I am writing this piece as a grape grower with a heavy heart. It will take us years to recover from the devastating effect the resent bushfires have had on our agricultural sector and our people. The damage and losses encompassed all sectors of agriculture and has had a profound effect on our rural communities.
My thoughts go out to all our growers and pastoralists effected by the recent tragedy. I hope the article below can contribute in a small way to the understanding of the effect of fire on grape vines, the resulting damage and equip our growers with strategies to mitigate the effects left over from the fires.
Grapevines can be damaged by fire in several different ways. The predominant factor driving the extend of the damage is the Intensity and duration of exposure to fire and radiant heat. Some effects of fire are obvious and quite noticeable on the surface such as burned bark, blackened shoots and dehydrated leaves.
However, it is the damage that occur below the surface that leads to long term vine decline and death. Off particular interest is the extent of damage to the vascular system, comprising of phloem (transporting sugars and hormones from the leaves to the roots) and xylem (transporting water and nutrients from the roots to the leaves). Below is a cross section of a healthy vine shoot.
It is important to understand the impacts and extend of the damage on the long-term viability of vines in order to make decisions about how they should be managed after fire exposure. This will assist growers to adopt the best management strategies to maximise long-term recovery of the vines.
Healthy undamaged vascular tissue is white and/or green. Any tissue that is yellow, creamy or light brown has been damaged and is in the process of deterioration. Tissue that is dried out and dark brown is dead and can’t be revived. Different levels of damage may occur irregularly around the trunk depending on the duration and location of the fire exposure. Alongside is a photograph provided by Agricultura Victoria.
Unfortunately damage to the vascular tissues is permanent. Once the cambium layer is destroyed there is no further vascular tissue produced at that location during the current growing season and the effected part of the vine dies. Depending on the location on the vine and the degree of damage, the long-term grapevine viability can be adversely affected.
Most internal damage can be readily checked within a few days of a fire event but the full extent of damage may not be obvious until the following season. As the vascular system is obscured under the bark, it is not always possible to determine the health of the cambium layer from external observation.
Below are the two methods recommended by Agriculture Victoria to determine the extent and severity of fire damage to the vines. There are two different ways to check the viability of the internal tissue.
The first one is non-destructive and each plant can be checked. Using a grafting or sharp knife, make a small cut into the wood, much like that used in T budding, to reveal the health of the cambium layer and vascular tissue. Healthy and undamaged tissue will be white and/or light green. If the tissue is moist and creamy white with a greenish tinge, then the cambium suffered minor to moderate damage and is still alive. In this instance the vine has a good chance of recovery. If the cambium is dry and brownish in colour then regrettably it is dead and that part of the vine will not recover or grow. Fire may damage one side of the vine but not the other so it may be necessary to check a few sections around the trunk.
The second method of checking viability is destructive and involves cutting completely through the vine and staining the tissue with methylene blue. Live tissue stains bright aqua-blue and dead tissue stains a brownish-blue colour. The latter technique is more useful for assessing the viability of a large area of vines being considered for removal.
Vine recovery and vineyard re-development:
It is highly recommended to perform a thorough assessment of each vineyard before deciding on a course of action. Recovery of vines after fire is variable and it can take several months for the full extent of the damage to become apparent.
It is vital to get the vines rehydrated as soon as possible. Temporary irrigation systems such as furrow irrigation, movable sprinklers or even just driving down the midrow dispersing water can be instigated as an interim solution until the original irrigation infrastructure has been repaired. Consideration need to be taken as to soil moisture levels to avoid soil moisture saturation and possible water logging. As the viable leaf area has been significantly reduced, transpiration levels also need to be taken into account.
2. Vineyard rejuvenation
Pay close attention for the appearance of new buds and shoots within weeks after fire damage. Continues growth displaying strong new canes and adequate leave development should serve as an indication that the vascular system is still functional. It must be noted however that there have been incidences where vineyards that displayed initial strong re-growth deteriorated and died off in the following year. In most cases, and certainly when fire damage occurs after veraison, the vines should be left to regrow during which time viability assessments can be made to determine the best strategy for dealing with the vineyard. Application of a nominal level of nitrogen through fertigation will assist regrowth and development.
3. Vineyard redevelopment
Before deciding on a course of action, growers should consider the length of time left in the growing season for adequate regrowth and retraining to occur and the availability of funds, material and labour to complete the process.
There are three levels of redevelopment available when fire damage has been so severe that retention of the existing cordon is not an option and the decision was made to remove and re-develop the cordon.
A: Own rooted vines severely damaged by fire can still retain healthy tissue at the base of the trunk or below the ground. New shoots can appear from the base within a few weeks after the fire. After 4 weeks select the three strongest shoots and remove the rest if any. The new growth should be allowed to grow and lignify. In winter, the damaged trunk and cordon can be removed and the strongest shoot from the base retained to form a new trunk. This method is recommended if there is less than 12 weeks left in the growing season and the decision has been made to replace the cordon.
B: If there are more than 12 weeks left in the growing season and there are clear signs of healthy tissue at the base of the trunk or below the ground, it may be desirable to immediately remove the damaged trunk and cordon. This will promote the development of new shoots below the cut that can be trained up to the cordon in the current growing season. Ensure that equipment used during rework is sterilised between vines, especially if Eutypa is present in the vineyard. Application of a wound seal solution is highly recommended when trunks are removed.
Grafted vines growing on rootstocks need to be treated in a different manner. If the grafted vine has been severely damage and the rootstock regrow, this may be left to redevelop a new trunk and a scion of the desired cultivar can then be grafted onto this in the following year. Alternatively, if there is sufficient rootstock remaining after the damaged tissue is removed, a new scion can be grafted close to the ground level in the current growing season and trained up to the cordon.
C: Where the fire damage to vines is severe, if the block has a history of poor performance or if a new variety, clone or rootstock is desired, the best action may be to completely remove the vines and replant the block. Consideration to the availability of funds, material, labour and lost revenue need to be taken into account.
It is highly recommended to perform a thorough assessment of each vineyard before deciding on a course of action as there is no standard approach. Recovery of vines after fire is variable and it can take several months for the full extent of the damage to become apparent. Rehydration of the vineyard should be the first priority.
Engage with your local grower liaison to align new vineyard development with the strategic direction of your buyer and/or winery. This can provide the opportunity for long term contracts, securing revenue and stability for the road ahead.
– Pete Breugem – Thomas Elder Consulting – Viticulturist/Winemaker