DURING the Christmas and new year period, extensive rainfall, and hail in some regions, and unusual humidity before and after the rain events, affected several grapegrowing regions.
This created conditions conducive to the development of diseases such as downy mildew, powdery mildew and botrytis in grapevines.
Looking ahead, Robyn Dixon said the forecast to March suggested a likelihood of above-median rainfall in the south-east of Australia – including Sunraysia and further east along the Murray through Robinvale and Swan Hill.
Ms Dixon said immediate action was crucial to mitigate the risk of significant crop losses from disease, and that meant strategies to reduce both the risk and severity of fungal infections during periods of heightened disease pressure.
Bob Emmett agreed, saying while most growers would have completed their major spraying programs, grapes (table as well as wine) would still be at various stages of ripening and that would go on for at least another month.
He said growers would “probably be in the late part of the season, past veraison, and trying to withhold sprays, especially for residue reasons, or changing to ones without residue those challenges”.
“Some growers could now be facing a downy mildew infection, although this will vary by region – and within regions,” Dr Emmett said.
“Downy mildew is an infection where the organism relies heavily on moisture and rain – and is compounded by excessively wet soil at the same time.
“At the right temperatures I would expect patchy primary infections at the moment.
“While quite a number of growers now use their own weather station information for infectious conditions as a guide for spraying, the most recent events would have been significant for further infections.
“But decisions about one more spray will probably be driven, as I mentioned, by residue levels at vintage and the size of infections on individual properties.”
Mildura and Wentworth’s 33mm in two days was up as much as 30 per cent on long-term records for the whole month, while Robinvale copped 66mm on January 8 and 9 (monthly average 32.7mm).
On the same two days Swan Hill got 42mm – while further east towards Kerang, between January 3 and 9, the region was drenched by almost 130mm, compared to a long-term January average of just 31.7mm.
In terms of powdery mildew, Dr Emmett said there was a lot of evidence to show it is nowhere near as dependant on wet weather or temperature for establishment, but it will spread rapidly in mild weather and humidity if not sprayed.
“Growers should have, would have, sprayed early during flowering, and probably again post flowering, so most of that should have been done,” he said.
“However, growers not planning to produce a crop this season could control any problems with early and systematically spraying throughout the year.
“Powdery mildew can even start from small levels in vine bark or on trellis posts, and if it is wet after bud-burst they will release spores and infections will pop up all over the place but it has to be more than 10 degrees and wet to do that.
“The fungus will grow on green tissue and the spores will be spread by wind – if it is very hot they will die, but in the right humid/mild conditions their spread can be very quick.”
Ms Dixon also offered some general tips for best-practice disease control.
She said that in high-vigour blocks, consider shoot thinning, trimming, leaf removal and bunch thinning to enhance canopy openness, allowing better light and air penetration.
“This practice not only accelerates canopy drying post-rainfall, but also facilitates improved spray penetration and chemical deposition,” she said.
“Vineyard floor management, such as keeping mid-row cover crops low to support airflow and canopy drying, is another strategy to put in place.
“In addition, if a neighbouring vineyard has opted to rest their block, a strategic approach could involve treating a few boundary rows to mitigate disease pressure stemming from those vines.”
Chemical options for powdery mildew, downy mildew and botrytis control are listed in the Dog Book (a booklet titled Agrichemicals registered for use in Australian viticulture, published annually by AWRI).
Chemicals applied to fruit destined for the domestic wine market have shorter withholding periods and these are found on the label.
Ensure you consult with your grape purchaser or the AWRI if you are considering chemical options that do not follow the Dog Book recommendations for export wine.
Ms Dixon said effective spray coverage was critical for successful disease control.
She said you should test your spray coverage using water sensitive papers, or a kaolin-based clay sunscreen, and once your equipment is set up well, check you’re applying the correct chemical rate to avoid underdosing.
“Operating a spray unit in the same direction for every spray application runs the risk of missing the same hard-to-reach parts of the canopy each time, potentially creating a disease hotspot,” she said.
“Spraying in a different pattern, so the opposite direction is travelled up the row, has potential to improve overall coverage.”
Ms Dixon said growers should consult their suppliers about available products and their features, particularly regarding rain-fastness (the ability of a product to remain on the target after rain or heavy dew).
She said to check the compatibility of these products with adjuvants designed to improve rain-fastness.
“The relationship between spray coverage and retention is a function of agrichemical mixes and rates, spray application volumes, canopy type and the target canopy tissues (leaves, wood or bunches of fruit).
“Choosing the right product for the current conditions is crucial, as it might differ from what’s suitable during drier years.
“In situations where resources and time are constrained, prioritisation becomes crucial.
“Identify your most profitable blocks and concentrate on securing a healthy crop from those areas. Monitoring is important, along with recognising powdery mildew, downy mildew and botrytis.
“With downy mildew, applying preventative sprays prior to a downy mildew infection event is crucial to controlling it in these wetter years.
“Fungicides are in high demand during wet seasons, so keeping a chemical supply available is wise.
“Most registered products for downy mildew act as protectants but always avoid using these on an active infection to prevent resistance, which could lead to losing the efficacy of the product in the future.
“If you have had a downy mildew infection event and are using a metalaxyl (group 4) for curative control, then a second group 4 spray on a seven to 10-day interval is required.
“However, while the AWRI website provides information on phosphorous acid use, it is crucial to communicate with your winery or grape purchaser before applying this product.
“Some export markets maintain low tolerance for phosphorus acid residues, necessitating careful consideration before its use.”
