Protect that wound

Whether they are cordon pruned or cane pruned, all vines are susceptible to a range of diseases that will eventually kill them if nothing is done.

The two major diseases are eutypa and botryosphaeria dieback. Both occur when fungal spores infect vines through pruning wounds. The spores can be spread by wind or rain splash, and given they can survive on dead wood, infected vine prunings can be a source of infection.

South Australian Research and Development Institute (SARDI) scientist Dr Mark Sosnowski often refers to trunk diseases as the “silent assassins” of the vine world. That’s because they are doing their damage quietly and effectively without providing any visual indications. In other words your vines could be suffering from some form of trunk disease and you wouldn’t know it because there is nothing to see - for the first few years at least. 

In a joint research project between NZW and SARDI, Sosnowski and Plant and Food scientist Dion Mundy have been researching trunk diseases in both Marlborough and Hawke’s Bay. In 2013 they surveyed 697 vineyards spread across the two districts. Each vineyard was visited, with 200 random vines looked at, in an effort to assess whether there were foliar symptoms of eutypa dieback and general dieback.

“We certainly found foliar symptoms of eutypa dieback in Hawke’s Bay and Marlborough, although the incidence was relatively low.”

It was a different story when it came to general dieback.

“Dieback is much more concerning. We found vines as young as four years old with dieback in a cordon pruned vine. At least two spurs had dieback on one arm. We know that from the time a vine gets infected through the pruning wound until you see a symptom, can be anywhere from three to eight years. So you can imagine on a four or five year old vine, that infection must have happened on the first or second year of pruning.”

While this proves any vine of any age can be affected, Sosnowski says there is a tipping point when vines all of a sudden become more vulnerable – between 15 and 20 years of age.

From the research here in New Zealand many of the vineyards with 15-year-old vines had 50 percent infection – some with 20-year-old vines had 80 percent. (See above).

“We see that in Australia in a more serious way, where vineyards of 20 years have 100 percent infection.”

Cordon versus cane pruning

There has been a lot of discussion about whether one or the other is more at risk of trunk disease. Sosnowski says they have compared the two methods and at first glance it appears that cordon pruned vines are the more susceptible.

“The incidence of infection in cordon pruned vines in most varieties was significantly higher, but we have to be careful of this. The reality is that the age of the vines in the survey was in the 10 to 20 year period. And what a French study comparing the same thing has shown, is that before 20 years of age they saw a lot more symptoms in cordon pruned vines, than they did in cane pruned vines. 

“Then the French study reported that over 20 years of age, cane pruned vines had a greater mortality rate. So they were just dropping dead without showing any symptoms. 

“So it doesn’t mean that cane pruned vines are more resilient or less likely to be infected. It just means that you are not going to be able to see the symptoms before the vines start to die. And in reality, the cane pruned vine will probably die first, just because you didn’t see the symptoms, so you didn’t do anything about it.” 

Why is Sauvignon Blanc more susceptible?

That is the question many Marlborough growers will be asking. And it’s a good question Mark Sosnowski says, although it’s hard to provide an answer.

“We don’t completely understand why, we just know that when we compare it with other varieties it is the most susceptible visually and when we inoculate the vine. It is probably physiological and has to do with the vine’s cell structure.” 

He says according to recent research by student Jessica Hamblin at the University of Adelaide, Sauvignon Blanc has been shown to have large xylem vessels when compared to some of the more tolerant varieties. It is easier for the fungus to grow through it. 

“Then there are other natural biochemical reactions going on in a vine, which we are finding less of in Sauvignon Blanc than we are in say Merlot which is more tolerant of the disease.”

It is one of the areas of research Sosnowski is planning to follow up. He will also undertake further research into the susceptibility or tolerance of certain clones and rootstocks.

Prevention – cheaper than curing  

You need to prevent the disease getting in in the first place, Sosnowski says. That means you need to protect all pruning wounds, as soon as possible. 

“The susceptibility of those wounds decreases over time, but it can be up to four to six weeks before it decreases.”

You can either manually paint the wounds with a protectant or spray the vines. That has proved effective in Australia and on trials here at home.

But there are other more cultural ways of preventing the disease from striking.

Given the spores are dispersed during rainfall, avoid pruning in the rain. 

Remove all old wood from the vineyard – spores can also be transferred by wind, meaning you are also at risk from vines and other hosts around the district.

Choose the time you prune. There is some thought that by pruning later in the season, when the sap begins to flow, susceptibility decreases.

Choices if you already have trunk disease.

The eutypa fungus can grow at a rate of 50mm a year, so the sooner you get on to removing the infected parts of the vine, the better.

If you are going to cut out the infected area, cut at least 10 cm below the infected wood if possible to ensure no residual fungus is left.

Cut back to allow water shoots to grow, replacing the original vine. Australian research has shown if you cut too high up the trunk, there is more likelihood of the fungus reoccurring than cuts made lower down the trunk.

Remember that the fungus in general grows downwards – not up.

And if all else fails, you may have to remove the vine completely – which is why prevention is by far the best and most economical solution. Sosnowski says, research is currently underway with the Australian Grape and Wine Authority to determine if reworking vines can also control botryosphaeria dieback.