Fungal Diseases

  1. Powdery Mildew
  2. Downy Mildew
  3. Botrytis
  4. Trunk Diseases

Powdery Mildew

Images of Early Stages of Grape Powdery Mildew mycelium (before formation of conidia).

The following images are taken through a cellphone microscope (Konus clip 60-100x Handheld and Smartphone Microscope available from Jacobs Digital Auckland) in the third week of December 2020 in a Martinborough vineyard. Images show early stages of Grape Powdery Mildew on leaves and on berries / rachis.

Powdery Mildew on Leaf

The image below shows the yellow discolouration (blotch) on the top side of the leaf. Generally in the early season, the easiest place to find PM infections like these are on small basal leaves in the head.

<Typical Topside leaf PM>


The image below shows the greyish discolouration on the underside which that corresponds to the yellow blotch on the surface.

<Typical underside leaf PM>


The image below shows the mycelium on the leaf.

<Typical Underside leaf PM mycellium>


The image below shows the mycelium on the leaf. Note the immature leaf hairs of the leaf in the centre of the photo.

<Leaf hairs and mycellium 1>

<Leaf hairs and mycellium 2>


Powdery Mildew on berry/rachis

The image below shows the mycellium on the surface of the berry.

<Berry showing mycellium>


The image below shows the mycellium on the rachis.

<Rachis showing mycellium>


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:

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

View the Powdery Mildew, Downy Mildew and Botrytis Prevention Spray Programme for 2020–21.

View the Grape Powdery Mildew Prevention Trial 2017-18, Ngatarawa Rd (Handsprayed).

View the Grape Powdery Mildew Prevention Trial 2016-17, Ngatarawa Rd (machine sprayed).

View the Grape Powdery Mildew Prevention Trial, 2015–16.

View the Grape Powdery Mildew Eradication Trial, 2016.

View the Farmlands Adjuvant Trial – Preparation of Trial Site 2015-16.

View our Best Practice - Eradication of Powdery Mildew Infection 2017.

View the full report on the HML32 powdery mildew eradication trial.

View the Farmlands presentation/report on their independent powdery mildew eradication screening trial, which includes HML32 formulations.

Downy Mildew

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

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.

View the Enhanced Maturity and Botrytis Trial 2016.

View the Powdery Mildew and Botrytis Prevention Spray Programme for 2020-21.

View the Presentation ‘Botrytis, Slip Skin and Water’ - Dr Gareth Hill Plant and Food Research 2017.

Trunk Diseases

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.

View the presentation Pruning to limit trunk disease, Francois Dal