Guide to Grapevine Abnormalities in South Africa - P.G. GOUSSARD

Virus associated phenomenon (1); thickened graft union (2). VIRUS AND VIRUS-LIKE DISEASES Fanleaf 4.1

T his group of viruses which occurs throughout the world in all winemaking countries is associated with fanleaf, yellow mosaic and veinbanding – these three biological strains are grouped together based on induced abnormalities and often occur in conjunction with each other. Following transfer to plant material (whether by grafting or feeding actions of nematodes as soil-borne vectors), fanleaf- associated virus particles (Photo 165) move easily from cell to cell in all directions and a situation is created that impacts seriously on the productivity and life expectancy of grapevines. At the same time the ingress of characteristic, visually perceptible abnormalities (especially during the early part of the growing season) is induced – which is dependent, however, on the virus strain(s) involved, and may vary considerably. In fanleaf per se leaf discolouration and deformation are very characteristic and associated with bigger/wider petiole sinuses, asymmetrical leaf blades and main veins being drawn towards each other to impart a fanlike appearance to leaves (Photos 166 & 167), while different patterns of chlorotic spots also occur (Photo 168). In contrast yellow mosaic and veinbanding are not associated with leaf deformation, but are characterised by various patterns of bright yellow discolourations that range from irregularly distributed spots to overall yellowing of leaves – as with yellow mosaic (Photos 169 & 170). In veinbanding this phenomenon is sometimes localised around the veins, or may even occur in the form of yellowish chlorotic lines or rings (Photos 171 & 172). It goes without saying that such discolouration patterns can be very confusing (Photos 173 & 174) in terms of successful problem identification and it is recommended that suspect material be subjected to specific virus detection techniques for confirmation. In association with the above-mentioned phenomena, dwarfed shoot growth with a “zigzag” growth pattern and the appearance of shortened internodes, as well as double nodes, are considered characteristic traits of fanleaf infection. Abnormal shoot branching is also prevalent (Photo 175). The incidence of severe millerandage (Photos 176, 177 & 178) can be considered the most important contributing factor to reduced productivity in fanleaf-infected grapevines. Although such situations can be ascribed to various causes, in this case it can be associated with smaller/underdeveloped root systems in conjunction with a reduced photosynthetic capacity caused by leaf deformation/discolouration. It is known that cytokinins, gibberellins and auxins especially are closely involved in the set of berries and subsequent growth and development. These hormones do not act in isolation; a very fine balance and interaction is required to ensure eventual optimal grape quality and quantity. It can be argued that due to the smaller number of root tips in infected grapevines, fewer cytokinins are produced, thereby causing the balance to be disturbed. This, together with insufficient reserves – as aggravated by a reduced photosynthetic capacity – may result in smaller berries and bunches in conjunction with dwarfed shoot growth (smaller grapevines).

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