How to Control Powdery Mildew in Vines

When it comes to managing powdery mildew in grape orchards, prevention is better than cure, and the key is to act early in the season.

Powdery mildew mainly attacks the green parts of the vine, such as the leaves.
Photo: Prof. PG Goussard

During the 2021/22 season, wet spring and summer conditions have led to an explosion of powdery mildew infestations at wineries in various areas of the Western Cape and Southern Cape.

Not only did the fungal disease have a negative effect on yields, but it also drove up production costs, as farmers were forced to use more fungicides than usual in an attempt to control the disease.

Moreover, before the harvest could take place, they also had to send workers into the vineyards to remove the infected berries by hand, at the risk of having an adverse effect on the quality of the wine.

De Wet du Toit, crop protection specialist at Winfield United South Africa, told farmers at a Winetech Vinpro information day held in May in Rawsonville, Western Cape, that early action was key to successful management of powdery mildew.

“Instead of waiting for signs of powdery mildew to appear before taking action, it is better to follow a spray program early in the season to prevent the disease from spiraling out of control. You cannot destroy powdery mildew, so prevention is much cheaper and more effective than trying to eradicate the disease,” he explained.

Although there are no physical signs when plants are first infected with powdery mildew, farmers should be aware that poor management and high disease pressure in a season is a indication of further problems to come the following.

Du Toit said the fungus overwintered in dormant buds, which released spores after bud break. These spores were carried by the wind to nearby vines, where they germinated to form new infections within five to ten days. Once a vine was infected, it took between seven and 28 days, depending on climatic conditions, for the characteristic powdery white spots to form on the leaves.

He added that powdery mildew usually develops at temperatures between 20°C and 27°C (see table 2), with relative humidity greater than 75%. The fungus can be destroyed if exposed to temperatures above 40°C for at least six hours. However, such temperatures were rare in canopies, especially those of vigorous growers with lots of foliage.

Other factors contributing to the development of powdery mildew include periods of reduced ultraviolet radiation, such as when days are overcast or when shading occurs in leaf canopies; as well as a lack of air movement through canopies, especially thicker ones.

The spores do not need free water to germinate, so rainy conditions are not a prerequisite for disease development.

“Some growers using tunnels use irrigation to wash powdery mildew off their vegetables, but that won’t work outdoors because the fungus can be present in multiple stages in the vineyard,” Du Toit said.

The fungus mainly attacks the green parts of the plant. Once the berries have reached a Brix value of 12 to 13, they are no longer sensitive; however, leaves and stems remain prone to infection.

Du Toit said the grape varieties most susceptible to the disease were Chardonnay, Chenin Blanc, Riesling, Semillon, Pinotage, Muscadel, Hanepoot and Ruby Cabernet.

Arms

Speaking at the information day, Gustav Rademeyer, crop protection specialist at Wenkem, said farmers should have proper plans in place for their spraying programs before the start of the season. Depending on disease pressure and farmer access to tools, applications should be made every two to three weeks.

“If you are growing in a high-risk area, a two-week schedule is recommended, while a three-week schedule will suffice in a low-risk area,” Rademeyer explained.

“Spraying every three weeks will require higher doses and is riskier than applying fungicides every two weeks. the environmental conditions are conducive to the development of powdery mildew,” he added.

Rademeyer said mistakes made in a powdery mildew spray program could be corrected by applying dusted sulfur between the flowering stage and when grape berries have reached pea size, but added that time had to be taken into account during this operation.

“In windy areas, like Worcester and Rawsonville, conditions are not always favorable for sulfur dusting.”

Besides sulfur, a wide variety of products are available for powdery mildew management (see table 1). Some farmers use products with the same active ingredients twice in a row; however, Rademeyer advised against this due to the risk of the fungus becoming resistant to the fungicide when an active ingredient was used repeatedly.

“There are enough products with different chemical groups to allow farmers to use a product only once per season,” he said.

To reduce this risk, he advised farmers to use four or five products from different chemical groups and to rotate them. He added that sulfur was a valuable weapon in this arsenal, as there was no risk of powdery mildew becoming resistant to it.

The fight

As with all pesticides, products used in powdery mildew control had to be used as directed in order to achieve the desired results. Thus, farmers must read product labels to ensure they are using the correct mixes and application methods, and apply the products under the right climatic conditions.

Rademeyer said it doesn’t pay to spray fungicides if the spray nozzles aren’t clean or if they’re worn, as it could cause blockages or incorrect droplet sizes that can lead to poor fungicide coverage. . In turn, this would negatively affect the effectiveness of the product.

“The nozzles should be cleaned every morning before spraying. If the nozzles continually clog, the filters can break, which means they need to be replaced,” he added.

The correct size nozzle should also be used with the correct pressure, as this will give good spray coverage and ideal sized droplets.

Rademeyer said it’s best to use four to 10 nozzles per spray pump, depending on growth stage and canopy size. “For example, more nozzles and water are needed to accommodate increased leaf cover and vegetative growth at different phenological stages of vine development.”

He said the spray droplets should be between 145 and 325 microns in size. “In the past, farmers were advised to spray fungicides at high pressure, which meant using smaller nozzles.

“Since then, we’ve realized that it’s better to use larger nozzles at lower pressure because they don’t clog and wear out as quickly as smaller nozzles,” he said. declared.

Rademeyer recommended using orange or red Albuz nozzles at a pressure range of 6 to 12 bar to achieve the optimum water volume and droplet size.

For good penetration of thick canopies, he advised that fungicides should be applied at a speed not exceeding 80s/100m, which amounts to a maximum tractor speed of 4.5km/h to 5km/h.

Given the huge escalation in input costs, farmers may be tempted to forego fungicide applications. To save on costs, Rademeyer said growers should instead apply the more expensive products early in the season, when low water volumes are needed, and use the cheaper ones later, when higher water volumes are needed. larger ones were needed.

Email Gustav Rademeyer at [email protected].

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