It is called grapevine vein clearing virus or GVCV, which is a virus that is a great threat to your grapevine's growth and productivity. It is often identified as translucent clearing of veins on the secondary and tertiary veins of young leaves and mosaic-like patterns on mature leaves. Here are some information regarding the GVCV virus and what measures you can do to prevent them.

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Source: Gardening Know How