Ms Dixon suggested the optimal strategy for combating powdery mildew involved the consistent application of protective chemicals early in the season to protect all growth from this disease.
Preventing its establishment in the initial stages is pivotal for minimising the disease load later on.
“Unfortunately, this season posed challenges, hindering the application of early sprays in numerous vineyards or diluting those already applied due to heavy rain periods.
“These conditions have heightened the risk of powdery outbreaks. In such scenarios, taking preventive action to avoid a widespread powdery disease epidemic becomes absolutely essential.”
Ms Dixon said elemental sulfur is a multi-site fungicide which is not susceptible to resistance and has some curative activity.
To gain control of an existing infection using sulfur, good spray coverage is essential as an active powdery mildew infection may repel water, so ensuring adequate spray coverage is crucial.
“Consider using an adjuvant to enhance coverage on challenging-to-wet surfaces,” she said.
“Some sulfur products include built-in adjuvants, enhancing their spread on plant surfaces. For other sulfur products, consult your chemical supplier to determine if an adjuvant is needed to improve spread.
“Some production zones are experiencing earlier ripening conditions compared to previous seasons. Ensure applications of sulfur adhere to the pre-harvest cut-off timeframes if used for late-season powdery mildew management.
“Some single-site chemicals exhibit curative activity against powdery mildew, but using them on active infections is not advised due to the heightened risk of resistance development.
“CropLife Australia recommends applying single-site fungicides preventatively before colony formation for effective powdery mildew control.”
The third risk this season is botrytis bunch rot, as late season botrytis expression can create significant challenges for growers formulating strategies for variable site locations and varietal mixes.
Ms Dixon said this required a sound understanding of the pathogen and how it interacts with various late-season fungicide modes of actions.
She said many biological fungicides provide optimal protection when used prior to visual expression of disease symptom, meaning disease control may be less than optimal when products are employed reactively for curative purposes.
All spray options for late-season botrytis management require good coverage of bunches and berries. Strategic canopy and fruit zone management not only enhance spray droplet coverage but also promote better airflow around the bunch zone.
“When botrytis is already evident in tightly clustered bunches, causing juice leakage from the berries, spraying might not be effective.
“In such cases, hand removal of affected bunches or selective harvesting may be an option.
“Controlling for downy and powdery will also help manage botrytis, and research from New Zealand suggests maintaining 70 to 90 per cent bunch exposure by selectively removing leaves from late flowering to pre-harvest significantly reduces botrytis risk.
“However, caution is necessary with leaf plucking to avoid sunburn, especially in hot regions, if excessive leaf removal occurs later in summer.”
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Downy mildew
CAUSED by plasmopara viticola, downy mildew is a major disease of grapevines that originates from North America. It was first detected in a commercial Western Australian vineyard in 1998.
It is characterised by the presence of oil spots on the surface of leaves and white down that can be seen on the underside of the leaves, canes and bunches in periods of high humidity. It can cause significant impact on yield if control measures are not implemented.
Downy mildew attacks all green parts of the grapevine.
The symptoms vary with leaf age. On young leaves (in spring), the disease will appear on the upper surface as small yellow spots referred to as oil spots. They are about 10mm diameter, often with a chocolate halo. These spots tend to grow to about 50mm diameter as they mature and the halo fades. As they enlarge they may appear to cover most of the leaf, especially if there is more than one spot on the leaf. On red varieties the oil spot can appear red.
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Powdery mildew
POWDERY mildew, common across Australian vineyards, appears as a characteristic grey-white powdery growth on green tissue of the grapevine.
Infections around time of flowering or shortly after cause the most damage to grapes.
Main symptoms on leaves are irregular yellow blotches best seen on the upper surfaces and, on varieties like chardonnay, with associated browning of the smallest (tertiary) veinlets on the under sides.
The blotches soon show the typical white mildew as spots expand and merge across both surfaces of leaves. Eventually the whole surface is covered.
The disease develops in the absence of free moisture and generally progresses independently of weather, though mild, humid conditions are favoured and cooler temperatures may be limiting.
Control of powdery mildew in vineyards is largely by chemical fungicide sprays, with many vineyards being sprayed six to seven times per season.
With improved knowledge of how the disease spreads, the need to time sprays appropriately, improved spray technology and access to an excellent array of fungicides, direct crop loss from powdery mildew tends to be minimal.
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Botrytis
BOTRYTIS cinerea is one of number of different fungi that can cause bunch rot in grapevines.
A weather-driven disease, botrytis rot can cause significant loss of grape yield and quality, even after application of a full program of fungicides.
Botrytis spores are almost always present in vineyards.
Infection can be initiated from spores carried over from the previous season, in sources such as cane debris, bunch remnants, tendrils, leaf petioles and leaf blades. Important sources of infection in growing vines are infected damaged leaves, decaying floral parts (mainly caps) and aborted and rotting berries.
The fungus can rest in a quiet (latent) state and then resume growth when the developing berry begins to soften.
After latent botrytis resumes its invasion of a grape berry, it can then spread from berry to berry, which is especially rapid in compact bunches of thin-skinned varieties. Botrytis also spreads readily from bunch to bunch in crowded fruit zones.
Botrytis infects grapevine tissue via wounds and natural openings, including microfissures in the berry skin and wounds made by insects, powdery mildew, berry splitting, loose pedicels or other physical damage.
Any decaying grape tissue, especially damaged leaves, dead floral parts and ripe berries, is a prime target for botrytis colonisation and subsequent spore production.
Botrytis thrives in weather conditions which are consistently wet or humid.
