Dr Mark Sosnowski, a Research Scientist with the South Australian Research and Development Institute (SARDI) focused on Eutypa Dieback at recent New Zealand Winegrowers workshops in both Hawkes Bay and Marlborough.
Don't be fooled into thinking this is not a major issue for growers in New Zealand. Sosnowski was quick to point out that it is here and is prevalent in many older vines.
One of the major problems with this fungal disease is that by the time you pick up the foliar symptoms, the disease has already made a home for itself in the wood of the vine. During research in Adelaide Hills no signs of the disease appeared before the vines were six years old – although it is believed it was present well before then.
"The older the vines, the more symptoms you see."
The first signs will be in the foliage and the symptoms include; stunted shoots, cupped leaves, yellow in colour and often with necrotic margins. (See Photo below.)
The symptoms are most noticeable during the spring months, when the shoots are 30 – 70 cm long. Once the foliage starts to reach full growth, the healthy leaves can mask the unhealthy ones.
Beware that it's easy to confuse the symptoms with a range of other issues, such as bud mite or herbicide damage, frost injury and zinc deficiency.
In terms of fruit, the symptoms in severe cases mean inflorescence may not initiate at all, while in less severe cases buds can shrivel and drop off after flowering. There may be uneven berry ripening or bunch size, which will affect long term fruit quality.
It is in the wood that the most tell tale signs of Eutypa are seen. But these don't tend to emerge until the disease has taken hold of the vine. Cankers will form in the wood and when dissecting the trunk or cordon an obvious wedge of dead wood will be apparent.
Eutypa is spread by spores via wind, or water - such as rain splash back. If a vine has an open wound and spores land on that wound, the fungus has been transferred.
So to prevent Eutypa, Sosnowski says growers have to be meticulous with wound protection during pruning and thinning.
"You need to prevent the disease getting in in the first place, which is done by wound protection. If you do that, you don't have to think about the second option which is controlling the disease once it is established, which is a pretty drastic and costly thing to do."
Pruning is the most dangerous time of all, as no matter whether you mechanically or manually prune, the vine will end up with exposed wounds.
"The susceptibility of those wounds decreases over time but it can be up to four to six weeks before that susceptibility decreases. The other thing to think about is pruning time. If you cut in the dead of winter you have the longest window of susceptibility. But if you make the cuts closer to spring when the weather is warming up and you have sap flow, the window of susceptibility does decrease."
Basic rule number one – don't prune in the rain.
Rule number two – remove all dead wood from a vineyard, in case it is carrying the fungus.
Also you need to be aware that Eutypa has a multitude of host species besides grapevines. You may have the disease in plants within the vineyard – if so, remove those as soon as possible.
Most importantly is the need to protect the vine wounds – there are only two registered fungicide products in New Zealand. And both have to be applied by painting straight on to the wound – a laborious job.
Protecting may prevent, but dealing with Eutypa already established is a different kettle of fish. In that case, the diseased wood must be cut out. Given the fungus grows through the wood at an average rate of 50 mm a year, the sooner you get on to the removal the better.
"Also the distance we have been able to isolate in front of the stained material of healthy tissue is 80mm, so really if you are going to be cutting out the infected material, you need to cut another 10 cm below, to be sure there is no residual fungus left. And bear in mind that the fungus does grow down towards the ground predominantly."
By cutting back you can allow water shoots to grow, replacing the original vine. Cuts can be made at low to the ground, mid vine or high.
"Different varieties often have water shoots at very different rates. In Sauvignon Blanc there is not much problem getting water shoots, it's the same in Pinot or Merlot. But in some of the other red varieties we have problems getting water shoots, particularly when we cut the vine down to 20 or 30 cm."
In some varieties cutting back so far means the shoots may be minimal, around 40% in the first year, but over a period of time, (3 - 4 years) the shoots will increase.
Just where you cut the vine back to may play a significant role in the re-emergence of the disease. Research in Australia proved high cuts were almost a waste of time.
"As the water shoot origin got higher and higher we saw more and more symptoms reoccurring, to the point that the high cuts were probably a waste of money. By cutting low you are increasing your chances of getting past that disease that has spread down into the trunk."
Given the way spores can be transferred several kilometres in the right conditions, there is always the danger of cross infection between vineyards, which means all growers have to be vigilant and take preventative measures annually, Sosnowski says.
"It is certainly more prevalent in older vines and it is certainly present here in New Zealand. We haven't found a cultivar yet where the fungus doesn't grow. Just because you can't see the symptoms, doesn't mean you don't have the disease."
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The workshop presentation is available to be viewed on the NZWine website.
In the next issue of NZ Winegrower we will focus on Botryosphaeria disease ■
