April (No. 495)
Hot water treatment: cooling the confusion
Helen Waite , Faculty of Land and Food Resources, Dookie Campus, University of Melbourne
Although hot water treatment (HWT) of grapevine cuttings and rootlings has been widespread since the mid 1990s it has been the subject of ongoing controversy in both the nursery and grapegrowing industries. Hot water treatment (HWT) is used by nurseries and vine improvement schemes to control a range of external and internal pests and diseases in cuttings and rootlings, but there is still some confusion in industry about its efficacy and safety, fuelled by sporadic reports of cutting and rootling failure attributed to HWT. This article endeavours to clarify misconceptions surrounding the role and practice of HWT so that nurseries and growers can make informed decisions about when and how it should be applied to propagating and planting material.
Hot water treatment is used to control a number of external and internal pests and pathogens in dormant cuttings and rootlings including nematodes, crown gall, Australian Grapevine Yellows (AGY) and Petri disease (black goo), but it does not control viruses.
Control of viruses is achieved by growing vines in tissue culture at very high temperatures for many weeks and propagating from the apical meristem (stem tip) to produce clean mother vines that can be used as a source of cuttings for propagation. Unlike HWT it is not a technique that is applied to commercial batches of cuttings and rootlings.
There are two different HWT regimes in common use; the short duration regime, 54degC for five minutes for the control of external pests and pathogens and the long duration regime, 50degC for 30 minutes for the control of external and internal pests and pathogens. The short duration HWT (54degC for five minutes) controls nematodes on rootlings and phylloxera* on cuttings and rootlings, but the heat does not penetrate the tissue sufficiently to control Petri disease, crown gall or AGY which are found inside the wood. However, long duration HWT (50degC for 30 minutes) allows heat to penetrate the wood for long enough to control the more heat-sensitive Petri disease, crown gall and AGY organisms without killing the vine tissue. Both regimes satisfy the quarantine regulations of Australian States and territories that govern the movement of cuttings and rootlings within and between States.
Although both the long and short duration HWT regimes can be safely applied to healthy, dormant cuttings and rootlings, the relatively high temperatures of HWT are a significant stress, and poor handling practices both before and after HWT can result in incremental damage to the tissue making cuttings and rootlings vulnerable to failure when planted in the nursery or field. There have been a number of GWRDC and CRCV-sponsored research projects in recent years that have investigated the effects of HWT and the surrounding handling practices on different Vitis vinifera varieties, particularly Pinot Noir, Chardonnay and Cabernet Sauvignon since these three varieties cover the range of sensitivities to HWT. Pinot Noir is more likely to suffer a loss of quality in HWT than other varieties and Cabernet Sauvignon is the least likely to be affected while Chardonnay is neither particularly sensitive nor the most robust.
The reasons for the apparent failure of HWT cuttings are numerous, but HWT itself does not kill cuttings or rootlings if the equipment is properly designed and calibrated and the treated material is dormant. Accredited HWT plants are tested annually by an independent testing authority to ensure that they are correctly calibrated and operate properly in accordance with standards set by the Australian Vine Improvement Association (AVIA) or the Vine Industry Nursery Association (VINA) in their nursery accreditation schemes.
The results of the research projects have shown that HWT cuttings are often slower to establish than cuttings that have not been HWT and suffer retarded growth until mid summer when the rate of growth accelerates and equals that of untreated material. This means that HWT cuttings are more likely to be damaged by adverse weather conditions when planted into field nurseries. Cuttings that are grown on in protected environments such as glasshouses or shade houses are protected from adverse weather conditions and unlikely to be damaged. Source area and source area management also affect the capacity of cuttings to withstand HWT. Cuttings sourced from well-managed vineyards in hot climates that have received post-harvest irrigation perform better than cuttings from vineyards in cool climates or vineyards that have suffered water stress.
Pre- and post-HWT handling is also extremely important. Cuttings or rootlings that have been allowed to dehydrate are unlikely to do well regardless of HWT, as are cuttings and rootlings that have been soaked in water for long periods. Soaking cuttings or rootlings for long periods is detrimental on two counts, first, soaking allows potentially pathogenic organisms in the water or on the surface of cuttings and rootlings to move into the vine through cut ends and trimmed roots. Second, soaking saturates the tissue and because the water is low in oxygen, the tissue may become fermentative and die. Recent research has also shown that the respiration rate of HWT material is greatly elevated in the 24 hours following HWT, increasing the need for oxygen at a time when the tissue is saturated as a result of the HWT. This results in a short-term oxygen deficit and forces the tissue into anaerobic respiration (fermentation).
If cuttings and rootlings are enclosed in sealed plastic bags for storage before the tissue has been able to dry out and resume normal aerobic respiration, the low oxygen environment will be maintained and the tissue will continue to ferment, the toxic by-products of fermentation (ethanol etc.) will accumulate and the material will die. If cuttings and rootlings need to be cool stored after HWT they should be allowed a 24 hour recovery period before bagging and the bags should have several evenly distributed ventilation holes. Storage should be a consistent 1-2degC, there should be no additional water in the bags and material should not be moved in and out of storage. Removing rootlings from storage, trimming roots and returning the rootlings to storage is detrimental.
Further damage can be caused to cuttings and rootlings by exposing them to ethylene in storage. Ethylene is generated by stored fruit and can penetrate plastic bags. Consequently coolrooms used to store apples and pears should be avoided as these fruits generate ethylene during storage. Holding packaged cuttings or rootlings at room temperature for more than 24 hours increases the respiration rate and very quickly results in damaging oxygen deprivation. If growers are unable to find suitable coolrooms it is better to heel the vines in by placing them upright in a trench and loosely covering the roots with soil or clean granite sand.
If the procedures outlined above are followed the chances of damage to HWT cuttings and rootlings are much reduced and nurseries and growers should be able to reap the benefits of using clean, healthy propagating and planting material. However research into the effects of source area management, the interaction between HWT and cold storage, stages of dormancy and the effects of HWT on vine tissue is ongoing and although the causes of failure of HWT material can usually be identified it is not yet fully understood. The results of this and other research pertinent to grapevine propagation will be the subject of the next ‘Making Every Stick Count’ workshop to be held at Merbein in early June.
For further information regarding the workshop or any of the information in this article please contact Helen Waite at the above address, or on (03) 5833 9285.
Crocker, J., Waite, H., Wright, P. and Fletcher, G. (2002) Source area management: avoiding cutting dehydration and good nursery management may be the keys to successful hot water treatment. The Australian & New Zealand Grapegrower & Winemaker, 461a:33-37.
Waite, H., Crocker, J., Wright, P., Fletcher, G. and deLaine, A. (2001) Hot water treatment in commercial nursery practice – an overview. The Australian Grapegrower & Winemaker. 449a:39-43.
Waite, H., Jaudzems, G. & Faragher, J. (2004) Recent advances in grapevine propagation research. The Australian & New Zealand Grapegrower & Winemaker. 485a:39-41.
Editor's note: This article contains several figures and photographs which are not reproduced in the on-line version. These are contained in the printed version of Australian & New Zealand Grapegrower & Winemaker.