Grapevine leafroll-associated virus in Western Australian vineyards

Background

Grapevine leafroll-associated viruses (GLRaV) are a significant problem for grape growing regions worldwide. They are the most common grapevine viruses present in Australia. Infected vines may not show immediate visual symptoms or yield loss and the virus infection can go unnoticed. This leads to more significant problems as the infected vines mature. GLRaV is an issue for both wine and table grapes as it causes yield loss (up to 50%) and delayed or variable ripening, impacting on timing of harvest and grape quality. GLRaV infection can also cause graft incompatibility resulting in high mortality in nurseries and vineyards. The disease is caused by a number of different viruses belonging to the same virus family (Closteroviridae). They can only be differentiated by molecular or serological tests. In WA, GLRaV 1, 2, 3, 4, 5 and 9 are present however GLRaV 1 and 3 are the most common. Despite the potential for yield loss some wines with desirable characteristics are made from vines known to be infected with GLRaV in WA.

Hosts

Vitis vinifera (grape varieties/cultivars) are all susceptible to infection although cultivars and clones differ in their symptom expression. Rootstocks, which are derived from other Vitis or related species, can differ in their symptom expression, with some remaining symptomless.

Symptoms

One of the issues in identifying virus infection is the variability in symptom expression. Environmental conditions, virus strain variation and cultivar susceptibility can create differences in symptom appearance. It can also take several years from the initial infection event before infected vines show visual symptoms and they may not be seen every season.

The most obvious symptom of a GLRaV-infected vine is that the leaves roll downwards at the margins during late summer and autumn (Figure 1). Leaves can change colour, being red-purple for red and black varieties or yellow for white varieties, whilst the area around the veins remains green (Figures 2 and 3). Mature leaves (those lower on the shoots) are the first to show symptoms of infection.

Figure 1 Severe leaf roll symptoms on Chardonnay. Such symptoms will not be this pronounced in the early stages after the initial infection and therefore difficult to subsequently determine when the infection occurred

Figure 2 Cabernet sauvignon vines infected with GLRaV. Note the red leaves and the veins remaining green through to the leaf margin

Figure 3 Semillon vines infected with GLRaV. Note the yellowing of the leaf between veins. The yellowing will get more prominent as the vines senesce

Infected vines have less vigour than healthy vines but this can be difficult to detect in situations where many vines in the block or the whole block of vines could be infected with virus making comparison difficult.

Bunches from infected vines take longer to ripen than those of healthy vines, sometimes this can be up to a four week delay. Berry colour can be uneven or reduced, in red and black cultivars. During the growing season leaf symptoms appear prior to berry symptoms.

When am I likely to see GLRaV?

During the post-harvest autumn period, when leaves begin to change colour, is the most likely time to see virus-infected vines (Figures 4,5,6).

Figure 4 Vines with red leaves amongst senescing vines may be a sign of GLRaV

Figure 5 GLRaV-infected table grape variety

Figure 6 GLRaV infection in flame seedless table grapes. Note the red leaves and the veins remaining

During winter, infected vines will retain leaves whilst leaves on healthy vines will have dropped (Figure 7).

Figure 7 Leaves of infected vines remain on the vine

GLRaV symptoms can be confused with several other disorders or natural physiology characteristics of grapevines. Some red and black cultivars have leaves that turn red-purple in autumn, one in particular is the glory vine that naturally changes colour in autumn (Figure 8) and another is a table grape cultivar (Figure 9). However these vines do not exhibit leaf rolling and will drop their leaves early in winter.

Nutrient deficiencies, particularly phosphorus, zinc, magnesium, potassium and iron, can cause symptoms which may look like GLRaV (Figures 10 and 11). These symptoms are likely to appear any time during the growing season and would not appear with leaf rolling.

Figure 8 Glory vine. Note complete red colour of leaves including veins and no rolling of leaf margins

Figure 9 Leaves of some black Vitis vinifera cultivars turn red in autumn. This is caused by very high levels of anthocyanin (colour pigment) in these varieties. Note red across veins and no rolling of leaves

Phosphorous deficiency

Figure 10 Phosphorus deficiency leaf symptoms. Note the red leaves and veins are similar to GLRaV symptoms but without the rolling of leaves

Magnesium deficiency

Figure 11 Symptoms of magnesium deficiency in grape

Six spotted mite

Six-spotted mite infestation can cause symptoms on vines that could be confused with GLRaV infection. Six-spotted mite is located sporadically in some south-west vineyards. The signs of feeding by the mite differ from symptoms of GLRaV because the mite causes early defoliation of vines (Figure 12). Also, mite feeding is initially restricted to areas adjacent to the leaf veins rather than in the middle of the leaf, essentially the opposite of a GLRaV infection (Figure 13 and 14). If the presence of six-spotted mite is suspected as the cause of leaf doscolouration or leaf fall, checking the underside of leaves adjacent to veins will confirm whether the mite is present. Refer to the six-spotted mite page for more information on this pest Six-spotted mite - a pest of grapevines and avocados.

Figure 12 Six-spotted mite infestation causing red leaves and early defoliation of vines

Figure 13 Six-spotted mite infestation on shiraz . Note the red veins, this is opposite to GLRaV infection

Figure 14 Six-spotted mite infestation on sauvignon blanc cultivars. Note the yellow veins, this is opposite to GLRaV infection

Spread of GLRaV

Initial introduction of GLRaV to a vineyard usually occurs through infected propagation material or via budding and grafting and accounts for the movement of the virus over large areas. Insect vectors such as mealybug and scale have the ability to vector the virus but spread is generally localised within a vineyard. Transmission via roots has also been recorded to occur so the disease can spread along vineyard rows as roots from different plants can graft together.

What do I do if I suspect my vines have GLRaV?

If you suspect your grapevines may be infected with GLRaV samples should be sent to the Department of Primary Industries and Regional Development's Diagnostic Laboratory Services (DDLS) for accurate diagnosis. Testing can differentiate which of the GLRaVs have infected your vines. Sample forms and sampling procedures are available from DDLS - Plant pathology services.

Management

There are no curative management options for GLRaV besides removal of the infected vines. Removal of vines should include as much of the root structure as possible. If you were to plant immediately after removal and vine fragments remained with virus this could transmit to the newly planted vine. It is recommended to leave the land fallow for at least 9-12 months prior to replanting.

When sourcing planting material for a new vineyard ensure the cuttings or rootlings are sourced from an accredited certification scheme or from plantings that have been tested regularly for GLRaV and shown to be free of the virus.

Managing vectors in mealybug and scale can help to reduce the ongoing spread of GLRaV within your vineyard. For more information refer to Aphids, mealybugs and scales.