Introduction

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Virus and virus-like diseases and other infectious diseases of grapevines (Vitis spp.) are induced by intracellular pathogens of various nature. These diseases are widespread throughout the world wherever grapevines, especially Vitis vinifera, are grown. Although their causal agents may be spread naturally by vectors (i.e. nematodes, pseudoccid mealybugs, leafhoppers), the major and most efficient means of dissemination of these diseases is through infected propagative material.

The following types of diseases are known:

TRUE VIRUS DISEASES

These diseases are induced by recognized viruses which have been isolated, identified and, in some cases, reinoculated into grapevines, reproducing the natural syndrome.

Some of the virus diseases known to date (Table 1) are caused by nepoviruses, of which two major groups can be recognized according to the geographical origin of the viruses and their nematode vectors. Notable exceptions are grapevine fanleaf virus (GFLV) and its major vector Xiphinema index, which are both probably native to ancient Asia Minor but now have a worldwide distribution because of unrestricted commercial trade.

Many additional viruses have been isolated by mechanical inoculation from grapevines (Table 2), and the list is growing steadily. Some of these viruses have no economic importance, but are occasional contaminants of vines grown in specific environments.

At least six different closteroviruses have been identified in vines affected by leafroll and or the rugose wood complex. The frequency of these records leaves little doubt that closteroviruses are involved in the genesis of one or more such diseases. However, proof of cause or Koch's postulates must be fulfilled before any of these viruses can be identified as the aetiological agent of a specific disorder.

VIROID DISEASES

Viroids were first discovered in grapevines in 1984, but they are now known to have an extremely high incidence and a worldwide distribution. There are at least six viroids reported to infect grapevines in nature (Table 3). Three of these, i.e. hop stunt viroid (HSVd), citrus exocortis viroid (CEVd) and Australian grapevine viroid (AGVd), do not appear to cause diseases. Yellow speckle viroids are the only pathogenic viroids recognized so far. They may be implicated in the aetiology of the vein banding disease. Viroids are spread by grafting, by propagating material and by pruning tools.

VIRUS-LIKE DISEASES

Virus-like diseases are induced by unidentified agents that occur in the host tissues. They are perpetuated through propagative material and transmitted by grafting. As yet, no virus particles have been found associated with them. Some of these diseases are latent (e.g. vein necrosis) or semi-latent (e.g. vein mosaic and enations) in European grapes (V. vinifera) and most American Vitis species, so they can only be detected by graft inoculation to appropriate indicators. None has a recognized vector or is known to spread naturally.

TABLE 1 Virus and virus-like diseases of the grapevine

DISEASES INDUCED BY PHLOEM-OK XYLEM-LIMITED PROKARYOTES

Two different groups of such diseases are known (Table 4), those caused by non-cultivable mollicutes formerly known as mycoplasma-like organisms (MLOs), i.e. flavescence dorée and other grapevine yellows not transmitted by the leafhopper Scaphoideus titanus; and those caused by intraxylematic bacteria such as Xylella fastidiosa, the agent of Pierce's disease.

TABLE 2 Geographical distribution and vectors of viruses known to infect grapevines

Note: Three additional closteroviruses have been found which, apparently, are serologically unrelated to all the above. Of these. one is reported to be associated with corky bark and the remaining two appear to be associated with leafroll.

TABLE 3 Viroids of grapevines

TABLE 4 Grapevine diseases induced by intracellular prokaryotes (mycoplasma-like organisms and bacteria)

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