Vine growers should take no chances with downy mildew and Botrytis this season, with correct fungicide choices applied well and at the right time key to keep the door shut on both pathogens.

In the UK’s maritime climate vine growers are up against it, as mild and wet conditions encourage downy mildew, an enemy that can lead to total crop loss when infection occurs during flowering.

This is compounded by the fact that many of the traditional grape varieties grown here, such as Pinot Noir and Chardonnay, are highly susceptible to the disease.

Agrii’s viticulture specialist Julian Searle said downy mildew infection crept in late last season, perhaps after growers believed the risk was over, and showed how problematic it can be.

That is why he considers RIMpro’s downy mildew model as a very important management tool for growers, as it provides a quantitative risk assessment across the season and can help avoid such oversight.

He added that knowing the key stages when the crop is most at risk, and ensuring there is adequate protection at those times, are also vital to avoiding any yield loss.

Susceptible periods

The downy mildew pathogen survives in the soil and leaf litter as oospores. When wet weather and temperatures above 10°C arrive in the spring, it triggers the germination of zoospores that infect the new and rapidly developing crop canopy.

This tends to happen from mid-May onwards and Julian said the most susceptible period will be the weeks leading up to flowering and the flowering period itself.

“About six or seven weeks after flowering, there is less risk to the fruit as it toughens, but if the crop is moving through the phenological stages slowly, crops can be susceptible for a week to 10 days longer than expected,” he explained.

Julian believes that is what happened in 2023, with growers believing crops were over the worst risk only for infection to arrive suddenly in the middle of August.

“The criteria for a downy mildew infection event are quite complex, so using models is the best way to assess risk at those times and ensures you maintain protection when it is needed,” he explained.

With Botrytis, greatest infection risk is at flowering and fruit set, then at pre bunch closure, where detritus from flowers can stick within the bunch and provide a source of Botrytis infection.

Botrytis risk then recedes until veraison, the period when carbohydrates in the grapes are converted into sugars.

During this time, the fruit swells and creates pressure within the bunch, which can cause berries to split or become loose at the stalk end, providing an access point for disease.

“Here in the UK, this tends to occur in September and fruit isn’t harvested until October, when there is a lot of damp weather and relateively warm temperatures, so conditions for disease are almost perfect.

“It’s important to maintain protection with fungicides right up to picking,” he explained.

Programmed approach

For downy mildew control, the most successful growers take a programmed approach, applying a fungicide every seven to 14 days, depending on crop growth stage and disease pressure, as dictated by models.

Products with activity on Botrytis can then be dropped into the mix during susceptible periods throughout the season.

Julian said that utilising biological products that help reinforce the plant’s own defence mechanisms is important early in the programme, with Frutogard a good example.

It works in three ways against downy mildew. First, it provides a physical barrier against the pathogen through the formation of callus. Second, its algae extract offers some anti-microbial effect.

Finally, the main active, potassium phosphonates, promotes the synthesis of defence proteins within the plant, providing resistance to the disease.

Agrii has been working with Plant Health Care to develop its harpin protein products for vines, and the foliar spray Innocul8 – which contains zinc, manganese, and peptides – also helps to build natural tolerance to disease when used early in the programme.

“When considering conventional fungicides, during periods of high disease pressure we are ideally tank mixing a protectant material with something with a little bit of curative activity, then rotating those modes of action through the season,” he explained.

Conventional products with activity against downy mildew include Percos (ametoctradin + dimethomorph), Shinkon (amisulbrom), Option (cymoxanil), Karamate (mancozeb) and SL 567 A (metalaxyl-M).

Julian said: “Copper oxychloride product Cuprokylt is an effective material on downy mildew and is a useful inclusion throughout a programme as an anti-resistance strategy.”

He added that the formulated potassium bicarbonate product Karma is another product that shouldn’t be underestimated in its importance.

“It’s one of the few eradicant products we have. If you have an outbreak of powdery mildew, downy mildew or Botrytis, a couple of back-to-back applications and you would find that it clears things up.

“It’s a clever formulation and unlike commodity substance, it doesn’t need anything else with it, as it has its own wetting system. Its rate of 5kg/ha in water volume of 500L/ha is about the right concentration as well,” he explained.

Useful biologicals

A final consideration for fungicide programming is covering Botrytis risk and Julian stressed the importance of getting something robust on vines at pre bunch closure to prevent infection developing within the bunch.

“Later, during veraison, biorational products are useful due to their short pre-harvest interval,” he added, with Amylo X (Bacillus amyloliquefaciens D747) and Botector (Aureobasidium pullulans strain DSM 14940 and Aureobasidium pullulans strain DSM 14941) key.

“Both are active on Botrytis and we have Amylo X that will have a go at powdery mildew in organic programmes, too,” he explained.

Alex Cooke, technical account management for horticulture at Certis Belchim, adds that when using biorational products, application quality is key to getting the most out of products like Karma and Amylo X.

“Often, biorational products are contact-acting and rely on good coverage of the plant to be most effective. That makes correct sprayer set up and water rate critical,” he noted.

To find out more about powdery mildew and other diseases and pests of vines and their management and control, download a pdf of Certis Belchim’s Successful vine-growing recently updated for 2024 which is available at