Current state of knowledge
Powdery Mildew has evolved from being a relatively easy disease to overcome to the most serious of grape diseases in New Zealand - ahead of botrytis and other rots. It has the ability to impart off-flavours that cannot be overcome in wine making, decimate yields, as well as creating injury on berries that provide an entry for botrytis and other end of season rots.
The powdery mildew we dealt with a few years ago was clonal and infection came from a defined early period - from flag shoots and their offspring. Now powdery mildew has the ability to produce fruiting bodies (chasmothecia), bringing the additional burden of ascospore release from chasmothecia which are retained close to the bunch zone in bark. That release is a function of climate, mainly rain events. Early scientific study in New Zealand indicates strongly that ascospore release occurs after rain events through flowering and after flowering. The spores from chasmothecia are thought to act more aggressively in bunches than flag shoot derived spores, but it may be simply that ascospore release that occurs through flowering and after flowering have temperatures more suitable to rapid colonisation of the disease.
If you are interested in Grape Powdery Mildew, the following links are worth checking out:
- A lecture by Dr David Gadoury delivered at Oregon University 2011. He is regarded by many as the global leading expert on chasmothecia. This lecture also goes into the effects of diffuse powdery mildew - which is an area under-reported and important. There are some excellent images of chasmothecia and of its development. The presentation is called 'Everything you need to know about grape powdery mildew and several things you need to forget'.
- A collection of the scientific papers arising from a meeting organised by Dr Rob Beresford at Plant and Food Research Auckland (May 2015) which benchmarks our current scientific knowledge of grape powdery mildew in New Zealand. There is some wonderful information here in respect of genetics, chemical resistance here and overseas, product research etc.
- 'Effects of Sunlight Exposure on Development and Management of Grape Powdery Mildew' by Professor Wayne Wilcox - Cornell University, New York State, USA
- A presentation on the genetic Characteristics of New Zealand Grape Powdery Mildew by Dr Peter Johnston et al.
- Online purchase of 'Compendium of Grape Diseases, Disorders and Pests Second Edition - Edited by Gubler and Wilcox and Uyemoto 2015.
Key facts and principles:
- Powdery Mildew disease (chasmothecia) are present in most New Zealand vineyards and it is now a disease that requires suppression all season long – including after harvest.
- Early New Zealand data strongly indicates that rain events trigger ascospore release (from chasmothecia). These releases can occur during flowering or later where temperatures provide Powdery Mildew the ability to move quickly from spore to infection
- Greatest care must be taken to protect the current season’s crop from Powdery Mildew when it is at its most susceptible - from the onset of flowering (E-L19) to about one month after fruitset (E-L27). Not only for the disease itself, but for the close connection that exists between it and botrytis infection and other secondary rots.
- Powdery Mildew is a disease of proximity and while it is unlikely that the disease can ever be eliminated from a vineyard, it is certainly possible to suppress it to very low levels through good viticultural practice and the use of protectant sprays, and at times of need, the use of protectant sprays that also possess eradicant activity.
Research and trials
Downy Mildew disease as it relates to grapes in New Zealand is certainly one of the most serious challenges a grower can face if allowed to move through to a secondary infection – growers can be left with bunches on wines and no leaves to ripen them.
In the North Island, the disease is a normal part of the disease challenge from Auckland north, with decreased challenge in Gisborne and Hawkes Bay. Understood to not be present in Martinborough.
In the South Island, primary infection has been seen in Marlborough and it is understood that in some seasons the crop there have been on the cusp of a secondary infection. Understood not to be present in Waipara or Central Otago.
In New Zealand, if your growing area has any history of downy mildew, then preventative sprays should be applied for it.
Our research was always undertaken with the aim of eliminating the use of copper, which is a highly effective material for prevention of the disease. In this we have not been successful, so have focussed research on establishing the minimum rates of copper to provide effective prevention of the disease depending on the degree of challenge. Refer to the Protectorhml product page for more detail.
There are many good outlines of the grape downy mildew life cycle on the internet. In simple terms, the disease shows first as a ‘grease spot’ (called primary infection) following a weather occurrence which exceeds 10 10 24 (10mls rain, temperature greater than 10 degrees for a 24-hour period). Other ‘grease spots' can occur on grape leaves, but downy mildew can be identified from those by the fact that it generally has a brown halo around the grease spot.
You only need a few grease spots and the right weather conditions following to move to a secondary infection. This is not a disease to just ‘keep an eye on’!
Botrytis is a disease that is present in the environment wherever there is dead plant material. In the New Zealand context, it uses multi-pathways to establish infection in wine grapes and therefore broad strategies are required to prevent it.
Direct infection during flowering has perhaps the most potential to be damaging, where the infection remains dormant/latent awaiting sugar accumulation within the berry and rain event/s to set it off – leading to end of season bunch botrytis or slip skin. Click here for a presentation from Dr Gareth Hill, Plant and Food Research with excellent images of latent botrytis infection before any visible symptoms and also examines between botrytis, slipskin and water.
Another major pathway is where botrytis is a secondary disease where the berry skin has been previously weakened say following a powdery mildew infection, or also commonly where there is physical damage caused by insects, such as leafroller caterpillar.
Good cultural control begins with good pruning, including early shoot thinning of unwanted shoots in the head or doubles/unders on canes. This leads to well balanced and even fruiting through the vine, avoiding heavy growth in the head or at end of canes. Blowing out flowering trash from the bunches, leaf plucking to allow air movement and quicker drying times, and even mechanical trunk shaking can all assist to lower botrytis risk.
Well timed fungicidal sprays are also important, and this is where HML32 with additives can be particularly useful as infections from both powdery mildew and botrytis are directly prevented.
In addition, trial work in the 2015/2016 season very much supports that if HML32 is used at the correct plant growth timing, thickening of the berry skin occurs resulting in considerable resilience to botrytis including slip skin.
Strategies to thicken berry skin have a part to play as have strategies to loosen bunch structure reducing berry to berry pressure.
Watch this video to see the difference that applications of HML32 at particular timings make to markedly reduce end of season slipskin infection in a vineyard in Hawke's Bay.
Use of HML32 around flowering
The label recommendation is to apply HML32 with sulphur plus additives just before flowering and when flowering is complete. If flowering is protracted and cool or wet, and there is a desire to apply additional cover, use Protectorhml at 2% concentration instead of HML32.
Henry Manufacturing has undertaken very little work in the area of grape trunk diseases.
At the 2017 New Zealand Organic Winegrowers Conference, the following presentation (listed with permission of its author) was given by Francois Dal. It provides an unusual degree of insight into how trunk diseases (short and long term) can be dealt with by well planned and executed pruning.
We list this presentation here as we wish to support good viticultural practice.