Leafroll Virus versus Magnesium Deficiency

There is, however, a note of caution required. For those growers with limited exposure to leafroll virus there is a risk they may confuse the visual symptoms of the virus with those of mineral deficiencies such as magnesium.

Vaughn Bell from Plant & Food Research in Hawke’s Bay has been working with NZW as part of their jointly funded research programme with the Sustainable Farming Fund. The focus of the research has been to develop control strategies for this economically important virus. Now into its second three-year term, the project has previously restricted its research activities to Hawke’s Bay and Martinborough. Both regions have vineyards where leafroll virus has adversely impacted on vine performance (e.g. delayed fruit ripening, reduced yields and elevated berry acidity), and with no known cure, the only means of control is to remove the infected vine. However, such actions are costly, involving not only removal and replanting, but also the downtime before the replacement vine is fully productive. 

Given these factors, growers need to be sure that the vines marked for removal are in fact infected with leafroll virus, and not expressing visual symptoms of a mineral deficiency – particularly magnesium. The reason for any confusion is understandable once you realise that in both cases, the visual symptoms of the virus and the deficiency appear in the leaves of the vine from mid-summer onwards.

 “The differences between them are often very subtle,” Bell recently told a group of Marlborough growers. “However, when you place the leaf of a magnesium deficient vine beside the leaf of a vine with leafroll virus, the difference is stark.”

In the case of a magnesium deficient vine, the leaf colour has a pinkish/orange tinge in the inter-vein, while areas immediately beside the main veins remain green. Leafroll virus symptoms visually express in the inter-vein areas, and are typically a dark red. Areas close to the main veins remain green. For those vines that have been infected for several years, the margins of leaves also cup downwards, hence the reference to ‘leafroll’.

While in Marlborough, Bell visited a number of Pinot Noir blocks. In one of those blocks, the grower had tagged a large number of vines he suspected as being infected with leafroll virus. 

“I would say his identification of the virus was correct for just 50% of the tagged vines. Across the 20 rows monitored, 236 vines had been tagged as being infected with leafroll virus, when they were actually expressing symptoms of magnesium deficiency.” 

If the grower had not been alerted to the differences, those vines would have been removed later this year – incurring a significant financial cost for no good reason.

“I accept that making the correct diagnosis can sometimes be confusing, particularly if growers have minimal experience of leafroll virus, but it’s critical they not remove healthy vines,” Bell said. 

The confusion isn’t helped by the fact that both the virus and deficiency start to express visual symptoms during the summer months. If at all practicable, this is the time Bell says, that growers need to be out in their blocks monitoring for signs of leafroll virus.

“The timing of when you look for the disease is really important. It is present now, (mid March at the time he spoke in Marlborough), but as time goes by, the number of symptomatic vines in your vineyard is likely to progressively increase. The timing of the visual symptoms though, depends on what time in the previous season the actual vine became infected. There is a lag phase of 12 – 18 months, maybe even two years from the point at which a vine is infected with the virus, and the point at which the visual symptoms start to express in the leaves. If you can only go into a block once, then make it as late in the season as you possibly can.”

Bell urges growers to be mindful of rogue frosts defoliating vines in April and therefore impeding accurate identification before monitoring has concluded. “Ideally, you want to identify as many of those infected vines as you can, so that they can be removed in the winter immediately following identification.”

Bell said it is critical to get on top of the virus as soon as possible, which means identifying the symptomatic vines this season, not waiting until next. 

There was a further issue that was discussed with the growers at the Marlborough field-day – and that was the potential impact of bird netting. Bell said it was much easier to observe symptoms of leaf virus through black netting compared with the white netting, so if at all possible, he urged growers to attach black nets to red berry varieties (Pinot Noir, Cabernet Sauvignon, Merlot etc) whilst deploying the white nets onto white varieties, where visual identification of leafroll virus is not undertaken because of the inability to accurately identify symptomatic vines.

Leafroll virus is spread by mealybugs, which are present in all North Island wine growing regions. While surveys in recent years have been limited in the South Island, there is growing evidence that mealybugs are also present in some Marlborough vineyards, although research planned over the next three years will advance understanding of this important aspect. While removing infected vines is one way of slowing down the spread of leafroll virus, controlling mealybug numbers is also vital. In emphasising the point, Bell spoke of three blocks in Hawke’s Bay that had been infected with leafroll virus.

“One block started out with 10% virus incidence in 2009. By 2011, the disease spread had infected 37% of vines in the block, with the decision made to remove all the remaining vines in mid 2011. On the back of that, there was, on average, one to two mealybugs per leaf from vine inspections undertaken in March 2010 and 2011, respectively. Mealybug numbers of that magnitude are not conducive to successfully controlling leafroll virus.

“In the second block, virus incidence was 16% in 2009.Currently, virus incidence is 45% but by the end of the 2013 season it will probably be closing in on 50%. In 2011, there was, on average, 1.5 mealybugs per leaf. The spread of leafroll virus has been so rapid that the entire block is due to be removed in July 2013.

“In another block leafroll virus incidence was also 16% in 2009; however, by 2012 it had reduced to 0.4%, with an average of just three mealybugs found for every 100 leaves inspected. The critical difference between these two blocks was not the virus identification or vine removal strategies – in both blocks, these tasks were undertaken well. What most distinguished the blocks was the abundance of the mealybug vector – high numbers where virus control failed; consistently low numbers where control succeeded.”

The take home message from the Marlborough workshop was – remove all infected vines and maintain mealybug populations at consistently low numbers. Implemented together, both protocols will help prevent a future epidemic. ν

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Note: Issues related to leafroll virus identification are specifically covered in the grower Fact Sheet “Leafroll symptoms on red varieties” (NZVE 101) available via the NZW website (www.nzwine.com)