An aerial operation to control possum numbers in the Silver Peaks, an area of rough forest and tussock and scrub covered hill country near Dunedin, will get underway this month.
Generally it is on vineyards where the extent of disease is not recognised. Of all the diseases which must be managed in New Zealand vineyards, trunk disease is different from the others. Why is this? Trunk disease is “insidious” whereas the others we deal with are “in your face”.
It will take several years for a vine infected with trunk disease fungi to show symptoms, if at all. One of the major trunk diseases, Botryosphaeria has no conspicuous symptoms until the vines are quite sick. But growers are aware of the symptoms of powdery mildew, botrytis and downy mildew, among others. Trunk diseases are like the cancer of grapevines, slowly spreading until it may be too late to treat.
This article will tell growers how to deal with trunk disease in their vineyards. The procedure is simple and is not costly, and is proven overseas to work well, as it has been in New Zealand. The problem is that not many growers and vineyard managers are aware of what to do. So, they do nothing, and this is what you do if you want to allow trunk disease to destroy your vineyard. The choice is yours.
Older vineyards show more trunk disease damage. This is not surprising, as older vines accumulate pruning wounds. The following figure is taken from a Winegrowers article by Sosnowski and Mundy (2014, Aug-Sept) and shows the pronounced vineyard age effect on disease incidence. Note that varieties differ in susceptibility, and that Sauvignon Blanc is one of the most susceptible varieties known.
Why is trunk disease a problem now?
Trunk disease is a growing problem around the world, whereever grapes are grown. It seems worse now than it has been for several decades. Part of the reason is that newly-grafted grapevines now are infected with the disease and eventually infect vineyards where they are planted. This is a common problem around the world, including New Zealand. Mature grapevines are infected through winter pruning wounds, especially in wet weather. The first line of defence against pruning wounds is to paint or spray an appropriate fungicide on all fresh pruning wounds. New Zealand is more lucky than many countries in the range of products which are registered for this purpose. Protecting pruning wounds is the first line of defence against trunk disease.
The second line of defence is the subject of this article, which is Timely Trunk Renewal, or TTR. As will be discussed this procedure should be used early in the life of the vineyard when trunk disease effects are first noticed. Modelling studies in California have shown that this helps reduce the spread of disease and also brings about the most profitable outcome of disease control.
How to control trunk diseases in your vineyard?
The trick is to identify trunk diseases early, before the vine has become diseased and loses production. The two main diseases in New Zealand are Eutypa and Botryosphaeria, and the latter is more widespread. Botryosphaeria typically will show up just before harvest with a yellowing of the foliage, and sometimes characteristic nutrition deficiency symptoms. These are unusual, as they may appear on only one or two vines, as distinct from the normal soil effects, which can affect many vines in a zone. Potential symptomatic vines should be marked, with spray paint or a tape. Eutypa shows up in old vines with characteristic shoot growth symptoms in the spring.
The response to a potential symptomatic vine is to encourage sucker growth from the base, as low as possible, but not from the rootstock. Suckers should be trained on bamboo cane to the fruiting wire, and within a plastic shield to prevent herbicide damage. If possible, select a sucker on each side of the vine, for two potential new trunks, and in line with the row.
After one or two years the existing vine may have the old trunk removed. And full production can be obtained from the one or two new trunks.
Make sure when you cut off the old trunk you do with an angle of around 45° facing North and immediately paint with wound protection fungicide. If you catch the trunk disease early, you can remove the old trunk below fungal staining which is working its way down the trunk, indicating the progress of trunk disease.
Remove and burn the old trunk parts as this will reduce infection. On subsequent vineyard surveys for symptoms, be particularly aware of vines nearby those treated with trunk renewal, as trunk disease tends to occur in patches, down and across rows.
Follow these simple guidelines and you will save your vineyard from the ravages of trunk disease. Why not let your vineyard enjoy a life of 50 or more years, rather than let it be terminated by a disease that you can easily control. For the growers of Sauvignon Blanc, don’t let a few nasty fungal diseases take away New Zealand’s gift from Mother Nature.