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 and at best requires two seasons to control.
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 spore release from chasmothecia which are retained close to the bunch zone in bark; that release being a function of climate conditions (perhaps closer to flowering). The spores from chasmothecia are thought to act more aggressively in bunches than flag shoot derived spores.
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
Preventative Programme for Powdery Mildew Control
The 2014 and 2016 season trials of HML32 and the results of its use by growers has proven HML32 with sulphur and copper to be a very effective preventative/eradicative regime for powdery mildew.
The Fungal Spray Programme based on Protector, HML32, sulphur and copper has been updated as a result. From budburst to just before veraison, maintain a 10-14 day spray programme with Protectorhml and sulphur then at the critical six period from the start of flowering, bracket flowering at 5% capfall and 80% - 100% capfall with HML32, sulphur and Copper and another application at pre-bunch closure and just before veraison. For higher challenge situations, such as previous history, disease pressure or susceptible varieties continue applications of HML32 + sulphur + copper after flowering at 10 day intervals and recover after any major rain event.
View the seminar by Dr David Gadoury for more information on this critical period where the plant is at risk of powdery mildew infection.
Eradication of Powdery Mildew Infections
If you haven't managed to prevent Powdery Mildew infections this season, these can be successfully managed using HML32 with copper and potassium bicarbonate if you catch them early enough and the application is in accordance with our recommended best practice. Severe infections are more difficult to manage but you should at least be able to suppress it. Go to our updated Best Practice for advice in the recipe and application methods.
The 2016 Grape Powdery Mildew Eradication Trial and the Farmlands eradication of powdery mildew in preparation for its Adjuvant Trial confirmed the excellent efficacy of the HML32, Copper and Potassium Bicarbonate recipe for the eradication.
Updated Research from 2016 Eradication Trial
- DO NOT include sulphur. It reduces the efficacy of the eradication sprays
- Eradication sprays provide significant forward protection, sufficient that protectant sprays can be resumed subsequently at normal timings, if required.
- Two applications (7 days apart) are better than one
- If a single application is all that can be managed – the efficacy can be improved by raising the metallic copper rate from 45g/100l to 67-90g/100l.
Severe infections are more difficult to manage but you should at least be able to suppress it. Go to our updated Best Practice for advice in the recipe and application methods.
For a quick reminder about key tactics for powdery mildew control:
- Battlefield Tactic: Strategy Wins Battles (2015) [PDF/423KB]
- Battlefield Tactic: Defend Flowering, Eradicate Insurgents (2015) [PDF/955KB]
- Battlefield Tactic: Act Quickly, Eradicate Immediately (2015) [PDF/441KB]
- Battlefield Tactic: Post Harvest - Clean out Pockets of resistance (2014) [PDF/568KB]
Effective botrytis control generally relies on good viticultural practice combined with well-timed fungicidal sprays.
Botrytis is a multi-pathway disease making it very hard to control. One of the main pathways is through berry injury caused by powdery mildew, hence our focus on controlling powdery mildew as a priority.
Another major pathway is through direct infection during flowering which remains latent until late season conditions enable it to manifest. Wet conditions of sufficient duration are likely to trigger botrytis as ‘slip skin’.
Botrytis, including the potential impact of latent infections, can be minimised by good viticultural practice. Botrytis control begins with good pruning, leading 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.
Strategies to thicken berry skin have a part to play as have strategies to loosen bunch structure reducing berry to berry pressure.
Trial work in the 2015/2016 season very much supports that if HML32 is used at appropriate times it brings a strong mix of direct and indirect fungicidal attributes leading to very good end of season outcomes, including botrytis.
Watch this video to see the difference applications of HML32 at particular timings made to an end of season slipskin infection in a vineyard in Hawke's Bay.