Controlling Powdery Mildew

At the recent NZW Grape Days, the issue was discussed in great length. With Gisborne and Hawke’s Bay having suffered in the past three years, it was Marlborough’s turn in 2015 to come under the hammer. As Stuart McLagen from Delegat told the Grape Day audience in Marlborough, the region is “locked and loaded” if growers don’t get on top of the issue early on this coming season.

“If we get another year (in 2016) with the right conditions, the fuse will be lit and if we don’t control it very early, we are going to be in trouble.”

To understand how to control powdery, you need to understand how the pathogen works, Trevor Lupton from Lewis Wright Valuation and Consultancy says. 

As most people now know, there are two types of powdery. One the asexual type, which has been in New Zealand for more than 100 years. It over winters in buds infected the previous spring and produces flagshoots in the next spring . It is the flagshoots which produce spores to start the infection cycle for the next season.

Type 2 is sexual, which gives the pathogen genetic diversity. It over winters as Chasmothecia, releasing acospores in spring.

“One of the key things about this guy, is its genetic diversity and the risk of resistance management with the modern single site fungicides if they are not used properly,” he said.

Condition wise, powdery infection is favoured by warm days, temperatures between 20 and 28 degrees. It loves low ultra violet light, which means it develops on the inside of vines. It enjoys high humidity – but not rain.

“So it develops faster in warm springs and warmer regions, and areas with dry seasons,” Lupton said.

In terms of when the vine is most at risk, Lupton said berries become resistant to attack three to five weeks after fruit set.

“Probably five weeks in our maritime environment.”But the rachis remains susceptible. 

Leaves are highly susceptible at 50 percent maximum size, then they become a little more tolerant – although never immune to attack.

“So it’s important to protect the leaf, particularly in the pre flowering period, because that is the source of the infection onto the berries. Therefore bunch-line spraying isn’t an option with powdery.”

Spray intervals need to be carefully considered. In research undertaken in Gisborne, Lupton said the growers who managed to control powdery had closed up their spray intervals.

“They closed them up to seven to 10 day intervals particularly over flowering to bunch closure. Gone were the 10 – 14 or 14 – 21 days with DMIs.” 

Water rates have also changed, he mentioned. In the past, rates had dropped to 250 litres a hectare, on three metre rows.

“It is more common now to see 450 to 500 litres a hectare. That is not to say that you can’t do it with 250 – 300 using a super wetter, but in my mind, water rates hide a thousand sins. You get a bit of wind and coverage becomes more difficult, once your water rates get down.”

Those having success in controlling powdery in Gisborne were also using a higher rate of sulphur per hectare.

“So on a three metre wide row they were using four to five kg a hectare. Historically they had been using three.”

Lupton said in Australia, it was common to see six kg a hectare used, and he had wondered why.

“But remember they have been working with Chasmothecia for 30 years now. So what is natural to them may have been a response to those conditions.”

As reported in earlier stories here in NZ Winegrower, the Gisborne growers who were having good control over powdery mildew were using lower numbers of DMIs.

“The growers who were having problems were often using one or two DMIs over flowering and one or two in January. That was an observation of mine.”

So given Gisborne has been a hot seat for powdery mildew infection, Lupton along with David Manktelow started looking at growers who were suffering to see if they could see any patterns emerging.

Two growers involved had suffered severe powdery in 2014. So severe that the crops were either rejected or 20 to 50 percent had to be cut prior to harvest.

In 2015, the percentage of bunches infected in those exact vineyards had dropped to between 0.4 percent and 2.3 percent. Lupton says this shows that powdery mildew is a management issue and it can be managed in just one season.

But how? That is the question many growers will be asking as we prepare for budburst 2015.

“The key change was the sprayer set up,” he says. “We dropped the DMIs. We could possibly have got away with one, but I would have liked to mix it with sulphur.

“We also mixed protectants with eradicants during the season. There were some minor changes to the canopy (opening it up) to ensure we got spray coverage. But the overall thing about controlling powdery mildew was about the coverage and getting that sprayer set up right.”

Also growers need to take into account their own individual vineyard, what the row widths are and whether or not they are adjusting their water rates to fit their row width and/or length.

“The label rates we work on were developed around three metre rows and 1000 litres of water a hectare. We have worked from that and said, modern sprayers don’t need 1000 litres, we can get away with 300 or 400 litres a hectare.

“But we have also gone to the situation where more than 50 percent of our vineyards are less than 2.5 metre row widths.
So we have to think about adjusting our product rate per hectare to suit.

“If you have been using 400 litres per hectare for 3 metre row, then you need 500 litres per hectare for a 2.4 metre row.”

The same applies to sulphur rates. 

“If you were using 5kg of sulphur for 3 metre rows, you need to up that to 6.25 kg for 2.4 metre rows.”

And his final advice?

Never use a single site fungicide as an eradicant or on an established powdery mildew infection. Use a durable such as sulphur.

“One thing we have done historically which we should never do again, is use two DMIs back to back seven days apart. If you want to build a research population (of powdery mildew) with great resistance, that’s a good way to do it. But don’t do it in your vineyard.”